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Tense and aspect systems
ARTIFICIAL INTELLIGENCE
407
Book Review Oesten Dahl, Tense and Aspect Systems (Basil Blackwell, Oxford, 1985) 193 pages, £22.50. Reviewed by' Alexander Nakhimovsky
Colgate Umversity, Department of Computer Sctence, Hamtlton, NY 13346, U.S.A. Dahl's Tense and Aspect Systems (henceforth TAS) lS a comparative study of the time-related grammatical categories of the verb in 64 typologically diverse languages. Although written primarily for linguists, TAS might be of interest to those in AI who work on commonsense knowledge representation and temporal reasoning, especially tn the context of natural language understanding. Both Dahl and this reviewer suggest that Comrle's superlative introductions [1,2] should be read before, or instead of, TAS TAS consists of a preface, list of abbrevmtions (which has to be consulted frequently), six chapters and a conclusion. The appendix lists the 156 sentences and 9 short narratives whose translations into the languages of the sample constitute the main body of Dahl's data. By reading this questionnaire one can get an immediate feel for the range of distinctions grammaticallzed in various languages of the world. The first six narratives, for example, all tell the same story (x stands for ' T ' or " h e " ) : x WALK in the forest. Suddenly x STEP on a snake. It BITE X in the leg. x TAKE a stone and THROW at the snake. It DIE. The narrative context is varied as follows: (1) Do you know what happened to me yesterday? (2) Do you know what happened to me once when I was a child? (3) Do you know what happened to my brother yesterday? I saw it myself. (4) Do you know what happened to my brother yesterday? He told tt himself. (5) Once upon a time there was a man. This is what happened to him one day. (6) (The speaker is right back from a walk in the forest) Do you know what just happened to me? It turns out that most languages are like English and use the same tenses in all six narratives. However, there are languages that grammatically (and thus obligatorily) encode in their verb forms the differences between today past, yesterday past and the more remote past, or between directly observed events Artificial lntelhgence 32 (1987) 407-410 0004-3702/87/$3 50 © 1987, Elsevier Science Pubhshers B V (North-Holland)
408
BOOK REVIEW
and " q u o t e d " events. The double achievement of TAS is that ~t provides reliable statistics for what ~s and ~s not typical, while bringing together in one place a rich collection of data on the more exotic possibilities. Chapter 1, "General Background," starts out by observing that a TMA (tense, mood or aspect) category is characterized by "the set of contexts in which the category is found in the language, rather than the set of objects which the term denotes " (p 3) The boundaries of such a set are, of course, imprecise, and Dahl goes into a long discussion of how best to deal with imprecision: he rejects fuzzy sets in favor of prototypes and "dominant'" (l.e. necessary) features which form a subset of the prototypical features (pp. 3-11). After several &gresslons Dahl gets down to defining TMA categories (pp. 22-31), making the familiar distinctions between grammatmal and lexical categories, tense and aspect, aspect and "inherent aspectual meaning." (All of these have been discussed m [1] and other recent works on tense and aspect [3-6], but, as Dahl observes, people still confuse perfect with perfective and mlsclassify progressive as a tense.) Finally, the chapter presents the major claim of the book' grammatmal categories in individual languages are instances of cross-linguistic category types, each identified by a set of prototyplcal uses Dahl does not "claim that all languages use the same TMA categories but only that the overwhelming majority of all categories found in the TMA systems of the world's languages are chosen from a restricted set of category types" (p 31) Still, the claim is stronger than s~mply postulating a fimte set of universal features that can characterize TMA categories of different languages: Dahl suggests that prototypmal bundles of features go together in the majority of their language-specific reahzations. An additional claim is that some crosslinguistic categories are consistently expressed by periphrastic means, while others are usually marked morphologically. Chapter 2, " T h e Investigation," presents the data on which the clmms are tested and the methods of analysis. The data are primarily the questionnaire responses, occasionally supplemented with examples from lingmstic descriptions. Dahl uses only native speakers as informants because TMA categories are notoriously difficult for adult learners to lnternahze. Finding the informants proved to be a major problem, especially since Dahl wanted a sample that would not be dominated by the Indo-European family. This part of the book shows how xmpossibly difficult cross-linguistic investigations are: a few years' work and a grant from the Swedish government have yielded only 64 languages, of which 23 are Indo-European and most others are represented by only one informant. (To keep things in perspective, there are more than 5,000 languages altogether ) Since the number of informants was limited, the size of the questmnnaire had to be kept down and the number of contrasts it could test was correspondingly reduced. As a result, the investigation is restricted to the categories found m the main clauses of active declarative affirmative sentences.
BOOK REVIEW
409
Choosing examples that could be translated into all 64 languages also proved difficult. Sentences that were too trivial or uniform quickly got the translators bored (only linguists and philosophers never tire of finding inspiration in them). A great number of examples (e.g. The boy drank a glass of milk) had to be rejected as culture bound. Even after allowing the informants to replace problematic expressions (e.g. forest by desert), Dahl reports complaints about cultural biases and is prepared to "announce an award to anyone who is able to come up with, say, two examples of completely non-culture bound transitive sentences with a human subject and a predicate phrase denoting an activity." (p. 48) The collected responses have been analyzed according to the following procedure: Identify language-specific categories that cover about the same range of contexts. Take the union of all the contexts covered by these categories and rank them according to the number of categories that check out for each context. Compare the distribution of each category with the ranking list. (To be safe, Dahl uses two statistical similarity measures and reports that they agree quite well on his data.) After the comparison, some categories may have to be struck out from the list and new ones added, which leads to a modified set of contexts and a new ranking list. Repeat the procedure until "we think that we have arrived at the optimal lists." (p. 60) (Surprisingly, nothing is said about whether the number of modifications decreases with each iteration or not.) Finally, assume that the top examples of the ranking list represent the "focus" of a cross-linguistic category. The main hypothesis of the book becomes restated as: most language-specific categories show great statistical similarity to relatively few ranking lists. The next three chapters report the results for aspectual categories (Ch. 3), tense categories (Ch. 4) and the category PERFECT and its relatives (Ch. 5). Chapter 6 presents an overview of the TMA systems of the languages in the sample. The book concludes with a table that lists all cross-linguistic categories (about 50), the number of instantiations for each, and their marking type (morphological or periphrastic). The majority of tenses are found to belong to PAST, IMPERFECTIVE-PAST, FUTURE, PERFECT, PLUPERFECT and DEFAULT (i.e. present of the default aspect and mood). Most aspectual systems come in one of two main types, PERFECTIVE-IMPERFECTIVE and PROGRESSIVE-DEFAULT. An unexpected finding is that the majority of perfects, pluperfects and progressives are expressed periphrastically, while the majority of perfectives are expressed morphologically. This is suspicioulsy similar to what one finds in Germanic and Slavic languages, but if the result is confirmed on a larger corpus, linguists will have another fact on their hands for which no explanation is in sight. There are many quibbles one can find with both Dahl's linguistics and statistics. Still, in a field that has seen a good deal of universalist theoretizing over a small and skewed corpus of data, TAS is a welcome example of a
410
BOOK REVIEW
hard-nosed empirical study that sets for ~tself the standards of methodological rigor rarely found m comparative hnguistics. Its limitations stem from the disparity between those standards and the available methods of hngmsttc data gathering and analysis. REFERENCES 1 Comne, B , Aspect (Cambridge Umverslty Press, Cambridge, 1976) 2 Comne, B , Tense (Cambridge University Press, Cambridge, 1985) 3 Dowty, D , Toward a semantic analysis of verb aspect and the Enghsh qmperfectlve' progressive, Lmgulsttcs and Phtlo6 1 (1977) 45-77 4 Dowty, D (Ed.), Lmgutsttcs and Phtlosophy 9 (1) (1986) A special issue on tense and aspect 5 Hopper, P, Tense-Aspect Between Semanttcs and Pragmattcs (John Benjamms, Amsterdam, 1982) 6 Tedeschl, P J and Zaenen, A (Eds), Tense and Aspect, Syntax and Semantws 14 (Academtc Press, New York, 1981)
407
Book Review Oesten Dahl, Tense and Aspect Systems (Basil Blackwell, Oxford, 1985) 193 pages, £22.50. Reviewed by' Alexander Nakhimovsky
Colgate Umversity, Department of Computer Sctence, Hamtlton, NY 13346, U.S.A. Dahl's Tense and Aspect Systems (henceforth TAS) lS a comparative study of the time-related grammatical categories of the verb in 64 typologically diverse languages. Although written primarily for linguists, TAS might be of interest to those in AI who work on commonsense knowledge representation and temporal reasoning, especially tn the context of natural language understanding. Both Dahl and this reviewer suggest that Comrle's superlative introductions [1,2] should be read before, or instead of, TAS TAS consists of a preface, list of abbrevmtions (which has to be consulted frequently), six chapters and a conclusion. The appendix lists the 156 sentences and 9 short narratives whose translations into the languages of the sample constitute the main body of Dahl's data. By reading this questionnaire one can get an immediate feel for the range of distinctions grammaticallzed in various languages of the world. The first six narratives, for example, all tell the same story (x stands for ' T ' or " h e " ) : x WALK in the forest. Suddenly x STEP on a snake. It BITE X in the leg. x TAKE a stone and THROW at the snake. It DIE. The narrative context is varied as follows: (1) Do you know what happened to me yesterday? (2) Do you know what happened to me once when I was a child? (3) Do you know what happened to my brother yesterday? I saw it myself. (4) Do you know what happened to my brother yesterday? He told tt himself. (5) Once upon a time there was a man. This is what happened to him one day. (6) (The speaker is right back from a walk in the forest) Do you know what just happened to me? It turns out that most languages are like English and use the same tenses in all six narratives. However, there are languages that grammatically (and thus obligatorily) encode in their verb forms the differences between today past, yesterday past and the more remote past, or between directly observed events Artificial lntelhgence 32 (1987) 407-410 0004-3702/87/$3 50 © 1987, Elsevier Science Pubhshers B V (North-Holland)
408
BOOK REVIEW
and " q u o t e d " events. The double achievement of TAS is that ~t provides reliable statistics for what ~s and ~s not typical, while bringing together in one place a rich collection of data on the more exotic possibilities. Chapter 1, "General Background," starts out by observing that a TMA (tense, mood or aspect) category is characterized by "the set of contexts in which the category is found in the language, rather than the set of objects which the term denotes " (p 3) The boundaries of such a set are, of course, imprecise, and Dahl goes into a long discussion of how best to deal with imprecision: he rejects fuzzy sets in favor of prototypes and "dominant'" (l.e. necessary) features which form a subset of the prototypical features (pp. 3-11). After several &gresslons Dahl gets down to defining TMA categories (pp. 22-31), making the familiar distinctions between grammatmal and lexical categories, tense and aspect, aspect and "inherent aspectual meaning." (All of these have been discussed m [1] and other recent works on tense and aspect [3-6], but, as Dahl observes, people still confuse perfect with perfective and mlsclassify progressive as a tense.) Finally, the chapter presents the major claim of the book' grammatmal categories in individual languages are instances of cross-linguistic category types, each identified by a set of prototyplcal uses Dahl does not "claim that all languages use the same TMA categories but only that the overwhelming majority of all categories found in the TMA systems of the world's languages are chosen from a restricted set of category types" (p 31) Still, the claim is stronger than s~mply postulating a fimte set of universal features that can characterize TMA categories of different languages: Dahl suggests that prototypmal bundles of features go together in the majority of their language-specific reahzations. An additional claim is that some crosslinguistic categories are consistently expressed by periphrastic means, while others are usually marked morphologically. Chapter 2, " T h e Investigation," presents the data on which the clmms are tested and the methods of analysis. The data are primarily the questionnaire responses, occasionally supplemented with examples from lingmstic descriptions. Dahl uses only native speakers as informants because TMA categories are notoriously difficult for adult learners to lnternahze. Finding the informants proved to be a major problem, especially since Dahl wanted a sample that would not be dominated by the Indo-European family. This part of the book shows how xmpossibly difficult cross-linguistic investigations are: a few years' work and a grant from the Swedish government have yielded only 64 languages, of which 23 are Indo-European and most others are represented by only one informant. (To keep things in perspective, there are more than 5,000 languages altogether ) Since the number of informants was limited, the size of the questmnnaire had to be kept down and the number of contrasts it could test was correspondingly reduced. As a result, the investigation is restricted to the categories found m the main clauses of active declarative affirmative sentences.
BOOK REVIEW
409
Choosing examples that could be translated into all 64 languages also proved difficult. Sentences that were too trivial or uniform quickly got the translators bored (only linguists and philosophers never tire of finding inspiration in them). A great number of examples (e.g. The boy drank a glass of milk) had to be rejected as culture bound. Even after allowing the informants to replace problematic expressions (e.g. forest by desert), Dahl reports complaints about cultural biases and is prepared to "announce an award to anyone who is able to come up with, say, two examples of completely non-culture bound transitive sentences with a human subject and a predicate phrase denoting an activity." (p. 48) The collected responses have been analyzed according to the following procedure: Identify language-specific categories that cover about the same range of contexts. Take the union of all the contexts covered by these categories and rank them according to the number of categories that check out for each context. Compare the distribution of each category with the ranking list. (To be safe, Dahl uses two statistical similarity measures and reports that they agree quite well on his data.) After the comparison, some categories may have to be struck out from the list and new ones added, which leads to a modified set of contexts and a new ranking list. Repeat the procedure until "we think that we have arrived at the optimal lists." (p. 60) (Surprisingly, nothing is said about whether the number of modifications decreases with each iteration or not.) Finally, assume that the top examples of the ranking list represent the "focus" of a cross-linguistic category. The main hypothesis of the book becomes restated as: most language-specific categories show great statistical similarity to relatively few ranking lists. The next three chapters report the results for aspectual categories (Ch. 3), tense categories (Ch. 4) and the category PERFECT and its relatives (Ch. 5). Chapter 6 presents an overview of the TMA systems of the languages in the sample. The book concludes with a table that lists all cross-linguistic categories (about 50), the number of instantiations for each, and their marking type (morphological or periphrastic). The majority of tenses are found to belong to PAST, IMPERFECTIVE-PAST, FUTURE, PERFECT, PLUPERFECT and DEFAULT (i.e. present of the default aspect and mood). Most aspectual systems come in one of two main types, PERFECTIVE-IMPERFECTIVE and PROGRESSIVE-DEFAULT. An unexpected finding is that the majority of perfects, pluperfects and progressives are expressed periphrastically, while the majority of perfectives are expressed morphologically. This is suspicioulsy similar to what one finds in Germanic and Slavic languages, but if the result is confirmed on a larger corpus, linguists will have another fact on their hands for which no explanation is in sight. There are many quibbles one can find with both Dahl's linguistics and statistics. Still, in a field that has seen a good deal of universalist theoretizing over a small and skewed corpus of data, TAS is a welcome example of a
410
BOOK REVIEW
hard-nosed empirical study that sets for ~tself the standards of methodological rigor rarely found m comparative hnguistics. Its limitations stem from the disparity between those standards and the available methods of hngmsttc data gathering and analysis. REFERENCES 1 Comne, B , Aspect (Cambridge Umverslty Press, Cambridge, 1976) 2 Comne, B , Tense (Cambridge University Press, Cambridge, 1985) 3 Dowty, D , Toward a semantic analysis of verb aspect and the Enghsh qmperfectlve' progressive, Lmgulsttcs and Phtlo6 1 (1977) 45-77 4 Dowty, D (Ed.), Lmgutsttcs and Phtlosophy 9 (1) (1986) A special issue on tense and aspect 5 Hopper, P, Tense-Aspect Between Semanttcs and Pragmattcs (John Benjamms, Amsterdam, 1982) 6 Tedeschl, P J and Zaenen, A (Eds), Tense and Aspect, Syntax and Semantws 14 (Academtc Press, New York, 1981)