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Reasoning by analogy: rational foundation of natural analogue studies
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Applied Geochemist ry, Suppl. Issue No. I. pp. 9-1 1,1 992 Printed in Great Britain
Reasoning by analogy: rational foundation of natural analogue studies JEAN-CLAUDE PETIT
DRDD , CEN -FAR , BP 6, 92265 Fontenay aux Roses Ccdex , France
REASONING BY ANALOGY
INTRODUCTION
IT HAS been recognized very quickly that long-term extrapolations concerning the safety of a nuclear waste repository cannot be satisfactorily made on the sole basis of short-term laboratory investigations. Most nuclear countries have hence developed an approach relying on the following research directions:
It may be necessary at this point to recall a widely accepted definition of analogy: this concept is linked to the structural resemblance of systems (or sets) based on the isomorphism of the relations between their constituent parts (or elements) and of properties deriving from these relations; such systems (or sets) are nevertheless recognized to be different in 1. laboratory experiments, which allow the identifi- substance (nature of components, etc.) . Historically, cation of individual mechanisms (in particular this concept , invented by the ancient Greeks in "rapid" ones) and the quantitative evaluation of geometry , is strongly associated with the notion of the influence of particular parameters in well- proportion ("analogia" in Greek corresponds to the word "proportio" in Latin). It has been discussed by designed controlled systems; 2. in situ testing, which permits the investigation of major philosophers for more than 2500 years, from ruling mechanisms or global phenomena in a Plato, Aristotle, Plotin and Thomas Aquinus to, medium-scale system of appropriate complexity more recently, Leibnitz and Kant. Their main objective was to decipher whether something pertinent but only on the short term ; could be said of the unknown by analogy with the 3. modeling, which, in spite of the often necessary known. relative simplification of the system components The distrust expressed by some scientists for the and functions, allows the rational extrapolation of natural analogue field stems in great part from the short-term data to much longer periods of time; negative connotation of reasoning by analogy. Such a 4. natural analogues, which are the only means by term is currently associated with the feeling that which very slow mechanisms can be identified and information derived by such a means is necessarily by which long-term predictions of models can be tested for pertinence (if not truly validated). vague, fuzzy, etc. , in contrast to the more rigorous Among the pioneers of this field, one tan mention induction and deduction processes on which science is assumed to be based. In a way, analogy would BROOKINS (1976), EWING (1979) and more rehence be relegated to the obscurantist reasoning of cently CHAPMAN et al. (1984). ancient times. In my opinion , this view is far too Although the field of natural analogues has grown schematic and suffers from serious misunderstandvery rapidly in recent years, receiving support from ings. It is not the place here to discuss in detail such a varied specialists (e.g . earth scientists, chemists and philosophical matter but I would just very briefly modelers to experts in safety assessment) and insti- point out some ideas that interested readers could tutions involved in radioactive waste disposal, there then study more thoroughly by themselves. First, it is is not yet a full consensus on their actual usefulness. worth knowing that there is a renewed interest nowaMore problematic is the criticism sometimes made days by philosophers, logicians and particularly cogthat analogical reasoning is not "true science" and niticians for analogy. For these experts in these that information retrieved from the study of natural problems, discrimination between processes like analogues will always remain questionable. induction/deduction and analogy does not seem as The present paper givessome clues about the exact clear cut as it appears to most of us and is, at least, a status of reasoning by analogy, compared to more subject of debate . Second, it is now clear that analogy "scientific" ways of deriving information from inves- cannot substitute for deduction, which is an elementigated systems. It is not a thorough discussion of this tary logical operation, and to a lesser extent for very complex, and by far too philosophical issue but induction. However , it is a key psychological process we hope , at least, to present to readers of papers of scientific discovery (heuristic role). Indeed, numerous empirical studies in the history devoted to natural analogue studies arguments showand sociology of science have demonstrated that ing that this approach has some sound foundation.
9
10
J.-c. Petit
analogy is one of the important mental processes that lead scientists to discoveries as well as to the elaboration of pertinent theories and models. I refer here to Holton's "private science" (1975), i.e. the research process as it actually takes place in the privacy of scientists' minds, and not to "public science", i.e. a corpus of widely accepted knowledge after due criticism by the scientific community and consolidation with time. ROOT-BERNSTEIN (1988), in his empirical work on scientists of the last two centuries, has identified several mental qualities which appear to be essential to the process of scientific discovery. The "facility for recognizing patterns", which is a concept very close to or associated with analogy, is acknowledged to be very important. He concludes that "any mental activity that contributes directly to scientific discoveries should be recognized as scientific method". In addition, in a recent overview on the role which information systems can play in scientific discovery, GARFIELD (1989), citing the work of BAWDEN (1986), notes that major advances in this field are linked to the ability of modern computers to process literature data. In effect, they allow one to "detect analogies, patterns and exceptions and the expansion of browsing capabilities, which enhance not only the interdisciplinary nature of the information obtained but also the serendipitous use of the literature". Finally, one should mention that current scientific procedures, such as the classification of natural objects into categories (e.g. Linne's classification of flora and fauna) make use of reasoning by analogy because they assume that, given a set of criteria that helps specify resemblances and dissimilarities, objects which are obviously different have more in common with their category members than with other objects. In particular, similar properties or behaviours are expected for all members of a category although (sometimes great) differences are easily detectable. EWING and JERCINOVIC (1987), in a study on natural sciences and the nuclear waste disposal issue, were the first in our field to pinpoint the ubiquitous and very useful role of analogy as a heuristic tool. There is thus nothing special in applying this type of reasoning to nuclear waste disposal, except that the use of analogy is now explicitly mentioned as a particular research methodology. Indeed, a few years ago a debate was raised among concerned scientists on the opportunity to better define what natural analogues were. Some of us (CHAPMAN et al., 1984), were looking for the best criteria to identify among natural objects those which were analogues. But it was soon acknowledged that, provided a good similarity exists with a disposal or part of it (e.g. nature of materials, geological or geochemical situation), it is only the way information is derived from a natural object that defines an analogue. For instance, the Oklo reactors (Gabon) are just highly concentrated V-ores for the mining company (although depleted in 235V), exciting natural nuclear phenomena for specialists in
neutronics, but unique analogues for the geochemists interested in studying the migration of scarce isotopes 39pu, 99Tc, 137Cs, etc.) in natural systems. Parodying the famous sentence of the reknowned eighteenth century geologist Hutton, who founded the geological method ("the present is the key to the past"), R. C. Ewing (pers. commun.) claims that the past is also the key to the future, meaning that our ability to predict the likely evolution of a repository must be evaluated against our capability of thoroughly explaining past geological events (defined as the process of postdiction, as opposed to prediction, in reference to the work of Simpson on historical sciences). He finally argues that such reasoning does not "demonstrate that our understanding is correct, but it does add to the demonstration of the general applicability and acceptability of the approach" (R. C. Ewing, pers. commun.). Another point against the use of natural analogues is their supposedly qualitative character, which would make them useless compared with the quantitative data on which modeling (and hence prediction) must be based. I would first like to stress that earth sciences have evolved during the last 25 years toward an ever growing state of quantification of natural processes, in particular as a result of the systematic injection of physical and chemical concepts, methods and analytical tools. In particular, it should be noted that some well-conducted analogue studies have produced quantitative data. Of course, boundary conditions are often poorly defined in the natural environment which results in great uncertainties about the validity of measured or inferred data. But, one should realize that the extreme difficulty of predictions in the future is precisely due to such fuzzy conditions with which one has to deal (initial conditions in the repository are assumed to be well defined). This is actually the challenge of the geoforecasting approach, which has only developed in recent years with well known limitations as regards, for instance, the prediction of earthquakes, volcanic eruptions and glaciation periods. Also, the statement on the qualitative character of natural analogue would implicitly imply that a quantitative piece of information is intrinsically of more value than a qualitative one for the scientific work. I do not agree, because it is true only within the frame of an accepted paradigm as clearly demonstrated by KUHN (1970). But, the change in paradigm itself, or more prosaically in our field the choices made for the construction of models, generally relies on qualitative information, because it necessitates a hierarchical and a structural organisation of concepts for which quantitative data are of limited help. Quantitative data for a given process (e.g. the value of a diffusion coefficient) can only be injected into the model subsequently to the decision that such a process (e.g. molecular Fickian diffusion) must be taken into account, and at a particular place in the modeling scheme. In this choice it is clear that qualitative
e
Rational foundation of natural analogue studies
aspects of our understanding of the overall system are decisive. Finally, I would like to note that the major heuristic role of natural analogues is not well acknowledged because it is in general not explicitly identified as such in the output of scientific research. For instance, the current evolution of ideas at the basis of the modeling of nuclear borosilicate glasses (source-term) clearly owes much to investigations on the dissolution of its natural analogue counterparts like basalt and rhyolitic glasses (choice of dominant processes, importance of secondary alteration phases, geochemical modeling with computer codes, etc.). However, it is very difficult "from the outside" to identify such a contribution whenever an important explanatory effort is not made. This issue of the poor representativeness of scientific output (e.g. articles) on the process of discovery itself has already been discussed in detail by the Nobel prize winner MEDAWAR (1963). In particular, he claims that science reported in papers, books, etc. (major recognised media of scientific communication) is very commonly "reconstructed" when compared to the actual process of discovery which takes place in the laboratory and in the scientist's brain! CONCLUSIONS
This short note shows that reasoning by analogy plays a very important role in the process of scientific discovery, which is fully acknowledged by numerous studies in the history and sociology of science. Clearly, true demonstrations cannot be obtained directly but only through logical operations such as deduction. Analogy is thus not in competition with normal science. It is just the only means at hand for obtaining unique qualitative and, in some cases, quantitative information on systems of equivalent
11
complexity to a nuclear waste repository, and for periods of time not accessible to laboratory or in situ experiments. It must be used with much caution and only in conjunction with laboratory and in situ experiments as well as with modeling. The role of analogy should not be underestimated, notably for the safety assessment of a disposal concept in a geological formation and for the communication of scientific results to the general public, who are not trained to the scientific method. Editorial handling: M. Gascoyne.
REFERENCES BAWDEN D. (1986) Information systems and the stimulation of creativity. J. Inform. Sci. 12,203-216. BROOKINS D. G. (1976) Shale as a repository for radioactive waste: the evidence from Oklo. Environ. Geol. 1, 225269. CHAPMAN N. A., McKINLEY I. G. and SMELL/EJ. (1984) The potential of natural analogues in assessing systems for deep disposal of high-level radioactive waste. NAGRA Tech. Rept. NTB 84-41. EWING R. C. (1979) Natural glasses: analogues for radioactive waste forms. Mater. Res. Soc. Symp. Proc. 1,5768. EWING R. C. and JERCINOVIC M. J. (1987) Natural analogues: their application to the prediction of the longterm behavior of nuclear waste forms. Mater. Res. Soc. Symp. Proc. 84,67-83. GARFIELD E. (1989) Creativity and science. Part 2. The process of scientific discovery. Curro Cant. 45, 3-9. HOLTON G. (1975) On the role of thernata in scientific thought. Science 188, 328-334. KUHN T. S. (1970) The Structure of Scientific Revolutions. University of Chicago Press. MEDAWAR P. B. (1963) Is the scientific paper a fraud? The Listener, 12 September, 377-378. ROOT-BERNSTEIN R. S. (1988) Setting the stage for discovery: breakthroughs depend on more than luck. The Sciences 28, 26-34.
Applied Geochemist ry, Suppl. Issue No. I. pp. 9-1 1,1 992 Printed in Great Britain
Reasoning by analogy: rational foundation of natural analogue studies JEAN-CLAUDE PETIT
DRDD , CEN -FAR , BP 6, 92265 Fontenay aux Roses Ccdex , France
REASONING BY ANALOGY
INTRODUCTION
IT HAS been recognized very quickly that long-term extrapolations concerning the safety of a nuclear waste repository cannot be satisfactorily made on the sole basis of short-term laboratory investigations. Most nuclear countries have hence developed an approach relying on the following research directions:
It may be necessary at this point to recall a widely accepted definition of analogy: this concept is linked to the structural resemblance of systems (or sets) based on the isomorphism of the relations between their constituent parts (or elements) and of properties deriving from these relations; such systems (or sets) are nevertheless recognized to be different in 1. laboratory experiments, which allow the identifi- substance (nature of components, etc.) . Historically, cation of individual mechanisms (in particular this concept , invented by the ancient Greeks in "rapid" ones) and the quantitative evaluation of geometry , is strongly associated with the notion of the influence of particular parameters in well- proportion ("analogia" in Greek corresponds to the word "proportio" in Latin). It has been discussed by designed controlled systems; 2. in situ testing, which permits the investigation of major philosophers for more than 2500 years, from ruling mechanisms or global phenomena in a Plato, Aristotle, Plotin and Thomas Aquinus to, medium-scale system of appropriate complexity more recently, Leibnitz and Kant. Their main objective was to decipher whether something pertinent but only on the short term ; could be said of the unknown by analogy with the 3. modeling, which, in spite of the often necessary known. relative simplification of the system components The distrust expressed by some scientists for the and functions, allows the rational extrapolation of natural analogue field stems in great part from the short-term data to much longer periods of time; negative connotation of reasoning by analogy. Such a 4. natural analogues, which are the only means by term is currently associated with the feeling that which very slow mechanisms can be identified and information derived by such a means is necessarily by which long-term predictions of models can be tested for pertinence (if not truly validated). vague, fuzzy, etc. , in contrast to the more rigorous Among the pioneers of this field, one tan mention induction and deduction processes on which science is assumed to be based. In a way, analogy would BROOKINS (1976), EWING (1979) and more rehence be relegated to the obscurantist reasoning of cently CHAPMAN et al. (1984). ancient times. In my opinion , this view is far too Although the field of natural analogues has grown schematic and suffers from serious misunderstandvery rapidly in recent years, receiving support from ings. It is not the place here to discuss in detail such a varied specialists (e.g . earth scientists, chemists and philosophical matter but I would just very briefly modelers to experts in safety assessment) and insti- point out some ideas that interested readers could tutions involved in radioactive waste disposal, there then study more thoroughly by themselves. First, it is is not yet a full consensus on their actual usefulness. worth knowing that there is a renewed interest nowaMore problematic is the criticism sometimes made days by philosophers, logicians and particularly cogthat analogical reasoning is not "true science" and niticians for analogy. For these experts in these that information retrieved from the study of natural problems, discrimination between processes like analogues will always remain questionable. induction/deduction and analogy does not seem as The present paper givessome clues about the exact clear cut as it appears to most of us and is, at least, a status of reasoning by analogy, compared to more subject of debate . Second, it is now clear that analogy "scientific" ways of deriving information from inves- cannot substitute for deduction, which is an elementigated systems. It is not a thorough discussion of this tary logical operation, and to a lesser extent for very complex, and by far too philosophical issue but induction. However , it is a key psychological process we hope , at least, to present to readers of papers of scientific discovery (heuristic role). Indeed, numerous empirical studies in the history devoted to natural analogue studies arguments showand sociology of science have demonstrated that ing that this approach has some sound foundation.
9
10
J.-c. Petit
analogy is one of the important mental processes that lead scientists to discoveries as well as to the elaboration of pertinent theories and models. I refer here to Holton's "private science" (1975), i.e. the research process as it actually takes place in the privacy of scientists' minds, and not to "public science", i.e. a corpus of widely accepted knowledge after due criticism by the scientific community and consolidation with time. ROOT-BERNSTEIN (1988), in his empirical work on scientists of the last two centuries, has identified several mental qualities which appear to be essential to the process of scientific discovery. The "facility for recognizing patterns", which is a concept very close to or associated with analogy, is acknowledged to be very important. He concludes that "any mental activity that contributes directly to scientific discoveries should be recognized as scientific method". In addition, in a recent overview on the role which information systems can play in scientific discovery, GARFIELD (1989), citing the work of BAWDEN (1986), notes that major advances in this field are linked to the ability of modern computers to process literature data. In effect, they allow one to "detect analogies, patterns and exceptions and the expansion of browsing capabilities, which enhance not only the interdisciplinary nature of the information obtained but also the serendipitous use of the literature". Finally, one should mention that current scientific procedures, such as the classification of natural objects into categories (e.g. Linne's classification of flora and fauna) make use of reasoning by analogy because they assume that, given a set of criteria that helps specify resemblances and dissimilarities, objects which are obviously different have more in common with their category members than with other objects. In particular, similar properties or behaviours are expected for all members of a category although (sometimes great) differences are easily detectable. EWING and JERCINOVIC (1987), in a study on natural sciences and the nuclear waste disposal issue, were the first in our field to pinpoint the ubiquitous and very useful role of analogy as a heuristic tool. There is thus nothing special in applying this type of reasoning to nuclear waste disposal, except that the use of analogy is now explicitly mentioned as a particular research methodology. Indeed, a few years ago a debate was raised among concerned scientists on the opportunity to better define what natural analogues were. Some of us (CHAPMAN et al., 1984), were looking for the best criteria to identify among natural objects those which were analogues. But it was soon acknowledged that, provided a good similarity exists with a disposal or part of it (e.g. nature of materials, geological or geochemical situation), it is only the way information is derived from a natural object that defines an analogue. For instance, the Oklo reactors (Gabon) are just highly concentrated V-ores for the mining company (although depleted in 235V), exciting natural nuclear phenomena for specialists in
neutronics, but unique analogues for the geochemists interested in studying the migration of scarce isotopes 39pu, 99Tc, 137Cs, etc.) in natural systems. Parodying the famous sentence of the reknowned eighteenth century geologist Hutton, who founded the geological method ("the present is the key to the past"), R. C. Ewing (pers. commun.) claims that the past is also the key to the future, meaning that our ability to predict the likely evolution of a repository must be evaluated against our capability of thoroughly explaining past geological events (defined as the process of postdiction, as opposed to prediction, in reference to the work of Simpson on historical sciences). He finally argues that such reasoning does not "demonstrate that our understanding is correct, but it does add to the demonstration of the general applicability and acceptability of the approach" (R. C. Ewing, pers. commun.). Another point against the use of natural analogues is their supposedly qualitative character, which would make them useless compared with the quantitative data on which modeling (and hence prediction) must be based. I would first like to stress that earth sciences have evolved during the last 25 years toward an ever growing state of quantification of natural processes, in particular as a result of the systematic injection of physical and chemical concepts, methods and analytical tools. In particular, it should be noted that some well-conducted analogue studies have produced quantitative data. Of course, boundary conditions are often poorly defined in the natural environment which results in great uncertainties about the validity of measured or inferred data. But, one should realize that the extreme difficulty of predictions in the future is precisely due to such fuzzy conditions with which one has to deal (initial conditions in the repository are assumed to be well defined). This is actually the challenge of the geoforecasting approach, which has only developed in recent years with well known limitations as regards, for instance, the prediction of earthquakes, volcanic eruptions and glaciation periods. Also, the statement on the qualitative character of natural analogue would implicitly imply that a quantitative piece of information is intrinsically of more value than a qualitative one for the scientific work. I do not agree, because it is true only within the frame of an accepted paradigm as clearly demonstrated by KUHN (1970). But, the change in paradigm itself, or more prosaically in our field the choices made for the construction of models, generally relies on qualitative information, because it necessitates a hierarchical and a structural organisation of concepts for which quantitative data are of limited help. Quantitative data for a given process (e.g. the value of a diffusion coefficient) can only be injected into the model subsequently to the decision that such a process (e.g. molecular Fickian diffusion) must be taken into account, and at a particular place in the modeling scheme. In this choice it is clear that qualitative
e
Rational foundation of natural analogue studies
aspects of our understanding of the overall system are decisive. Finally, I would like to note that the major heuristic role of natural analogues is not well acknowledged because it is in general not explicitly identified as such in the output of scientific research. For instance, the current evolution of ideas at the basis of the modeling of nuclear borosilicate glasses (source-term) clearly owes much to investigations on the dissolution of its natural analogue counterparts like basalt and rhyolitic glasses (choice of dominant processes, importance of secondary alteration phases, geochemical modeling with computer codes, etc.). However, it is very difficult "from the outside" to identify such a contribution whenever an important explanatory effort is not made. This issue of the poor representativeness of scientific output (e.g. articles) on the process of discovery itself has already been discussed in detail by the Nobel prize winner MEDAWAR (1963). In particular, he claims that science reported in papers, books, etc. (major recognised media of scientific communication) is very commonly "reconstructed" when compared to the actual process of discovery which takes place in the laboratory and in the scientist's brain! CONCLUSIONS
This short note shows that reasoning by analogy plays a very important role in the process of scientific discovery, which is fully acknowledged by numerous studies in the history and sociology of science. Clearly, true demonstrations cannot be obtained directly but only through logical operations such as deduction. Analogy is thus not in competition with normal science. It is just the only means at hand for obtaining unique qualitative and, in some cases, quantitative information on systems of equivalent
11
complexity to a nuclear waste repository, and for periods of time not accessible to laboratory or in situ experiments. It must be used with much caution and only in conjunction with laboratory and in situ experiments as well as with modeling. The role of analogy should not be underestimated, notably for the safety assessment of a disposal concept in a geological formation and for the communication of scientific results to the general public, who are not trained to the scientific method. Editorial handling: M. Gascoyne.
REFERENCES BAWDEN D. (1986) Information systems and the stimulation of creativity. J. Inform. Sci. 12,203-216. BROOKINS D. G. (1976) Shale as a repository for radioactive waste: the evidence from Oklo. Environ. Geol. 1, 225269. CHAPMAN N. A., McKINLEY I. G. and SMELL/EJ. (1984) The potential of natural analogues in assessing systems for deep disposal of high-level radioactive waste. NAGRA Tech. Rept. NTB 84-41. EWING R. C. (1979) Natural glasses: analogues for radioactive waste forms. Mater. Res. Soc. Symp. Proc. 1,5768. EWING R. C. and JERCINOVIC M. J. (1987) Natural analogues: their application to the prediction of the longterm behavior of nuclear waste forms. Mater. Res. Soc. Symp. Proc. 84,67-83. GARFIELD E. (1989) Creativity and science. Part 2. The process of scientific discovery. Curro Cant. 45, 3-9. HOLTON G. (1975) On the role of thernata in scientific thought. Science 188, 328-334. KUHN T. S. (1970) The Structure of Scientific Revolutions. University of Chicago Press. MEDAWAR P. B. (1963) Is the scientific paper a fraud? The Listener, 12 September, 377-378. ROOT-BERNSTEIN R. S. (1988) Setting the stage for discovery: breakthroughs depend on more than luck. The Sciences 28, 26-34.