- Email: [email protected]
Lexical reconciliation
Lingua
92 (1994) 41 l-430.
North-Holland
411
Lexical reconciliation* Jane Grimshaw Depurtnwnt of’ Linguistics und Center for Cognitive Science, Pluce. New Brunswick, NJO8903.
In the context about
lexical
of current
learning.
One
research holds
in lexical that
acquisition
of its syntax,
acquisition
of its semantics.
views. and
proposes
observation
is the input to the linguistic
checked
against
another
syntax.
representation
the semantics
University, 18 Seminary
examines
Learning
principles.
occurs
is critically
of the wcrd
the positive
of the two in which mapping
there are two fundamental
of a word
holds that the syntax
This paper
a reconciliation
observed
Rutgers
USA
results
is critically
in the
involved
in the
and limitations
a hypothesized
ideas
involved
meaning
These derive a predicted
of the two based
on
s-structure,
when the two match.
1. Introduction The now standard lines of reasoning concerning universal properties of language and language learnability lead inexorably to the conclusion that the lexicon that is eventually mastered by a language learner is every bit as much a function of the system mastering it as it is of the linguistic data the learner is exposed to. This represents something of a change ~ in earlier days of generative grammar, the lexicon was often defined as a collection cratic information. The assumption was that lexical information what was unpredictable and unprincipled, and what therefore must about the language. Since the lexicon was in no way determined
of idiosynwas simply be learned by UG, its
properties depended only on what was observed. In the past ten or fifteen years, we have achieved greater understanding the structure of lexical systems, as a result of work on the representation *
This paper
University
grew out of joint
Language
Development
work
with Steve Pinker,
Conference
influence. Many discussions with Lila Gleitman conversations with Barbara Landau. Thanks
in
which
1990, and
was presented owes
much
helped to clarify the arguments are also due to Alan Prince,
of of
at the Boston
to his input
and
immensely, as did the Rutgers-Penn
lexicon group, participants at the University of Pennsylvania Lexical Learning Conference in 1992, and the Workshop on Thematic Roles in Linguistic Theory and Language Acquisition held at the University
of Kansas
in 1993, and an anonymous
reviewer.
0024-3X41/94/507.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0024-3841(93)E0045-9
412
J. Grimshaw
I Lexical
reconciliation
lexical items, and the grammatical processes which involve them 1990, Jackendoff 1990, Levin and Rappaport 1992, Pinker 1989, press, Zubizarreta 1987). With this has come the recognition systems, just like other linguistic domains, are subject to universal
(Grimshaw Williams, in that lexical constraints
and a high degree of language-internal consistencey. (For a particularly interesting perspective on the relationship between the theory of words and the notion of lexical listing, see Williams 1992.) A change in perspective has resulted. If the endpoint of the acquisition process is highly principled, then the acquisition process itself must be interesting. The acquisition issues that have been addressed in recent research seem to fall into two subgroups. First, especially since Baker (1979) there has been an important series of studies which attempt to uncover the principles behind the acquisition of patterns of alternation shown by verbs: including Bowerman (1974, 1990) Choi and Bowerman (1991) Grimshaw (1989), Gropen et al. (1991), Pinker (1989), Randall (1990). These questions have been the concern in particular of theories which attempt to make use of mapping between semantics and syntax. Second, some recent work has investigated the question of how the relatively gross meaning of a word can be acquired (Landau and Gleitman 1985, Gleitman 1990, Naigles 1990, Fisher et al. 1991) looking into theories which make use of mapping of some kind between syntax and meaning. Research of the first kind asks how learners determine that gire participates in the ‘dative alternation’ (We gave a book to someone, We guvr someone a book) whereas donate does not (We donated a book to someone, *We donated somone a book). Research of the second kind asks how learners determine that a particular morpheme means ‘give’, rather than ‘hold’. Whether these are ultimately different questions, with different answers, remains
to be seen.
2. The linguistics of the lexicon The claim that observation of the world is inadequate for the acquisition of lexical syntax is no surprise - the argument here is just the same as the argument for any other kind of syntax. The theory of lexical syntax involves well-formedness principles for argument structure and its interface with lexical semantics and d-structure: the principles are universal, and not plausibly learned. With respect to lexical meaning, though, this position is perhaps more surprising, although arguments from lexical semantic universals, patterns of errors in learning and so forth, show the effects of UG in
J. Grimshaw / Lexical reconciliarion
413
lexical semantics as elsewhere. The theory of lexical semantics plays the same role in constraining possible word meanings and the range of solutions that the child will entertain, as the theory of Universal Grammar plays in syntax. For example, it seems to be the case that while a causative meaning can be expressed either by a single morpheme, as in (la), or by a phrase, as in (1 b), a causative-of-a-causative meaning can never be expressed by a single morpheme, even though it can be expressed by a phrase, as in (c). (1 a) The chemical killed the grass. cause-to-die (1 b) The chemical caused the grass to die. cause-to-die (lc) Overapplication caused the chemical to kill the grass. cause-to-cause-to-die This restriction is a consequence of the principles which organize lexical semantic structure and argument structure, some of which are discussed in the work cited above. Thus linguistic theory provides a constrained representational system which limits the solutions available to the learner of the lexicon, along lines familar from other domains. Starting from the simplest point of view, for each morpheme in the lexicon, there are two sorts of information to be determined by a learner. These are the syntax of the lexical item, which includes its syntactic category and its subcategorization properties, however they are expressed, and its semantics. Minimally, for example, a learner must determine that hope is a verb which occurs with a sentential complement, and that it expresses a relation of a certain kind between an individual, the hoper, and a proposition or situation (We hope that it won’t rain tomorrow, We hope for rain tomorrow). Much current work on lexical theory attempts to uncover the principles of UC which constrain the relationships among various aspects of the lexical representation of predicates and between the lexical representation and the syntactic realization. (On the latter see especially Baker 1988, Pesetsky 1992.) The picture that emerges from this research program is one in which the range of syntactic configurations associated with a verb is highly predictable from its semantics, once parametric syntactic variation is taken into account. The lexical semantic representation of a predicate determines its argument structures, its argument structures plus parametric properties of phrase structure entail the d-structure configurations of the predicate, and the
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/ Lexical
reconciliation
d-structure plus further parametric effects entail the s-structure configurations the predicate appears in. The existence of principles of these kinds guarantees that certain relationships will obtain or adjective, and its meaning.
between
the s-structure
of a verb, noun,
This basic point is fundamental to almost all current ideas about lexical learning. However, the principled character of the relationship between meaning and syntax is sometimes questioned (for a recent example see Bowerman 1990) so it is important to clarify the nature of the claim that the relationship is indeed principled. The key point is that the notion of mapping is a highly abstract one, in two important respects. First, mapping from lexical semantics is onto d-structure, not s-structure, although all that the language learner and the linguist can observe is the sstructure. S-structure of course is affected by other considerations also, some cross-linguistically variable, and thus reflects linking only quite indirectly. In fact, it is not possible to draw conclusions about linking directly from dstructure either, since d-structure itself is subject to parametric variation. All predictions of a mapping theory are made modulo parametric properties, both lexical and grammatical, of the language. Second, mapping does not take as its input an event, but a semantic representation. That is to say, the theory of UG does not say mything at all about how events are described syntactically. What it does say something about is how particular semantic representations are expressed syntactically (modulo the parametric properties of the language, as just discussed). To say, then, that a verb which ‘means x’ will map into the syntax in a particular way, is to say that a verb with a particular lexico-semantic representation will have a particular syntax. Another verb which superficially appears to have the same meaning, in that it describes (at least) approximately the same events, may have a very different syntax. A particularly neat example of this type is the contrast discussed by Levin and Rappaport Hovav (1992) between the English blush and Italian arrossire. Both are used to describe the same event, and in this sense they mean the same, yet the Italian verb has unaccusative syntax while the English verb has unergative syntax. Levin and Rappaport Hovav show that the semantic representations of the verbs are different: arrossire is a change of state predicate, while blush is not. Thus despite their similarity in terms of the events they describe, the verbs have different lexical semantic representations, and this is what determines their syntax. The principles which relate one level of representation to another relate the lexical semantics of a predicate to its (lexical) syntax: they do not relate events in the world to (lexical) syntax.
J. Grimshaw / Lexical reconciliation
415
3. Situations and sentences Idealizing considerably, we can pose the learning problem as in (2): (2) SITUATION SENTENCE
\ /
OBSERVER
>>>>> WORD REPRESENTATION
The input to the learning system is a pair of an s-structure, and a representation of a world situation. This representation will be an interpretation of a perceived event, for example, based on observation, previous observations, surrounding discourse, etc. How then does a learner arrive at the lexical representation for the verb in the sentence, given this kind of learning situation? Two ideas form the focus of a large amount of current research on the question. The first idea is that analysis of the situation can make it possible to determine the meaning of a word, and that the meaning of a word in turn makes it possible to determine its lexical syntax. This type of proposal has been very fully developed by Pinker (1989), in an important extension of earlier work, which concentrated on the question of how meaning might play a role in allowing the child to perform an initial syntactic analysis upon which the syntactic system of the language could be erected, and on ways in which the meaning of a verb could make aspects of its syntax predictable to a learner; Grimshaw (1981) Pinker (1984, 1989). The second idea which has emerged on how linguistic relations play their part in acuisition, is that analysis of the sentence makes it possible to determine (parts of) its semantics (see Landau and Gleitman 1985, Fisher et al. 1991, Fisher et al., this volume, Gleitman 1990). These two ideas about mapping are sometimes contrasted under the rubrics ‘semantic bootstrapping’ and ‘syntactic bootstrapping’ (but see Pinker, this volume, for discussion). ‘Semantic bootstrapping’ has one extremely important property: it makes direct use of the principles mapping from lexical meaning to lexical syntax discussed earlier. I will not illustrate this in detail - the works referred to above contain many examples. Nonetheless, it would probably be incorrect to maintain that an analysis of the situation alone is the input to the learner. Because of the indirect relationship between events and semantic representations of verbs, discussed in the previous section, it is not easy for an observer to determine a verb’s meaning from an event. Gleitman and colleagues (see Gleitman 1990 and Fisher et al., this volume, for examples) have looked into this point, showing that events typically have multiple construals, hence many
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/ Lexical reconciliation
verb meanings will be compatible with most events. As an example, consider a verb which has a caused change of state interpretation, with a semantic representation of the form: ‘x causes y to become -‘. Such verbs often have a change of state counterpart, which can utilize the same morpheme, as with melt, or a different morpheme, as in kill/die. The change-of-state semantic representation of the form: ‘y becomes -‘. (3) We melted the ice/the ice melted We killed the dragon/the dragon
version
has a
died
The problem for word learning from world situations is that circumstances that can be described by one member of the pair of verbs, can often be described equally well by the other: see the discussion of give and receive in Fisher et al. (this volume) for example. The causative entails the change of state, hence whenever the causative description is true of some state of affairs in the world, the change-of-state description is true also (although not vice versa). So if a learner guesses the causative (kill) when in fact the verb has a change-of-state meaning (die), and if word learning is based on world observation alone, recovery is only possible if some situation tells the learner that even when there is no possibility of construing the event as involving an agent, die is still used. If a learner chooses the change-of-state meaning (‘die’) for a morpheme which is in fact the causative (‘kill’), there is no way to correct the mistake, because there is no world situation where the inchoative is inappropriate and the causative appropriate. The correct meanings must therefore be assigned to the members of the pairs by some other means, since learning by world observation seems at worst to be impossible, and at best to require that juvenile speakers have access to a disambiguating situation for every pair of meanings like this. But if observation of the world is not enough, other means must be available for lexical learning. This is where key information contained in the sentence in (2) offers the most promising avenue for successful word learning, very roughly along the lines sketched by Gleitman (1990). The fundamental idea is that the linguistics of words itself makes learning words possible. It is the language, and not the world, that supports the process of word learning. Returning to the kill/die problem, although the situations in which the two morphemes are used are not good sources of information about the meanings of the verbs, the two are linguistically different in a crucial respect, and this linguistic difference makes word learning possible. What I will for now term, loosely, the ‘transitivity’ of kill and causative melt contrasts with the
J. Grimshaw1 Lexical reconciliation
417
‘intransitivity’ of die and inchoative melt, and this property is correlated with the meanings in a way that can be exploited by a language learner. What aspects of verbal meaning can in principle be deduced from the syntactic context a verb appears in? The proposal advanced by Landau and Gleitman (1985) is that learners use information about the surface syntax of a clause to determine (aspects of) the meaning of the verb in the clause. Specifically, they suggest that the subcategorization frame of a verb contains the critical information. It is easy to see that this idea will be useful in solving the kill/ die problem: since kill is subcategorized for an NP complement and die is not, a learner who knew the subcategorizations in advance could use them to choose the right morpheme for the right meaning. (Presumably variability resulting from parametric variation can be factored out of the situation, and a sufficiently abstract view of the subcategorization sets will make it possible to treat the subcategorization of a verb in one language as being the same as the subcategorization of a verb with the same meaning in another language, despite superficial differences in the syntactic systems in the two cases.) When the issue is considered in a more precise fashion, however, a number of considerations suggest that subcategorization frames are not the optimal source of information. (See Pinker, this volume, and Fisher et al., this volume, for additional remarks on the limitations of frames for learning semantics.) First, observable context alone cannot determine what the subcategorization frames of a verb are. Arguments figure in subcategorization frames, adjuncts do not. However both occur on the same side of the verb in a language like English, hence there is no positional evidence to distinguish one from the other. This problem arises wherever adjuncts and arguments have the same form, e.g. with PPs. Consider the examples in (4): while write takes an optional PP adjunct, put has an obligatory PP argument. Similarly, lust has a temporal argument, while wriggle occurs with a temporal adjunct. (4a) (4b) (4~) (4d)
He wrote a book in his room. He put a book in his room. The performance lasted for an hour. The performer wriggled for an hour.
Without knowing which expressions are arguments it is not possible to know what frames the verb appears in, but in order to know which expressions are arguments it is necessary to know what the verb means. Thus it is not clear what role the subcategorization frames could have in the acquisition of verbal
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meanings in cases where there is no clear independent indication of the argument or adjunct status of a phrase associated with the verb. A second kind of limitation arises because of the existence of large numbers of many-to-one semantics-to-syntax mappings. For example, the set of verbs which subcategorizes for NP is both enormous and extremely disparate semantically : (5a) He weighed (5b) He weighed
the tomatoes. 300 pounds.
(6a) He became a doctor. (6b) He shot a doctor. (7a) He asked someone (7b) He asked someone
the time. a question.
So the fact that a verb takes an NP complement is not very informative as far as its meaning is concerned. (Of course it is not completely uninformative, since it does make it possible to eliminate a number of candidates: ‘put’ and ‘die’, for example, are not possible meanings for any of the verbs in (5))(7).) The reason that the syntax here is comparatively uninformative is that many different meanings are mapped onto a syntactic expression like NP. Syntactic mapping based on a single subcategorization frame will be maximally effective when the mapping is one-to-one or just few-to-one. This is probably the case with, for example, sentential complements. A verb with a sentential complement must draw its meaning from a relatively small set of possibilities, which include verbs of communication (say, announce, state), verbs of logical relation (entail, presuppose), and verbs of propositional attitude (hope, believe). Being aware of the limitations and problems of mapping from subcategorization frames to meaning in cases such as the ones just discussed, the researchers working on syntactic mapping have investigated the idea that sets of subcategorization frames, and not just single frames, play a role in the acquisition of verbal meaning (see especially Landau and Gleitman 1985, Fisher et al. 1991, and the discussion of ‘frame range’ in Fisher et al., this volume). The learner will examine the set of subcategorizations that a verb appears in, and discover properties of its meaning in this way. Note that one strong disadvantage of this position is that it is not possible to learn a meaning for a morpheme on exposure, even repeated exposure, to a single sentence type. Analysis across sentence types will be required, since the
J. Grimshaw
/ Lexical reconciliation
419
entire set of frames that the verb appears in, or some approximation thereof, must be observed before its meaning can be determined. There is an important pre-condition for the success of the ‘sets of frames’ idea. In order for learners to be able to use a verb’s subcategorization set to determine its meaning, the learner must know in advance which set goes with which meaning. This is possible only to the extent that the subcategorization set/meaning mapping is cross-linguistically stable, or shows only parametric variation. So the question is whether UG determines the subcategorization set associated with a verb, or not. This issue turns out to be highly problematic the reason is that the total subcategorization set of a verb is a function of the set of subcategorizations in which each sense of the verb participates. And the way senses are distributed across morphemes is not uniform across languages. (This point is also made in Pinker, this volume.) Let us consider why this should be so. The relevant principles of UG govern the mapping between the meaning of a verb and its syntax in that meaning. In addition, UG regulates the set of possible semantic alternations that a morpheme may participate in, such as the causative/change-of-state relation. If each morpheme had exactly one sense, or if the alternative senses of a morpheme were always regulated by UG, this would exhaust the situation. But in fact, a morpheme can have several senses, each with its own UG determined syntax and each participating in alternations in the ways prescribed by UG. UG says little or nothing about the complete set of senses the verb has, and therefore little or nothing about the total set of subcategorizations of the morpheme. UG only determines the properties of the individual senses and those that are related grammatically. As an example, consider the verb shoot. It has at least two senses: one exemplified in She shot the burglar, and one in The burglar shot out of the room. In the first sense, shoot takes an NP complement, in the second a PP. In both cases the subcategorizations are highly predictable. shoot-l is like, say, stab, and takes an individual/NP as its complement, while shoot-2 is like, say raced, and takes a directional PP as its complement. The subcategorization of each sense is completely in accordance with the theory of grammar, but nothing about the theory of grammar determines that shoot will have these two senses. Hence the theory of grammar cannot possibly predict that shoot will have these two subcategorization frames, and a learner could not know this in advance. In cases like these, the senses of the verb do not seem to be related by UG at all, even though they are all realized by a single morpheme. Presumably they are related by association, which depends on semantic field and other
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cognitively real but grammatically irrelevant factors. Thus it is probably not an accident that shoot has a verb-of-motion meaning, and stab does not, since shooting involves a rapidly moving bullet. The point is that the relationship is associative in character, and not UG determined. Similarly, consider the case of know as in I know her and in I know that she is here. The two uses of the verb are cognitively close - both are state predicates and both are psychological. Hence it is presumably much more likely that these two senses will be
clustered together into a single morpheme than that say, eat and one of the two senses of know will be clustered together. The probabilities involved are not a matter of UG, however, and of course the clustering of senses together under a single morpheme is notoriously variable cross-linguistically. The two senses of know correspond to different morphemes in many other languages: French for one. What is involved in these effects is the spreading of morphemes across meanings. The results range in character from pure homonyms where no significant semantic relation can be seen at all, through relations that seem cognitively natural, and that probably reflect various clustering effects, to relations that are grammatical in character. The point is, though, that the entire subcategorization set associated with a morpheme is simply the sum of the subcategorization sets for each meaning of the morpheme. While it is true that the subcategorization set for each meaning is highly principled and strongly related to the meaning, the same is not true for the entire set. Each individual subcategorization frame reflects the systematic properties of the form-meaning correspondences, but the entire set of subcategorizations also reflects accidental combinations of subcategorizations, which result from a single morpheme’s appearing in several different senses. It follows that there is no stable mapping between the full set of subcategorizations that a morpheme appears in and the meaning of the morpheme. The full set of subcategorizations will depend, not just on UG, but also on the range of meanings that the morpheme assumes in a given language. The conclusion is, then, that a predictable relationship between subcategorizations and morphemes does not hold: the predictable relationship is between subcategorizations and a particular sense of the morpheme. It follows that it is not possible to use the entire subcategorization to learn meaning, because the entire subcategorization set is not associated with a single meaning in the jirst place. As a result, a learning mechanism based on subcategorization sets will give poor results on morphemes with many senses, typically the most common ones. It will predict many errors, or failure of learning. Suppose that the
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learning device tracks the syntactic context of the morpheme and attempts to determine its meaning based on this information. It will collect up all observed subcategorizations for a morpheme, disregarding the fact, which it cannot by definition be sensitive to, that verb’s meaning differs across these subcategorizations. It will then assign to the verb the meaning that would be assigned to a verb which, under a single meaning, occurred in this set of frames. But this will be incorrect, except in the case of a verb with exactly one sense. For verbs with multiple senses, if the procedure succeeds in assigning a meaning at all, it will give one meaning to the verb instead of several, and the meaning it gives might well be completely unrelated to any actual meaning of the verb being learned. Without knowing about meaning it is not possible to know which subcategorizations should be grouped together, and which should be kept separate. What of lexical entries in which the relationship between the alternatives is regulated by UG, such as the causative/change of state examples discussed above (e.g. melt/melt)? Suppose that there are two subcategorization frames associated with each of these verbs, one transitive and one intransitive. Can this set of frames be exploited to learn the meaning of the verb? (Note that the existence of the alternation cannot be crucial for learning the meanings of the members, because many verbs, such as kill and die do not alternate, but are still learned.) Here again the answer is negative, because the two occurrences of the morpheme have different meanings, under standard assumptions about lexical representation. 1 So there cannot be a single sense specified by UG as associated with this subcategorization set. Note that it also cannot be the case that the shared meaning, i.e. what the two cases of the verb have in common, is learned from the subcategorization set either, because all causative/change of state pairs have the same subcategorization set, even though each pair has its own meaning. Clearly what we must aim for is a learning procedure that uses the alternation as a clue to the semantic analysis of the verbs, that might tell the learner that these verbs have change and caused change meanings, However,
1 However, Grimshaw in work in progress (1993) denies these assumptions for all cases of alternations involving no overt morphology, proposing instead that there is only one meaning for e.g. mel/ in its causative interacting
with clausal
and inchoative structure.
Further
uses. The ‘alternation’ work is required
is just the result of the meaning
to see how this ultimately
bears on the
learning questions addressed here, but it seems that in this sense of ‘meaning’, observation of the two clause structures associated with me11 is essential for arriving at the correct analysis. This conclusion
will hold for the UG-governed
alternations
only.
422
just
J. Grimshaw
observing
because
subcategorization
/ Lexical reconciliarion alternations
there are other transitive/intransitive
will not alternations
achieve
this
result,
like eat and leave.
(8a) We melted the ice. (8b) The ice melted. (9a) We left the room. (9b) We left. (SC) *The room left. (10a) We ate the ice. (lob) We ate. (10~) *The ice ate. What will guarantee success is to take into account the properties of the arguments, and not subcategorization alone; it is the fact that the subject of one case of the verb corresponds to the object of the other cases that reliably distinguishes change/caused change pairs from eat and leave. One final point concerning the causative/change-of-state pairs. In fact both are ‘transitive’ in d-structure, according to the unaccusative hypothesis (Perlmutter 1978, Burzio 1986) which means that technically they both have Again, this suggests that subcategorization transitive subcategorizations. frames are not exactly the right place for the child to look for help in figuring out verb meaning. We can summarize the conclusions so far as follows. Mapping from meaning onto syntax can successfully exploit a set of principles of Universal Grammar which relate lexical meaning ultimately to surface syntax. However, determining the meaning just from observation of the world seems to require multiple exposures across situations in many cases, and may be impossible under certain circumstances, when the world is particularly uninformative. On the other hand, mapping from syntax onto meaning promises to successfully exploit linguistically encoded information about a verb’s meaning. However as formulated so far, it seems to require multiple exposures across sentences, and also may be impossible in many cases, when syntax is uninformative.
4. Reconciliation Clearly then we seek a model which preserves the advantages of both kinds of ideas: which makes it possible to use UG mapping principles to regulate
J. Grimshaw
syntax,
and to use surface
syntax
I Lexical
423
reconciliation
to regulate
the semantic
analysis.
One way
in which it is possible to combine the essential good effects of both types of mapping gives them different roles in the learning process: the semantics-tosyntax mapping principles provide a predictive mechanism, and the observed s-structure provides a checking mechanism. This is the basis of Reconciliation. (Wilkins 1993 explores a rather similar model for lexical learning, with particular emphasis on the acquisition of morphology.) Reconciliation (1) (2)
(3)
(4)
(5) (6) (7) (8)
The learner interprets a scene or situation, hears a sentence and detects the verb. The learner finds a relationship R among participants in the situation (entities, propositions etc.) that is sensible given the interpretation of the observed situation. The learner checks that R involves participants consistent with the content of the (candidate argument) expressions in the sentence, and rejects an R that does not meet this requirement. The learner constructs a lexical conceptual structure which is consistent with R, and assigns candidate argument expressions in the sentence to argument positions in the lexical conceptual structure. This lexical conceptual structure is fed through the semantics-to-syntax mapping principles of UG in their language particular instantiation. The s-structure predicted by step 5 is compared to the observed s-structure. If they do not match then no learning takes place. If they do match then the morpheme is entered into the lexicon with the hypothesized lexical conceptual structure,
A few comments
are required
about
the steps involved
in reconciliation.
Step 2 excludes situations where the interpretation of the event is one of throwing, say, and R is a relationship between propositions. Step 3 constrains the device to considering Rs that express relationships between the right kind of entitities: if the sentence contains two NPs, and one is the ball, then a verb meaning between haI, R Whether only on 3 is one
‘say’ is not a candidate, since it is not a possible relationship a ball and some other entity. Similarly, with the NPs John and the cannot be a verb meaning ‘dress’, although it could mean ‘throw’. this should depend on probabilities given real world knowledge, or strictly linguistic selectional restrictions, I leave open. Note that step way in which the sentence constrains the process of word learning,
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not through the syntactic form of the arguments of the predicate but through gross consistency between the (candidate) arguments and the relationship expressed by a candidate R. This step is pre-linguistic, in the sense that it does not rely on the linguistic representation of the arguments, just on their gross meaning. It is quite different, then, from the effects of the linguistic mapping and checking involved in later stages. An interesting question is whether some of the effects described in the literature on ‘syntactic bootstrapping’ are really due to this (non-linguistic) process, see Pinker (this volume). At Step 4, however, the procedure is now linguistic in character. If R is a causal relationship between two entities, a causative lcs will be constructed. The system is now working with a linguistic representation, rather than just with construals of events, and conceptual properties of relationships and entities. With respect to the final steps, note that an incorrect representation will be entered into the lexicon, if the match discovered in R6 is accidental. We must assume that general principles of decay will eventually result in eradication of mistaken entries (cf. Braine 1971), since they will result in matches less often than correct entries. Moreover, the notion of a ‘match’ requires further explication. The definition could require identity of form between the observed sentence and the predicted sentence, or it could allow for limited inconsistencies, in particular with respect to various ways of reducing the number of arguments that are actually expressed in a clause. Imperatives lack an overt subject, for example, there are elliptical forms, and there are verbs like wash and eat, which can be syntactically intransitive while apparently maintaining a two-place semantic structure. ‘Cognate objects’ (e.g. to die a peaceful death) and ‘fake reflexives’ (e.g. to behave oneself) pose the opposite problem, and a more detailed treatment is required than I will give here. The reconciliation model incorporates aspects of both semantic and syntactic ‘bootstrapping’. It crucially involves mapping from a posited meaning to a syntactic form. It also exploits the surface syntax to constrain solutions. A simple result of the model is that the number, the position, and the form of the syntactic arguments of a predicate will constrain the semantic representation it is given. This is because of the grammatical principles regulating the lcs-syntax relation. Suppose for example, that a learner hears a sentence containing the verb give: Mary is giving the package to the boy, and observes an event in which one individual hands a package to another individual. Suppose the learner interprets the event as involving a three-place logical relationship of transfer of possession. This ‘R’ will be consistent with the content of the candidate argument expressions in the sentence: Mary, the package, and the boy, since
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these are the right kind of entities to participate in such a relationship. Now the learner constructs an lcs for R (step 4) say x acts to transfer y to I, and assigns the candidate argument expressions in the sentence to positions in the lcs of the predicate. Suppose the learner assigns Mary to x, the package to y, and the boy to z. Then given the semantics-to-syntax mapping principles as they work out in English, the predicted s-structure is Mary verbs the package to the boy. Since this is the observed s-structure, this experience yields an lcs for give. (Note that the wrong assignment of candidate argument expressions to lcs positions would not have yielded a match, hence no learning would have occurred.) Suppose, on the other hand, that the learner construes the event as one of holding or getting, interpretations that are equally consistent with gross observation. Now the learner will posit a two-place ‘R’. At step 3 the procedure might already break down, if the learner can decide that all of the candidate arguments in the clause must be actual arguments, since there are 3 candidate arguments but only two actual arguments. Perhaps this is not so easy, however, since one or other of the phrases might be involved in some adjunct role. Assuming then, that the procedure will not halt at step 3, what happens? The learner now constructs one of the two lcs’s, for hold or get, and examines the predicted s-structures that correspond to them. Let us assume that the lcs’s are something like: x have y, x come to have y. There is no way for these lcs representations to yield the observed s-structure. They have the wrong number of arguments and since there is no way to treat the PP ro the boJ as an adjunct there is no way to reconcile the observed s-structure with the predicted s-structure. In addition, the arguments will be in the wrong syntactic position. Semantics-to-syntax mapping will place the ‘getter’ or ‘holder’ (the boy) in subject position, but this will contradict the observed s-structure which has the giver (Mary) in subject position. In general, the number of arguments in the observed sentence will have to match the number of arguments of R: if a predicate expresses an n-place relationship it will have n syntactic arguments, and if it has n syntactic arguments it will be a logical n place predicate. (I set aside here the disruptions to this generalization mentioned above.) This is, more or less, what the Theta Criterion and the Projection Principle guarantee (Chomsky 1981): the number of phrasal arguments in the syntax is the same as the number of (open) logical positions in the lexical representation of the predicate, and the syntactic derivation cannot change the number of arguments. By the same reasoning Reconciliation resolves the kill/die problem discussed above. A learner can conclude that die can be a change-of-state predicate but
426
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not a causative, that kill can be causative but not change-of-state, and that melt can be both. This can be determined just on the basis of a single observation each for kill and die, and two observations of melt, one in each complement structure. A verb which means to cause something to change state must have two arguments, a verb which means to change state must have one. Hence, since kill has two syntactic arguments, it must have two semantic arguments and cannot have a change-of-state meaning. Since die has one syntactic argument it must have one semantic argument and cannot be causative. The syntactic form provides the information that there are two semantic arguments, which provides the necessary information about possible verb meanings. As we saw in section 3, in order to properly identify instances of the alternation between causative and change-of-state meanings, it is necessary to take into account the fact that the subject of one case of the verb corresponds to the object of the other: We melted the ice, The ice melted. The properties of the arguments are essential to distinguishing these verbs from verbs like leave and eat. If intransitive eat is mistakenly analyzed as having a change of state meaning, its predicted s-structure will have what is eaten in subject position The ice ate. The observed s-structure will have the eater in subject position We ate. Hence the wrong analysis will be rejected. Similarly, if a learner fails to assign a change-of-state analysis to melt, treating it instead as having an lcs like intransitive leave or eat, the predicted s-structure will have the agent in subject position (We melted), while the observed s-structure will have the entity undergoing the change of state in subject position (The ice melted). Once again, the error will be avoided. The syntactic form of an argument will similarly constrain meaning. The syntactic form depends on the semantic properties of the argument, hence the syntax of an argument can provide information about its semantics, and hence about the semantics of a predicate. Sentential complements, for instance, will occur with verbs of propositional attitude (e.g. believe), verbs of logical relation (e.g. entail) and verbs of saying (e.g. announce). Reconciliation has the desired effect that this fact will prevent certain types of errors by learners. Suppose the event is one in which a child is playing roughly with a dog and an adult says either I think that you are being mean to the dog or That dog will kill you. If the verb means ‘think’ and the learner thinks it means ‘kill’, the predicted s-structure will contain an NP complement while the observed s-structure will contain a clausal complement. Similarly if the verb means ‘kill’ and the learner thinks it means ‘think’, the predicted sstructure will contain a sentential complement while the observed s-structure will contain an NP.
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In sum, under Reconciliation, the number, position, and form of the arguments of a predicate will all constrain the interpretations that can be assigned to that predicate. Of course, the same general point can be made for many other kinds of linguistic information: hearing a verb in the passive form, in the progressive, with a particular aspectual modifier and so forth, will similarly provide constraining information about the verb’s semantics, eliminating many posited, but incorrect, lexical semantic representations. This captures the essence of the issue addressed by syntactic bootstrapping: it provides a more precise characterization of the idea that the language can be used to map from observation to verb meanings. In so doing, however, it makes crucial use of the notions behind semantic bootstrapping, concerning the mapping between semantics and syntax.
5. Conclusion The important
properties
of Reconciliation
are these:
_ It does not depend on exposures to multiple sentence types, in the sense that neither cross-situational analysis, nor cross-sentential analysis is involved in setting up the lcs representations. Therefore the problems posed by the variable senses of a morpheme discussed in section 3 do not arise. _ It uses semantics to predict syntax where Universal Grammar makes this possible. _ It uses syntax to eliminate wrong semantic candidates where possible. I emphasize again that I have addressed here the question of learning basic lexical semantic representations, and not the learnability problem discussed in much of the literature, which concerns the problem of determining which verbs participate in which ‘alternations’, see especially Pinker (1989) and references therein. Within the terms of the present discussion we could see this as the question of how the system should proceed when a single morpheme would be assigned multiple representations, whether multiple lcs’s, or multiple syntactic configurations, but I will not explore the issue here. Also unexplored is the issue of how morphologically complex items are analyzed. Even with a procedure like Reconciliation, which exploits a full set of grammatical principles, there is no way to save a learner from having to learn some word meanings simply from observation. There are many are sets of
J. Grimshaw / Lexical reconcilialion
428
words which have absolutely identical linguistics. Such sets include the sets of causative verbs (kiN, melt, burn) and change-of-state verbs (die, melt, burn). Semantics-to-syntax mapping guarantees that each member of the set will be syntactically indistinguishable (just as the names of animals, cut versus dog for example, are not syntactically distinct). So examination of the surface syntax will not inform a learner as to whether a verb means ‘to become liquid’ or ‘to become solid’. The general situation is that it is possible to use the surface syntax to constrain analyses of the semantic structure of a verb, but not its semantic content: the fact that a verb is a change-of-state verb, but not the fact that it expresses a particular change of state. This is a matter of semantic content only and is not reflected in the syntax of a verb at all. Presumably, then, the semantic differences among members of these sets must be learned from observation about the world in some sense. However, this does not necessarily mean that the differences are perceptual in character. A vast quantity of information about the world can be encoded linguistically but is not linguistic itself. Thus a child can observe that melt is used of, for example, ice, while burn is used of, for example, paper. This is sufficient for the child to conclude that the meanings are as they are rather than the other way around. In this way, it is possible for a child to know what a word means without ever having observed an event which would count as an occurrence of what the verb describes. Indeed if this were not the case it would be impossible to understand how meaning differences among unobservable verbs are acquired: think, hope, imagine. For this reason, language is a source of essential information for lexical learning in two respects. As just discussed, language can convey information about word meaning which is orders of magnitude more informative than observation of the world can be. Second, by virtue of the grammatical principles that govern it, language constrains the possible representations of words in ways that learners can exploit in word learning. Reconciliation is one way in which this might happen.
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411
Lexical reconciliation* Jane Grimshaw Depurtnwnt of’ Linguistics und Center for Cognitive Science, Pluce. New Brunswick, NJO8903.
In the context about
lexical
of current
learning.
One
research holds
in lexical that
acquisition
of its syntax,
acquisition
of its semantics.
views. and
proposes
observation
is the input to the linguistic
checked
against
another
syntax.
representation
the semantics
University, 18 Seminary
examines
Learning
principles.
occurs
is critically
of the wcrd
the positive
of the two in which mapping
there are two fundamental
of a word
holds that the syntax
This paper
a reconciliation
observed
Rutgers
USA
results
is critically
in the
involved
in the
and limitations
a hypothesized
ideas
involved
meaning
These derive a predicted
of the two based
on
s-structure,
when the two match.
1. Introduction The now standard lines of reasoning concerning universal properties of language and language learnability lead inexorably to the conclusion that the lexicon that is eventually mastered by a language learner is every bit as much a function of the system mastering it as it is of the linguistic data the learner is exposed to. This represents something of a change ~ in earlier days of generative grammar, the lexicon was often defined as a collection cratic information. The assumption was that lexical information what was unpredictable and unprincipled, and what therefore must about the language. Since the lexicon was in no way determined
of idiosynwas simply be learned by UG, its
properties depended only on what was observed. In the past ten or fifteen years, we have achieved greater understanding the structure of lexical systems, as a result of work on the representation *
This paper
University
grew out of joint
Language
Development
work
with Steve Pinker,
Conference
influence. Many discussions with Lila Gleitman conversations with Barbara Landau. Thanks
in
which
1990, and
was presented owes
much
helped to clarify the arguments are also due to Alan Prince,
of of
at the Boston
to his input
and
immensely, as did the Rutgers-Penn
lexicon group, participants at the University of Pennsylvania Lexical Learning Conference in 1992, and the Workshop on Thematic Roles in Linguistic Theory and Language Acquisition held at the University
of Kansas
in 1993, and an anonymous
reviewer.
0024-3X41/94/507.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0024-3841(93)E0045-9
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J. Grimshaw
I Lexical
reconciliation
lexical items, and the grammatical processes which involve them 1990, Jackendoff 1990, Levin and Rappaport 1992, Pinker 1989, press, Zubizarreta 1987). With this has come the recognition systems, just like other linguistic domains, are subject to universal
(Grimshaw Williams, in that lexical constraints
and a high degree of language-internal consistencey. (For a particularly interesting perspective on the relationship between the theory of words and the notion of lexical listing, see Williams 1992.) A change in perspective has resulted. If the endpoint of the acquisition process is highly principled, then the acquisition process itself must be interesting. The acquisition issues that have been addressed in recent research seem to fall into two subgroups. First, especially since Baker (1979) there has been an important series of studies which attempt to uncover the principles behind the acquisition of patterns of alternation shown by verbs: including Bowerman (1974, 1990) Choi and Bowerman (1991) Grimshaw (1989), Gropen et al. (1991), Pinker (1989), Randall (1990). These questions have been the concern in particular of theories which attempt to make use of mapping between semantics and syntax. Second, some recent work has investigated the question of how the relatively gross meaning of a word can be acquired (Landau and Gleitman 1985, Gleitman 1990, Naigles 1990, Fisher et al. 1991) looking into theories which make use of mapping of some kind between syntax and meaning. Research of the first kind asks how learners determine that gire participates in the ‘dative alternation’ (We gave a book to someone, We guvr someone a book) whereas donate does not (We donated a book to someone, *We donated somone a book). Research of the second kind asks how learners determine that a particular morpheme means ‘give’, rather than ‘hold’. Whether these are ultimately different questions, with different answers, remains
to be seen.
2. The linguistics of the lexicon The claim that observation of the world is inadequate for the acquisition of lexical syntax is no surprise - the argument here is just the same as the argument for any other kind of syntax. The theory of lexical syntax involves well-formedness principles for argument structure and its interface with lexical semantics and d-structure: the principles are universal, and not plausibly learned. With respect to lexical meaning, though, this position is perhaps more surprising, although arguments from lexical semantic universals, patterns of errors in learning and so forth, show the effects of UG in
J. Grimshaw / Lexical reconciliarion
413
lexical semantics as elsewhere. The theory of lexical semantics plays the same role in constraining possible word meanings and the range of solutions that the child will entertain, as the theory of Universal Grammar plays in syntax. For example, it seems to be the case that while a causative meaning can be expressed either by a single morpheme, as in (la), or by a phrase, as in (1 b), a causative-of-a-causative meaning can never be expressed by a single morpheme, even though it can be expressed by a phrase, as in (c). (1 a) The chemical killed the grass. cause-to-die (1 b) The chemical caused the grass to die. cause-to-die (lc) Overapplication caused the chemical to kill the grass. cause-to-cause-to-die This restriction is a consequence of the principles which organize lexical semantic structure and argument structure, some of which are discussed in the work cited above. Thus linguistic theory provides a constrained representational system which limits the solutions available to the learner of the lexicon, along lines familar from other domains. Starting from the simplest point of view, for each morpheme in the lexicon, there are two sorts of information to be determined by a learner. These are the syntax of the lexical item, which includes its syntactic category and its subcategorization properties, however they are expressed, and its semantics. Minimally, for example, a learner must determine that hope is a verb which occurs with a sentential complement, and that it expresses a relation of a certain kind between an individual, the hoper, and a proposition or situation (We hope that it won’t rain tomorrow, We hope for rain tomorrow). Much current work on lexical theory attempts to uncover the principles of UC which constrain the relationships among various aspects of the lexical representation of predicates and between the lexical representation and the syntactic realization. (On the latter see especially Baker 1988, Pesetsky 1992.) The picture that emerges from this research program is one in which the range of syntactic configurations associated with a verb is highly predictable from its semantics, once parametric syntactic variation is taken into account. The lexical semantic representation of a predicate determines its argument structures, its argument structures plus parametric properties of phrase structure entail the d-structure configurations of the predicate, and the
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/ Lexical
reconciliation
d-structure plus further parametric effects entail the s-structure configurations the predicate appears in. The existence of principles of these kinds guarantees that certain relationships will obtain or adjective, and its meaning.
between
the s-structure
of a verb, noun,
This basic point is fundamental to almost all current ideas about lexical learning. However, the principled character of the relationship between meaning and syntax is sometimes questioned (for a recent example see Bowerman 1990) so it is important to clarify the nature of the claim that the relationship is indeed principled. The key point is that the notion of mapping is a highly abstract one, in two important respects. First, mapping from lexical semantics is onto d-structure, not s-structure, although all that the language learner and the linguist can observe is the sstructure. S-structure of course is affected by other considerations also, some cross-linguistically variable, and thus reflects linking only quite indirectly. In fact, it is not possible to draw conclusions about linking directly from dstructure either, since d-structure itself is subject to parametric variation. All predictions of a mapping theory are made modulo parametric properties, both lexical and grammatical, of the language. Second, mapping does not take as its input an event, but a semantic representation. That is to say, the theory of UG does not say mything at all about how events are described syntactically. What it does say something about is how particular semantic representations are expressed syntactically (modulo the parametric properties of the language, as just discussed). To say, then, that a verb which ‘means x’ will map into the syntax in a particular way, is to say that a verb with a particular lexico-semantic representation will have a particular syntax. Another verb which superficially appears to have the same meaning, in that it describes (at least) approximately the same events, may have a very different syntax. A particularly neat example of this type is the contrast discussed by Levin and Rappaport Hovav (1992) between the English blush and Italian arrossire. Both are used to describe the same event, and in this sense they mean the same, yet the Italian verb has unaccusative syntax while the English verb has unergative syntax. Levin and Rappaport Hovav show that the semantic representations of the verbs are different: arrossire is a change of state predicate, while blush is not. Thus despite their similarity in terms of the events they describe, the verbs have different lexical semantic representations, and this is what determines their syntax. The principles which relate one level of representation to another relate the lexical semantics of a predicate to its (lexical) syntax: they do not relate events in the world to (lexical) syntax.
J. Grimshaw / Lexical reconciliation
415
3. Situations and sentences Idealizing considerably, we can pose the learning problem as in (2): (2) SITUATION SENTENCE
\ /
OBSERVER
>>>>> WORD REPRESENTATION
The input to the learning system is a pair of an s-structure, and a representation of a world situation. This representation will be an interpretation of a perceived event, for example, based on observation, previous observations, surrounding discourse, etc. How then does a learner arrive at the lexical representation for the verb in the sentence, given this kind of learning situation? Two ideas form the focus of a large amount of current research on the question. The first idea is that analysis of the situation can make it possible to determine the meaning of a word, and that the meaning of a word in turn makes it possible to determine its lexical syntax. This type of proposal has been very fully developed by Pinker (1989), in an important extension of earlier work, which concentrated on the question of how meaning might play a role in allowing the child to perform an initial syntactic analysis upon which the syntactic system of the language could be erected, and on ways in which the meaning of a verb could make aspects of its syntax predictable to a learner; Grimshaw (1981) Pinker (1984, 1989). The second idea which has emerged on how linguistic relations play their part in acuisition, is that analysis of the sentence makes it possible to determine (parts of) its semantics (see Landau and Gleitman 1985, Fisher et al. 1991, Fisher et al., this volume, Gleitman 1990). These two ideas about mapping are sometimes contrasted under the rubrics ‘semantic bootstrapping’ and ‘syntactic bootstrapping’ (but see Pinker, this volume, for discussion). ‘Semantic bootstrapping’ has one extremely important property: it makes direct use of the principles mapping from lexical meaning to lexical syntax discussed earlier. I will not illustrate this in detail - the works referred to above contain many examples. Nonetheless, it would probably be incorrect to maintain that an analysis of the situation alone is the input to the learner. Because of the indirect relationship between events and semantic representations of verbs, discussed in the previous section, it is not easy for an observer to determine a verb’s meaning from an event. Gleitman and colleagues (see Gleitman 1990 and Fisher et al., this volume, for examples) have looked into this point, showing that events typically have multiple construals, hence many
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/ Lexical reconciliation
verb meanings will be compatible with most events. As an example, consider a verb which has a caused change of state interpretation, with a semantic representation of the form: ‘x causes y to become -‘. Such verbs often have a change of state counterpart, which can utilize the same morpheme, as with melt, or a different morpheme, as in kill/die. The change-of-state semantic representation of the form: ‘y becomes -‘. (3) We melted the ice/the ice melted We killed the dragon/the dragon
version
has a
died
The problem for word learning from world situations is that circumstances that can be described by one member of the pair of verbs, can often be described equally well by the other: see the discussion of give and receive in Fisher et al. (this volume) for example. The causative entails the change of state, hence whenever the causative description is true of some state of affairs in the world, the change-of-state description is true also (although not vice versa). So if a learner guesses the causative (kill) when in fact the verb has a change-of-state meaning (die), and if word learning is based on world observation alone, recovery is only possible if some situation tells the learner that even when there is no possibility of construing the event as involving an agent, die is still used. If a learner chooses the change-of-state meaning (‘die’) for a morpheme which is in fact the causative (‘kill’), there is no way to correct the mistake, because there is no world situation where the inchoative is inappropriate and the causative appropriate. The correct meanings must therefore be assigned to the members of the pairs by some other means, since learning by world observation seems at worst to be impossible, and at best to require that juvenile speakers have access to a disambiguating situation for every pair of meanings like this. But if observation of the world is not enough, other means must be available for lexical learning. This is where key information contained in the sentence in (2) offers the most promising avenue for successful word learning, very roughly along the lines sketched by Gleitman (1990). The fundamental idea is that the linguistics of words itself makes learning words possible. It is the language, and not the world, that supports the process of word learning. Returning to the kill/die problem, although the situations in which the two morphemes are used are not good sources of information about the meanings of the verbs, the two are linguistically different in a crucial respect, and this linguistic difference makes word learning possible. What I will for now term, loosely, the ‘transitivity’ of kill and causative melt contrasts with the
J. Grimshaw1 Lexical reconciliation
417
‘intransitivity’ of die and inchoative melt, and this property is correlated with the meanings in a way that can be exploited by a language learner. What aspects of verbal meaning can in principle be deduced from the syntactic context a verb appears in? The proposal advanced by Landau and Gleitman (1985) is that learners use information about the surface syntax of a clause to determine (aspects of) the meaning of the verb in the clause. Specifically, they suggest that the subcategorization frame of a verb contains the critical information. It is easy to see that this idea will be useful in solving the kill/ die problem: since kill is subcategorized for an NP complement and die is not, a learner who knew the subcategorizations in advance could use them to choose the right morpheme for the right meaning. (Presumably variability resulting from parametric variation can be factored out of the situation, and a sufficiently abstract view of the subcategorization sets will make it possible to treat the subcategorization of a verb in one language as being the same as the subcategorization of a verb with the same meaning in another language, despite superficial differences in the syntactic systems in the two cases.) When the issue is considered in a more precise fashion, however, a number of considerations suggest that subcategorization frames are not the optimal source of information. (See Pinker, this volume, and Fisher et al., this volume, for additional remarks on the limitations of frames for learning semantics.) First, observable context alone cannot determine what the subcategorization frames of a verb are. Arguments figure in subcategorization frames, adjuncts do not. However both occur on the same side of the verb in a language like English, hence there is no positional evidence to distinguish one from the other. This problem arises wherever adjuncts and arguments have the same form, e.g. with PPs. Consider the examples in (4): while write takes an optional PP adjunct, put has an obligatory PP argument. Similarly, lust has a temporal argument, while wriggle occurs with a temporal adjunct. (4a) (4b) (4~) (4d)
He wrote a book in his room. He put a book in his room. The performance lasted for an hour. The performer wriggled for an hour.
Without knowing which expressions are arguments it is not possible to know what frames the verb appears in, but in order to know which expressions are arguments it is necessary to know what the verb means. Thus it is not clear what role the subcategorization frames could have in the acquisition of verbal
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meanings in cases where there is no clear independent indication of the argument or adjunct status of a phrase associated with the verb. A second kind of limitation arises because of the existence of large numbers of many-to-one semantics-to-syntax mappings. For example, the set of verbs which subcategorizes for NP is both enormous and extremely disparate semantically : (5a) He weighed (5b) He weighed
the tomatoes. 300 pounds.
(6a) He became a doctor. (6b) He shot a doctor. (7a) He asked someone (7b) He asked someone
the time. a question.
So the fact that a verb takes an NP complement is not very informative as far as its meaning is concerned. (Of course it is not completely uninformative, since it does make it possible to eliminate a number of candidates: ‘put’ and ‘die’, for example, are not possible meanings for any of the verbs in (5))(7).) The reason that the syntax here is comparatively uninformative is that many different meanings are mapped onto a syntactic expression like NP. Syntactic mapping based on a single subcategorization frame will be maximally effective when the mapping is one-to-one or just few-to-one. This is probably the case with, for example, sentential complements. A verb with a sentential complement must draw its meaning from a relatively small set of possibilities, which include verbs of communication (say, announce, state), verbs of logical relation (entail, presuppose), and verbs of propositional attitude (hope, believe). Being aware of the limitations and problems of mapping from subcategorization frames to meaning in cases such as the ones just discussed, the researchers working on syntactic mapping have investigated the idea that sets of subcategorization frames, and not just single frames, play a role in the acquisition of verbal meaning (see especially Landau and Gleitman 1985, Fisher et al. 1991, and the discussion of ‘frame range’ in Fisher et al., this volume). The learner will examine the set of subcategorizations that a verb appears in, and discover properties of its meaning in this way. Note that one strong disadvantage of this position is that it is not possible to learn a meaning for a morpheme on exposure, even repeated exposure, to a single sentence type. Analysis across sentence types will be required, since the
J. Grimshaw
/ Lexical reconciliation
419
entire set of frames that the verb appears in, or some approximation thereof, must be observed before its meaning can be determined. There is an important pre-condition for the success of the ‘sets of frames’ idea. In order for learners to be able to use a verb’s subcategorization set to determine its meaning, the learner must know in advance which set goes with which meaning. This is possible only to the extent that the subcategorization set/meaning mapping is cross-linguistically stable, or shows only parametric variation. So the question is whether UG determines the subcategorization set associated with a verb, or not. This issue turns out to be highly problematic the reason is that the total subcategorization set of a verb is a function of the set of subcategorizations in which each sense of the verb participates. And the way senses are distributed across morphemes is not uniform across languages. (This point is also made in Pinker, this volume.) Let us consider why this should be so. The relevant principles of UG govern the mapping between the meaning of a verb and its syntax in that meaning. In addition, UG regulates the set of possible semantic alternations that a morpheme may participate in, such as the causative/change-of-state relation. If each morpheme had exactly one sense, or if the alternative senses of a morpheme were always regulated by UG, this would exhaust the situation. But in fact, a morpheme can have several senses, each with its own UG determined syntax and each participating in alternations in the ways prescribed by UG. UG says little or nothing about the complete set of senses the verb has, and therefore little or nothing about the total set of subcategorizations of the morpheme. UG only determines the properties of the individual senses and those that are related grammatically. As an example, consider the verb shoot. It has at least two senses: one exemplified in She shot the burglar, and one in The burglar shot out of the room. In the first sense, shoot takes an NP complement, in the second a PP. In both cases the subcategorizations are highly predictable. shoot-l is like, say, stab, and takes an individual/NP as its complement, while shoot-2 is like, say raced, and takes a directional PP as its complement. The subcategorization of each sense is completely in accordance with the theory of grammar, but nothing about the theory of grammar determines that shoot will have these two senses. Hence the theory of grammar cannot possibly predict that shoot will have these two subcategorization frames, and a learner could not know this in advance. In cases like these, the senses of the verb do not seem to be related by UG at all, even though they are all realized by a single morpheme. Presumably they are related by association, which depends on semantic field and other
420
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cognitively real but grammatically irrelevant factors. Thus it is probably not an accident that shoot has a verb-of-motion meaning, and stab does not, since shooting involves a rapidly moving bullet. The point is that the relationship is associative in character, and not UG determined. Similarly, consider the case of know as in I know her and in I know that she is here. The two uses of the verb are cognitively close - both are state predicates and both are psychological. Hence it is presumably much more likely that these two senses will be
clustered together into a single morpheme than that say, eat and one of the two senses of know will be clustered together. The probabilities involved are not a matter of UG, however, and of course the clustering of senses together under a single morpheme is notoriously variable cross-linguistically. The two senses of know correspond to different morphemes in many other languages: French for one. What is involved in these effects is the spreading of morphemes across meanings. The results range in character from pure homonyms where no significant semantic relation can be seen at all, through relations that seem cognitively natural, and that probably reflect various clustering effects, to relations that are grammatical in character. The point is, though, that the entire subcategorization set associated with a morpheme is simply the sum of the subcategorization sets for each meaning of the morpheme. While it is true that the subcategorization set for each meaning is highly principled and strongly related to the meaning, the same is not true for the entire set. Each individual subcategorization frame reflects the systematic properties of the form-meaning correspondences, but the entire set of subcategorizations also reflects accidental combinations of subcategorizations, which result from a single morpheme’s appearing in several different senses. It follows that there is no stable mapping between the full set of subcategorizations that a morpheme appears in and the meaning of the morpheme. The full set of subcategorizations will depend, not just on UG, but also on the range of meanings that the morpheme assumes in a given language. The conclusion is, then, that a predictable relationship between subcategorizations and morphemes does not hold: the predictable relationship is between subcategorizations and a particular sense of the morpheme. It follows that it is not possible to use the entire subcategorization to learn meaning, because the entire subcategorization set is not associated with a single meaning in the jirst place. As a result, a learning mechanism based on subcategorization sets will give poor results on morphemes with many senses, typically the most common ones. It will predict many errors, or failure of learning. Suppose that the
J. Grimshaw
/ Lexical reconciliation
421
learning device tracks the syntactic context of the morpheme and attempts to determine its meaning based on this information. It will collect up all observed subcategorizations for a morpheme, disregarding the fact, which it cannot by definition be sensitive to, that verb’s meaning differs across these subcategorizations. It will then assign to the verb the meaning that would be assigned to a verb which, under a single meaning, occurred in this set of frames. But this will be incorrect, except in the case of a verb with exactly one sense. For verbs with multiple senses, if the procedure succeeds in assigning a meaning at all, it will give one meaning to the verb instead of several, and the meaning it gives might well be completely unrelated to any actual meaning of the verb being learned. Without knowing about meaning it is not possible to know which subcategorizations should be grouped together, and which should be kept separate. What of lexical entries in which the relationship between the alternatives is regulated by UG, such as the causative/change of state examples discussed above (e.g. melt/melt)? Suppose that there are two subcategorization frames associated with each of these verbs, one transitive and one intransitive. Can this set of frames be exploited to learn the meaning of the verb? (Note that the existence of the alternation cannot be crucial for learning the meanings of the members, because many verbs, such as kill and die do not alternate, but are still learned.) Here again the answer is negative, because the two occurrences of the morpheme have different meanings, under standard assumptions about lexical representation. 1 So there cannot be a single sense specified by UG as associated with this subcategorization set. Note that it also cannot be the case that the shared meaning, i.e. what the two cases of the verb have in common, is learned from the subcategorization set either, because all causative/change of state pairs have the same subcategorization set, even though each pair has its own meaning. Clearly what we must aim for is a learning procedure that uses the alternation as a clue to the semantic analysis of the verbs, that might tell the learner that these verbs have change and caused change meanings, However,
1 However, Grimshaw in work in progress (1993) denies these assumptions for all cases of alternations involving no overt morphology, proposing instead that there is only one meaning for e.g. mel/ in its causative interacting
with clausal
and inchoative structure.
Further
uses. The ‘alternation’ work is required
is just the result of the meaning
to see how this ultimately
bears on the
learning questions addressed here, but it seems that in this sense of ‘meaning’, observation of the two clause structures associated with me11 is essential for arriving at the correct analysis. This conclusion
will hold for the UG-governed
alternations
only.
422
just
J. Grimshaw
observing
because
subcategorization
/ Lexical reconciliarion alternations
there are other transitive/intransitive
will not alternations
achieve
this
result,
like eat and leave.
(8a) We melted the ice. (8b) The ice melted. (9a) We left the room. (9b) We left. (SC) *The room left. (10a) We ate the ice. (lob) We ate. (10~) *The ice ate. What will guarantee success is to take into account the properties of the arguments, and not subcategorization alone; it is the fact that the subject of one case of the verb corresponds to the object of the other cases that reliably distinguishes change/caused change pairs from eat and leave. One final point concerning the causative/change-of-state pairs. In fact both are ‘transitive’ in d-structure, according to the unaccusative hypothesis (Perlmutter 1978, Burzio 1986) which means that technically they both have Again, this suggests that subcategorization transitive subcategorizations. frames are not exactly the right place for the child to look for help in figuring out verb meaning. We can summarize the conclusions so far as follows. Mapping from meaning onto syntax can successfully exploit a set of principles of Universal Grammar which relate lexical meaning ultimately to surface syntax. However, determining the meaning just from observation of the world seems to require multiple exposures across situations in many cases, and may be impossible under certain circumstances, when the world is particularly uninformative. On the other hand, mapping from syntax onto meaning promises to successfully exploit linguistically encoded information about a verb’s meaning. However as formulated so far, it seems to require multiple exposures across sentences, and also may be impossible in many cases, when syntax is uninformative.
4. Reconciliation Clearly then we seek a model which preserves the advantages of both kinds of ideas: which makes it possible to use UG mapping principles to regulate
J. Grimshaw
syntax,
and to use surface
syntax
I Lexical
423
reconciliation
to regulate
the semantic
analysis.
One way
in which it is possible to combine the essential good effects of both types of mapping gives them different roles in the learning process: the semantics-tosyntax mapping principles provide a predictive mechanism, and the observed s-structure provides a checking mechanism. This is the basis of Reconciliation. (Wilkins 1993 explores a rather similar model for lexical learning, with particular emphasis on the acquisition of morphology.) Reconciliation (1) (2)
(3)
(4)
(5) (6) (7) (8)
The learner interprets a scene or situation, hears a sentence and detects the verb. The learner finds a relationship R among participants in the situation (entities, propositions etc.) that is sensible given the interpretation of the observed situation. The learner checks that R involves participants consistent with the content of the (candidate argument) expressions in the sentence, and rejects an R that does not meet this requirement. The learner constructs a lexical conceptual structure which is consistent with R, and assigns candidate argument expressions in the sentence to argument positions in the lexical conceptual structure. This lexical conceptual structure is fed through the semantics-to-syntax mapping principles of UG in their language particular instantiation. The s-structure predicted by step 5 is compared to the observed s-structure. If they do not match then no learning takes place. If they do match then the morpheme is entered into the lexicon with the hypothesized lexical conceptual structure,
A few comments
are required
about
the steps involved
in reconciliation.
Step 2 excludes situations where the interpretation of the event is one of throwing, say, and R is a relationship between propositions. Step 3 constrains the device to considering Rs that express relationships between the right kind of entitities: if the sentence contains two NPs, and one is the ball, then a verb meaning between haI, R Whether only on 3 is one
‘say’ is not a candidate, since it is not a possible relationship a ball and some other entity. Similarly, with the NPs John and the cannot be a verb meaning ‘dress’, although it could mean ‘throw’. this should depend on probabilities given real world knowledge, or strictly linguistic selectional restrictions, I leave open. Note that step way in which the sentence constrains the process of word learning,
424
J. Grimshaw I Lexical reconciliation
not through the syntactic form of the arguments of the predicate but through gross consistency between the (candidate) arguments and the relationship expressed by a candidate R. This step is pre-linguistic, in the sense that it does not rely on the linguistic representation of the arguments, just on their gross meaning. It is quite different, then, from the effects of the linguistic mapping and checking involved in later stages. An interesting question is whether some of the effects described in the literature on ‘syntactic bootstrapping’ are really due to this (non-linguistic) process, see Pinker (this volume). At Step 4, however, the procedure is now linguistic in character. If R is a causal relationship between two entities, a causative lcs will be constructed. The system is now working with a linguistic representation, rather than just with construals of events, and conceptual properties of relationships and entities. With respect to the final steps, note that an incorrect representation will be entered into the lexicon, if the match discovered in R6 is accidental. We must assume that general principles of decay will eventually result in eradication of mistaken entries (cf. Braine 1971), since they will result in matches less often than correct entries. Moreover, the notion of a ‘match’ requires further explication. The definition could require identity of form between the observed sentence and the predicted sentence, or it could allow for limited inconsistencies, in particular with respect to various ways of reducing the number of arguments that are actually expressed in a clause. Imperatives lack an overt subject, for example, there are elliptical forms, and there are verbs like wash and eat, which can be syntactically intransitive while apparently maintaining a two-place semantic structure. ‘Cognate objects’ (e.g. to die a peaceful death) and ‘fake reflexives’ (e.g. to behave oneself) pose the opposite problem, and a more detailed treatment is required than I will give here. The reconciliation model incorporates aspects of both semantic and syntactic ‘bootstrapping’. It crucially involves mapping from a posited meaning to a syntactic form. It also exploits the surface syntax to constrain solutions. A simple result of the model is that the number, the position, and the form of the syntactic arguments of a predicate will constrain the semantic representation it is given. This is because of the grammatical principles regulating the lcs-syntax relation. Suppose for example, that a learner hears a sentence containing the verb give: Mary is giving the package to the boy, and observes an event in which one individual hands a package to another individual. Suppose the learner interprets the event as involving a three-place logical relationship of transfer of possession. This ‘R’ will be consistent with the content of the candidate argument expressions in the sentence: Mary, the package, and the boy, since
J. Grimshaw 1 Lexical reconciliation
425
these are the right kind of entities to participate in such a relationship. Now the learner constructs an lcs for R (step 4) say x acts to transfer y to I, and assigns the candidate argument expressions in the sentence to positions in the lcs of the predicate. Suppose the learner assigns Mary to x, the package to y, and the boy to z. Then given the semantics-to-syntax mapping principles as they work out in English, the predicted s-structure is Mary verbs the package to the boy. Since this is the observed s-structure, this experience yields an lcs for give. (Note that the wrong assignment of candidate argument expressions to lcs positions would not have yielded a match, hence no learning would have occurred.) Suppose, on the other hand, that the learner construes the event as one of holding or getting, interpretations that are equally consistent with gross observation. Now the learner will posit a two-place ‘R’. At step 3 the procedure might already break down, if the learner can decide that all of the candidate arguments in the clause must be actual arguments, since there are 3 candidate arguments but only two actual arguments. Perhaps this is not so easy, however, since one or other of the phrases might be involved in some adjunct role. Assuming then, that the procedure will not halt at step 3, what happens? The learner now constructs one of the two lcs’s, for hold or get, and examines the predicted s-structures that correspond to them. Let us assume that the lcs’s are something like: x have y, x come to have y. There is no way for these lcs representations to yield the observed s-structure. They have the wrong number of arguments and since there is no way to treat the PP ro the boJ as an adjunct there is no way to reconcile the observed s-structure with the predicted s-structure. In addition, the arguments will be in the wrong syntactic position. Semantics-to-syntax mapping will place the ‘getter’ or ‘holder’ (the boy) in subject position, but this will contradict the observed s-structure which has the giver (Mary) in subject position. In general, the number of arguments in the observed sentence will have to match the number of arguments of R: if a predicate expresses an n-place relationship it will have n syntactic arguments, and if it has n syntactic arguments it will be a logical n place predicate. (I set aside here the disruptions to this generalization mentioned above.) This is, more or less, what the Theta Criterion and the Projection Principle guarantee (Chomsky 1981): the number of phrasal arguments in the syntax is the same as the number of (open) logical positions in the lexical representation of the predicate, and the syntactic derivation cannot change the number of arguments. By the same reasoning Reconciliation resolves the kill/die problem discussed above. A learner can conclude that die can be a change-of-state predicate but
426
J. Grimshaw I Lexical reconciliation
not a causative, that kill can be causative but not change-of-state, and that melt can be both. This can be determined just on the basis of a single observation each for kill and die, and two observations of melt, one in each complement structure. A verb which means to cause something to change state must have two arguments, a verb which means to change state must have one. Hence, since kill has two syntactic arguments, it must have two semantic arguments and cannot have a change-of-state meaning. Since die has one syntactic argument it must have one semantic argument and cannot be causative. The syntactic form provides the information that there are two semantic arguments, which provides the necessary information about possible verb meanings. As we saw in section 3, in order to properly identify instances of the alternation between causative and change-of-state meanings, it is necessary to take into account the fact that the subject of one case of the verb corresponds to the object of the other: We melted the ice, The ice melted. The properties of the arguments are essential to distinguishing these verbs from verbs like leave and eat. If intransitive eat is mistakenly analyzed as having a change of state meaning, its predicted s-structure will have what is eaten in subject position The ice ate. The observed s-structure will have the eater in subject position We ate. Hence the wrong analysis will be rejected. Similarly, if a learner fails to assign a change-of-state analysis to melt, treating it instead as having an lcs like intransitive leave or eat, the predicted s-structure will have the agent in subject position (We melted), while the observed s-structure will have the entity undergoing the change of state in subject position (The ice melted). Once again, the error will be avoided. The syntactic form of an argument will similarly constrain meaning. The syntactic form depends on the semantic properties of the argument, hence the syntax of an argument can provide information about its semantics, and hence about the semantics of a predicate. Sentential complements, for instance, will occur with verbs of propositional attitude (e.g. believe), verbs of logical relation (e.g. entail) and verbs of saying (e.g. announce). Reconciliation has the desired effect that this fact will prevent certain types of errors by learners. Suppose the event is one in which a child is playing roughly with a dog and an adult says either I think that you are being mean to the dog or That dog will kill you. If the verb means ‘think’ and the learner thinks it means ‘kill’, the predicted s-structure will contain an NP complement while the observed s-structure will contain a clausal complement. Similarly if the verb means ‘kill’ and the learner thinks it means ‘think’, the predicted sstructure will contain a sentential complement while the observed s-structure will contain an NP.
J. Grimshanj I Lexical reconciliaiion
421
In sum, under Reconciliation, the number, position, and form of the arguments of a predicate will all constrain the interpretations that can be assigned to that predicate. Of course, the same general point can be made for many other kinds of linguistic information: hearing a verb in the passive form, in the progressive, with a particular aspectual modifier and so forth, will similarly provide constraining information about the verb’s semantics, eliminating many posited, but incorrect, lexical semantic representations. This captures the essence of the issue addressed by syntactic bootstrapping: it provides a more precise characterization of the idea that the language can be used to map from observation to verb meanings. In so doing, however, it makes crucial use of the notions behind semantic bootstrapping, concerning the mapping between semantics and syntax.
5. Conclusion The important
properties
of Reconciliation
are these:
_ It does not depend on exposures to multiple sentence types, in the sense that neither cross-situational analysis, nor cross-sentential analysis is involved in setting up the lcs representations. Therefore the problems posed by the variable senses of a morpheme discussed in section 3 do not arise. _ It uses semantics to predict syntax where Universal Grammar makes this possible. _ It uses syntax to eliminate wrong semantic candidates where possible. I emphasize again that I have addressed here the question of learning basic lexical semantic representations, and not the learnability problem discussed in much of the literature, which concerns the problem of determining which verbs participate in which ‘alternations’, see especially Pinker (1989) and references therein. Within the terms of the present discussion we could see this as the question of how the system should proceed when a single morpheme would be assigned multiple representations, whether multiple lcs’s, or multiple syntactic configurations, but I will not explore the issue here. Also unexplored is the issue of how morphologically complex items are analyzed. Even with a procedure like Reconciliation, which exploits a full set of grammatical principles, there is no way to save a learner from having to learn some word meanings simply from observation. There are many are sets of
J. Grimshaw / Lexical reconcilialion
428
words which have absolutely identical linguistics. Such sets include the sets of causative verbs (kiN, melt, burn) and change-of-state verbs (die, melt, burn). Semantics-to-syntax mapping guarantees that each member of the set will be syntactically indistinguishable (just as the names of animals, cut versus dog for example, are not syntactically distinct). So examination of the surface syntax will not inform a learner as to whether a verb means ‘to become liquid’ or ‘to become solid’. The general situation is that it is possible to use the surface syntax to constrain analyses of the semantic structure of a verb, but not its semantic content: the fact that a verb is a change-of-state verb, but not the fact that it expresses a particular change of state. This is a matter of semantic content only and is not reflected in the syntax of a verb at all. Presumably, then, the semantic differences among members of these sets must be learned from observation about the world in some sense. However, this does not necessarily mean that the differences are perceptual in character. A vast quantity of information about the world can be encoded linguistically but is not linguistic itself. Thus a child can observe that melt is used of, for example, ice, while burn is used of, for example, paper. This is sufficient for the child to conclude that the meanings are as they are rather than the other way around. In this way, it is possible for a child to know what a word means without ever having observed an event which would count as an occurrence of what the verb describes. Indeed if this were not the case it would be impossible to understand how meaning differences among unobservable verbs are acquired: think, hope, imagine. For this reason, language is a source of essential information for lexical learning in two respects. As just discussed, language can convey information about word meaning which is orders of magnitude more informative than observation of the world can be. Second, by virtue of the grammatical principles that govern it, language constrains the possible representations of words in ways that learners can exploit in word learning. Reconciliation is one way in which this might happen.
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