F PERSPECTIVES ON COGNITIVE SCIENCE Theories, Experiments and Foundations edited by Peter Slezak University of New South Wales Terry Gaelli University of Metbourne Richard Glark Flinders University ABLEX PUBLISHING CORPORATION NORWOOD, NEW JERSEY Copyright @) 1995 by Ablex pubtishing Corporatlon All rights reserved. No part of this pubrication may be reproduced, storedin a retrievatsystem, oi_transmitte{l; ;il;;; or by any means, etectronic,m_e,ch.anicar, photocopying, microfilming, d"oro,ng, or otherwise, withoutpermission of the publish-e Printed in the United States of America Library of Congress Cataloging_in-publication Data Perspecli,ves-on co-gnitive scienc^e : theories, experiments, and foundations /edited by peter Stezak, Terry Caelli, ni"f,"rj'bf"rf..p. cm. Papers oriqina'v orpglluo at the inaugurar meeting of the Austrarasian i:ilivo;il ?,,:J;:1"" Science, herd in No"v. rgso at tnE univJr;;,u ;i ll"ly9ur bibliographicat references and index. ISBN: 1-56750-1os_2 (cr.)._rSeN: r-sois6lrzr_+ tppr.l1' cognitive Science-congresses. -2. irr"n iniormation processing--Conqresses. 3. Coonition_congorr"r.' i sr"="r-p;;;;'9;;:""".Caelli, Terry. llt. Ctar[, Richard.-lV]Arrtr"iu.i"n Society forCognitive Science. 8F311.P355 1994 153-dc20 94-36549 CIP Ablex Publishing Corporation 355 Chestnut Street Norwood, New Jersey 0764g Contents lnlr orlrrt'littrr l"i. I,I'AIININ(;, MI'MOITY & COGNII'ION l. lrnplicil Lcarning in a Cucd ltcaction.fimc Task I Il .4. lloukes,, S.J. Roortenrys and B.W. Ilarrtes ' A scll-lVtoclil'ying productio' System Modcl or, Infcrc'cc St urlcgics 19 G.S. IIulford, M.T. Moybery, S.B. Smith, J.C. Dick,otr. urtrl .l.Ii.M. Stewart i A N.'li'car Associative Memory Moder For the Storagc anrr licl ricval of Complcx Spatiotemporal Sequences 3l ll.A. Ileath . ('orlltutational Issues in Associative Learning 45 Ii.J. Kehoe Mctacognitive processes and Learning with lntelligent EducationalSystcms 63 K. Crawford anct J. Kay l'hc Shapc ol Learning Functions C. Speelman 'l'hc Simon then Garfunkel Effect: Mind Il9 G. Rhodes and T. Tremewan Coal Inf'erence in Information_Seeking Environments l4S B. Raskutti and I. Zukerman l'lvo Data Structures in Cognition lii. P.L. Roberts and C. MacLeocl During Transfer jg Priming and the Modularity ol l(). il. 1. (;oN il N'l s 'I'ttcHN tQUtts & A|,PLICA,I.IONS Neurocognitive Pattern classification of Distributed Brain Electrical Activity 103 R. Clark, D.E. pomeroy ancl J. Tizerd Determining Light-Source Direction from Images ofShading 127 D. Gibbins, M.J. Brooks and lt. Chojnacki Connectionist, Rule-Based and Bayesian Decision Aids: An Empirical Comparison l6j S. Schwartz, J. Wites qnct S. phittips A Cognitive Approach to Autonomous Mobile Robot Development 181 A. Sowmya Categorization and protypes in Design lg9 M.A. Rosenman and F. Sudweeks Multiple Reasoning Contexts in Health Care planning Zl3 C.L Brqdburn ond J. Zelenikow FOUNDATIONS Some Reflections on procedural and Declarative for Cognitive Processes Z2S T. Caelli and R. Wales The "Philosophical" Case Against Visual Imagery 23.1 P. Slezak Communication and Uncertaintv Zi3 R.A. Girle Levels of Description 2g3 P.E. Griffiths Empty-Headed Animals? Eliminativist prose ancl Corrs 301D. Khlentzos Which Symbols have ,,Meaning for the Machinc",l 317 H. Clapin why Knowledge Engineers Should study thc Irrr'rir.irics 331 D. Laker Reduction and Levels of Explanation in Coltrrcctionisnr 347 J. Sutton 9. 12. 13. t4. t5. t6. t7. 18, 19. 20. 21. 22. 23. lntroduction l)ctcr Slczak, Terry Caelli and Richard Clark llr,' prrpers collcctcd hcrc are representative of the leading work being done rrr r\rrrlnrliir artcl Ncw Zealand under the banner of 'cognitive science', and rlrr'rrgrpcirllncc ol'the volume marks a significant occasion in the developrrrr,trt ol llrc intcldisciplinary field in this region. Although the present volume Ir,r', rrrore lhan parochial value in view of the character and quality of the rr',,t'rrrt'lr lcportcd here, nevertheless, its regional provenance is not without ,,orrrt'irrlercst. The papers have been selected from among those which were ' u ttlutitlly l)rcscnted at the inaugural meeting of the Australasian Society for t .1r11i11v. Scicrtcc held at the University of New South Wales in November l')',0. ('oruing cxactly ten years after the establishment of such a society in rlrl llrritccl States in 1980, this conference might be seen, in one sense, as 1l1r''t'orrrirrg ol'age'of cognitive science in the Australian region. This occasion n.r,, tlrc lirst self-conscious gathering of researchers under the banner of 'r n,'nilivc scicnce'in Australia and, in this sense at least, it was a significant ,,tr'p irr tlrc clircction of a genuine dialogue between scholars in different fields rn rr lralting pidgin tongue, if not yet in a true interdisciplinary creole' I , rllrrrvirrg the pattern elsewhere, in Australia and New Zealand there are now | (,lt(.t s irnc[ lrrograms in cognitive science emerging at several universities, ,rrrrl it is lropcd that this dialogue will continue to flourish through such centers ,rrr,l tlrrotrgh conferences like the one at which these papers were presented. ( )l ('oursc, these institutional developments come a full 30 years after the ,',r;rlrlislrnrcnt of the Center for Cognitive Studies at Harvard in 1960 by r; A. Millcr and J. Bruner, and there are grounds for suspecting that the rlvolrrlionary developments which swept the United States in the 1960s and l')/0s wclc somewhat attenuated by the time they reached the Antipodes. I o l;1liq only one significant example, the extraordinary phenomenon of the t lrorrrskian Revolution with its dramatic scientific and institutional impacts Ir,r,, lrccn little in evidence Down Under (see Newmeyer 1986, Gardner 1987). I lrt' slorvcr emergence of a truly interdisciplinary cognitive science can perhaps 1,,'r'rplaincd in this way, confirming the remarks of one American psycholnI'rst wlro writes "The extraordinary and traumatic impact of the publication ri ,\1111111r1i, Structures by Noam Chomsky in 1957 can hardly be appreciatr'rl lry orrc who did not live through this upheaval" (Maclay, 1971,p' 163). llrrttcr licltl, I l. (l9.ll). l ltt, ll'ltr.r: iult,tl)t(ttttion ol lu\lt)t r,. I orrtkrrr: llt.ll. ('oltcrt, II.lt.(l9ti4).()tturtli.l.rrrt,rlntttsit.. lltc\(it,nt.t,ol rttttsicrtl tltt,/u\t\tu.t:1,t,/ lltt,\ttt,nttltt revoluIiott, I 550,I650. l)orrlecht, (iernurrry: I). lierLlel. Collirrs, II.M. (l9ti'1). Conccltts arl(l l)ractice ol l)lllieil)irlory licltlrvork. Irr ('. llcll rt ll Robcrts (Ed.), Sttciu! rcscurthing (pp. 54-(r4). I-orrrlon: l{otrtletlllc turtl Kcll:rn l';rrrl Collins, H.M. (1985). Changing ortler; Rcplit'utiort utul iruluctiott irr scicttti.l:tc l,t(tt.t^.t., Bcverly Hills, CA: Sage. collins, H.M. (19ti7). Expert systems an<r rhq sciencc of knowrctrgc. rrr w.rr. rliiker, r',1,, IJughes, & T.T' Pinch (Eds.), Sociot construction o./ technologitnlrrlrl(r/r.r.. NL,rt, lirt,r.titutt in the sociology ond hisrory o./ technology (pp. 329-3ag). ca'rbriclgc, MA: lllll, l,rt,ss, Collins, H.M. (1990). Artif'icat Experts: Social Knowledge und Itttc!!igcttt A4uclirrtt, Cambridge, MA: MIT press. collins, H'M', Green, R.H., & Draper, R.c. (r9g6). whcre's rhe cxperrisc: Lxpca sysrcrrrr as a medium of knowledge trans{'cr. In M..I. Merry (Ed.), Expert systetns g5,1pp. :Z:t_.l.f .tt. Cambridge, UK: Cambridge University press. cooke, N.M., & McDonard, J.E. (r9g6). A formal merhodology Ior acquiri'g .rrr represenring experr knowledge. proceedings of IEEE, 74, 1422-1130. Darden, L. (1987). Viewing thc history of science as compiled hinrlsight. AI MogaTine, g (21, 33-4 l . Debenlram, J. K. (19S9). Knowledge slstems r/esrgn. Syclncy: prentice_IIall. Feyerabcnd, P.K. (1975). Agoi,st merhocr; outrine o.f an onarchisric theory o;f knowrcrrga. London: New Left Books. Frciling, M., Alexander,.l., Messick, S., Rehfuss, S. & Shulman, S. (19g5). Starting u knowledge cngineering project: A step-by_stcp approach. AI Magoline, 6 (3),150-164. Hall' R'P'' & Kibler, D.F. (1985). Diflering methoclological perspecrivcs in artificiai inrelligcncc research. AI Magaz.ine, 6 (3), 166_179. Kitakami, H., Kunifuji, S., Miyachi, T., & Furukawa, K. (l9S.l). A merhodology for implementation ol'a knowledge acquisition systcrn. 19B4 Internationn! Symposiunt ,rr Logic Programming, IEEE Computer Society. Kuhn, T.S. (1962). The structure of scientific revolutions. Chicago, II-: Chicago Universiry prcss. Latour, B. (1987). Science in o('tion: Iloht to fotktw scientists ond engineis through society. Cambridge, MA: Harvard University press. Latorrr' B', & woolgar, S. (r979). Lnboratory riJe; The socior construcrion oJ scienrific. /acts. Beverly Hills, CA: Sage publicarions. Mccarthy, J. (195u). Programs wirh common sense. In M. Minsky (Ed.), Semuntic informarion processing (pp. 403-410). Cambridge, MA: MIT press. McCarthy, J. (1987). Gencrarity in artificiar inrclligence. Comnunicotions o;f the ACM,30, I 030I 035. r]4(i I Al(t tt N{ettrey, W. (1987). An assessment of tools for AI Mugazine,8 (4), 8l-89. Oxman, R., & Gero, J.S. (1987). Using an experl synthesis. Expert Systems, l, 4-15. Cl lAl) l't';ll 23 Reduction and Levels of Explanation in Connectionism* John Sutton Department of Philosophy Macquarie University INTRODUCTION Recent work in the methodology of connectionist explanation has I'ocrrsccl on the notion of levels of explanation. Specific issucs in conncctionisrn hcrc intersect with rvider areas of debate in the philosophy of psychology and thc philosophy of science generally. The issues I raise in this chapter, then, are not unique to cognitive science; but they arise in new and important contexts when connectionism is taken seriously as a model of cognition. The general questions are the relation between levels and the status of levels which have no obvious relation to others. In speaking of levels, what is the connection, if there is one, between explanation and ontology? Which, if any, conccpt of reduction is applicable to connectionist systems? What kind of legitinrtcy can the constructs of common sense psychology, or of that vclsion ol intentional realism represented by classical symbol-systems n I, hirvc irr ir full-scale connectionist theory of mind? In this chapter I address the promising and sophisticatccl picture of connectionist explanation developed by Andy Clark in his book Microcognition (Clark, 1989a) and in a number of lcccnt papers (Clark, 1988a, 1989b, 1990a). The drift is to suggest that, while Clark makes clear the radical nature of the connectionist explanatory I'r'anrcwork, his view fails to account successfully for the value of high-lcvcl cxlrlanations and for why such explanations work. In particular, Clark clocsu't provide a sufficiently robust account of the kind of mental causation rvhish seems to be necessary if realism about propositional attitudes is to be maintained. A weak * Many thanks and my acknowledgements to Danicl Stoljar-, with whom I wrote and clclive r crl an ancestor of this paper at the University of Adclaiclc in March 1990. The outline of A l lteor y of Reduction and Levels of Explanation presentcd hcre was worked out in collaboratiort rvit lr him. Thanks too to George Couvalis and Grahanr Ncrlich for comments on that pal)cr , inr(l to Gerarcl O'Brien, Huw Price, and Doris Mcllwain l'or many helpful discussions. building largc knowledge-based systems. . system fbr design diagnosis antl design Plato (1969). The tost days of socrurres (Trans. by I{. Tredennick), Harmondsworth, UK: I,enguin. Polanyi, M. (1967). The tocit dimension. Lonclon: Routledge and Kegan paui. Popper, K.R. (1959). The togic of scientific discovery. London: Hutchison. Pylyshyn, Z. w . (1984). Compulotion ond cognilion; Towarcl a.foundation for cognitive st.ience. Cambridgc, MA: MIT press. Ringle, M. (1979). Philosophv and artificiar intelligence. In M. Ringre (Ect.), phitosophica! perspectives in artificial intelligence. Atlantic Highlancls, NJ: Humanities press. Snow, c.P. (1964). The two curtures und a second look: An exltanded y,ersion oJ the two cultures and rhe scientific revorurion. cambridge, UK: cambridge University press. wittgenstein, L. (1914). Trocratus logico-phitosctphicus ('rrans. D.F. pcars & B.F. Mccuinness.) London: Routledge and Kegan paul. 347 348 surroN requirement of reducibility on a level of explanation, which I will spell out and defend at some length, will explicate the relations between levels in a way Clark's position cannot. It will then serve as a defense of the "condition of causal efficacy" on explanations which Clark, following Jackson and Pettit, rejects. Finally I apply this weak reducibility constraint back specifically to explanation in connectionism, and discuss the status of high-level explanations of connectionist systems. I will deliberately be blurring some allegedly vital distinctions here. I won't be drawing sharp distinctions between levels of description and levels of explanation, between intertheoretic and interlevel reductions, nor between type and token reductions. The metaphysics of reduction I advance has a number of gaps of detail, but its general drift is so appropriate to connectionist explanation, and such a useful counterweight to Clark's thoroughgoing antireductionism, that its introduction to these specific debates might excuse the compressed form it takes here. To situate the issue I am addressing, consider the long-running dispute about the legitimacy of causal explanations in terms of propositional attitudes' between robust "big-R" realists and eliminativists (P.M. Churchland, 1981, 1988b, Fodor, 1987). From the perspective of this chapter, the differences between these two positions are relatively minor: The requirement of reducibility on explanation to be expounded is, I claim, weak enough to be accepted by Fodor and by philosophical functionalists as well as by eliminativists. But Clark sees both extremes as misguided (along with others whose specific views I won't be discussing here but who include Dennett, lg'18, 198'7,1988; Wilkes , 1984, 1986; Rorty, 1980, 1983; and Price, 1988). Concerning requirements for the legitimacy of common sense psychological explanation, Clark laments, We can find two sets of otherwise opposed philosophers united in mutual error. Both Fodor's (1987) defence of ordinary mental talk and various eliminativist attacks on it are committed to a principle of scientific legitimation which can be stated thus: The goodness of Folk Psychological talk depends on its being legitimised by the discovery ol an engineering story which shares its form. (Clark, 1988a, p.275) It is Clark's rejection of this principle which is open to disptttc. 'I'lte rartgc of reductive possibilities in the development of cognitivc scicncc incluclcs, as extremes, Fodorian realism and San Dicgo climinativisrn. lltt( ('lark's rvish to accept and justify ordinary ntcntal talk ancl its tltcol'cl icnl clctivittivcs without "scicntil'ic lcgilinralion" is, I nririrtlltiu, rtcilhcr tt llcltttittc tllllirltt ttot a goocl crt<ltrglr <lcl'crtsc. REDUCTION AND LEVELS OF EXPLANATION IN CONNECTIONISM 349 CI,ARK'S LIBERALISM ABOUT CAUSATION AND LEVELS Levels of Connectionist Explanation Clark offers a neat, innovative model of explanation in connectionist systems (1989a, especially pp. 83-105, 184), and discusses its departures from traditional theories of explanation in cognitive science (1990a). He acknowledges that connectionism simply fails to fit Marr's classical view of levels of explanation (Marr, 1977, 1982). As I am almost wholly in agreement with Clark on these points, I will only sketch, from his presentation, those levels of explanation the status of which make them relevant to my argument. Starting only with a general task specification, connectionist models with distributed representation are simply set running with random weights, and trained up to better performance, by a variety of supervised or unsupervised learning procedures. The first steps in connectionist explanation, then, are not detailed specifications of the function to be computed and the information on which algorithms are to draw, as in classical models (Peacocke, 1986). Rather they deal with a fully working system. At this stage, the network can be described only by precise mathematical specification of the connections and weights of its individual units. Differential equations can specify both the state of the system at a given time (by stating a vector of numerical values), and its dynamic learning pattern. Explanation of this sort is at what Clark calls "the numerical level" (1989a, pp. 188-189). At this level, as Smolensky notes, "the explanations of behavior are like those traditional in the physical sciences" (1988, p. l). Only at this stage can the observer of the connectionist system work backwards, up the levels, toward an understanding of larger-scale patterns of hidden-unit activity, through, for instance, network pathology caused by artificial lesions and hierarchical cluster analysis. I will be concentrating on cluster analysis as an example of an interesting intermediate level of analysis, because it is well-known and already much discussed in the present context. Cluster analysis reveals the "sorts of internal representations the network has developed to carry out a particular task" (Elman, 1989, p.5). For each class of input, a mean vector of hidden unit activation patterns is computed. These mean vectors are all then subjected to hierarchical cluster analysis. "The hierarchical interpretation is achieved through the way in which the spatial relations of the representations are organized. Representations which are near one another in representational space form classes, and higher level catcgories correspond to larger and more general regions of this space" (fllman, 1989, p. 7; 1990, pp.205-207). Through such analysis the observer can clisplay a hierarchy of partitions, portraying the shape of the rcl.rlcscntational space of the possible hidden-unit activations that power the nclwork's g.rcll<lrnrarrcc (l'or more examples and detail see Rosenberg & St,irrowski, l()tl7; l'.S. ('ltrucltlitncl & Scjnowski, 1989; P.M. Churchland, 350 l;t,t t()N 1989a, l9ti9b; Clerrk, l9tl9a, 1990u; l'ilnutrt, l9lJ9, 1990). Understanding of conncctionist systcnrs, thcrt, c<rtrcs ttol llttottglt rlclltilcrl prior analysis of the structure of the task domain, btrt tltlouglt stitlislicirl glimpses into the running of an already operational nctwork. 'l'lris "explanatory inversion," which Clark describes as "Marr-througlt-tltc-lookittll glass" (1990a , p.215), does not imply a lack of explanatory power. ln lctlirrtr the task organize the network, rather than "imposing the form ol' utrr conscious, sentential thought on our models of unconscious processitttr'," (p.218), the connectionist is able to avoid "ad-hoc organizing principlcs attrl sentential, linguistic bias" (p. 219) projected downwards from our consciorts understandingl. So far so good: Connectionism, in Clark's rendering, is a means to escll)c what Patricia Churchland called philosophers"'fetishism with respect to logit' as the model for inner processes" (1986, p.381). But what of the status ol the levels, discovered in such post hoc strategies as artificial lesioning arttl cluster analysis, which are higher than that of mere numerical specificatiorr of connection weights? There is an intuitive sense in which it is the lattcr', the base level of connection weights which, as Paul Churchland has put it, drives the dynamic cognitive evolution of the system over time (P.M. Churchland, 1989a, Section 5). What implications does this have l'rlr' explanation? This question is the focus of some debate, and is central to nty concerns in this chapter. For now I want to bring into play other high levels of description ol' connectionist systems. Firstly, what Clark calls "the symbolic-Al levcl": Connectionist systems are described at this level as if they were classical, as if they follow rules, access schemas, fire productions, and so on. These arc descriptions according to which the system seems to be satisfying "harcl, symbolic constraints in serial order" (1989a, p. 194). The descriptions will tend to break down, to fail to explain, under suboptimal conditions, for' "solutions" to ill-posed problems, or with curtailed processing time. In thesc conditions, connectionist systems will still give "sensible performance" by satisfying as many soft constraints as well as possible. So the formal descriptions of the symbolic AI level cannot provide a unified account ol' genuinely connectionist cognition, even if they appear to be accuratc descriptions of a system's behavior under ideal conditions (see Clark, 1989a, pp. 194-195, and Smolensky, 1988 for detail on this level). The final relevant level is that of common sense psychology. Here too there is a prima facie tension with the base numerical level. The words and concepts of ordinary language have, it initially seems, no obvious discrete analogs in distributed connectionist systems, since in any particular case they will be I Compare Clark's description of Fodor's search for in-the-hcad structures which mirror tho structures of conscious thought-ascription as "polishing the tip of the iceberg" (Clark 1988b, p.616). ilt t)lroltoNANI)IIvl lr;()l Ixl,l ANAil()NlN(;()NNl (;iloNll.iM 3tr1 Icptcscttlctl irs lr corttplex lrclivlr(irln vcct()r'acr'oss a lalgc sct ol'units in statc s1'racc. It is this point which clrivcs thc rccent suggestion that eliminative matelialisrn wcluld bc confirmed by the success of connectionism (Stich, 1988a, 1988b; Ramsay, Stich, & Garon, 1990; P.M. Churchland, 1988c, 1989c; disputed by O'Brien, 1991, 1993). Clark has been at pains to maintain the importance of all these levels of explanation: Cluster analysis, the symbolic AI level and the common-sense psychological level (1989a , pp. 199-201). Surely some at least of these higher levels are going to count as explanatory, even, as Clark says, for the antisententialist unsatisfied with the propositional bias of classical AI (1990a, p. 218). But what principled account can be given of the status of these levels of explanation? It is not with the question of whether such higher level explanations may be justified, but with that of how and why they are justified, and of what would count as a justification, that my disagreements with Clark lie. Clark's Causal Liberalism Clark's def'ence of high-level explanations relies on the grouping of systems into equivalence classes defined according to the purpose of the explanation. "Each such grouping requires a special vocabulary, and the constructs of any given vocabulary are legitimate just insofar as the grouping is interesting and useful" (Clark 1989a, p. 187). The interest or utility of a level of explanation is, for Clark, sufficient for its legitimacy. In particular, utility is sufficient for the legitimacy of the constructs of any equivalence class even when these constructs are not reducible to the constructs of a physical causal level of explanation. We should in psychological explanation expect no "neat mapping" between ascribed mental states and "scientific stories about the inner causes" ofbehavior (1989a, p. 51 ; p. 49, p. 94, p. 1 12, p. 196; I 988a, p. 267). Individual thoughts, Clark reiterates, "are perfectly real, but they are not the kind of entities that have neat, projectible, computational analogues in the brain" (p. 153). Clark takes this position partly on the basis of a holistic ascriptivism about mental states: "ascription of a thought is . . . ascription of a structured competence within a close-knit semantic domain" (1988a, p.27 1; cf. 1988b, 1990b). Beliefs, for instance, are holistically ascribed on the basis of sufficiently rich and flexible bodies of behavior (p.267).1 don't want to quarrel about this ascriptivism: One could consistently accept it while still running the objections I am about to. My queries are directed, rather, at the second strand of Clark's rejection of "in-the-head realism" (1988a, pp.267-273). This is his set of views on explanation, causation, and reduction. It is most clearly set out in his rejection of what he calls the "condition of causal efficacy" on psychological explanation. The condition of causal efficacy is as follows: "A psychological i 352 l;tJl l()N ascription is only warrantccl il thc itcnrs il posits lurvc clilcct rtttitlrtlrttt's itt the production (or possiblc production) ol' bcltaviot'" (Clar k l9|l9it, p. l()(r), Clark's stated aim is to provide a picturc ol'high-lcvcl cxlllitttitliott dissociated from this condition, one which will give "a Inorc libclltl, tttotc plausible, and more useful picture of explanation in cognitivc scicttcc lttttl daily life" (p. 196). Such a "liberalism about causation" deliberatcly cliv<lt ecs (not just causal explanation but) causation from reduction ( l9tlttit, pp.272-273). This distinction between causation and reduction should result, suggcsls Clark, in a radical division within cognitive science. On the one hand wottltl be an engineering project, seeking the in-the-head causes of behavior'. Ott the other will be what he calls "descriptive cognitive science," which takcs "sentential thought-ascriptions as its data and use[s] dynamic, computatiorritl models to chart the logical, epistemic or normative relations among thougltls so ascribed" (1988a, p.274).It gives "a formal theory or model of the slructtttc of the abstract domain of thoughts" (1989a, p. 153). Clark welcomes lltc prospect of a "rift" resulting from the lack of any "useful relation" betwcctt the two kinds of cognitive science (1988a, p.273, 1989a, p. 159). This leaves a puzzling lack of clarity concerning the relation between tltc two kinds of cognitive science, between formal description and engineeritt,.l story, and more specifically between higher level psychological explanatiorr and physical causal explanation. There are two ways to read Clark on thcsc topics. He could be making what he calls this "distinction between description and cause" in order to deny the causal powers of the constructs of the descriptive project, in particular to reject the implication of mental statcs in the causation of action. But unless you're a hardened Wittgensteinian, denying mental causation just won't do. Explanation in terms of beliel's, desires, and the rest must be causal explanation of some sort if it is to bc legitimate. Mostly, Clark acknowledges this requirement. His liberalism is, after all, a causal liberalism, and such an extreme brand of neobehaviorism is not his. As he puis it, "the lack of in-head, engineering analogues to individual beliel's and desires need not deprive us of the right to treat beliefs and desires as real, causally active factors in the etiology of human action" (1988a, p.273). He criticizes Fodor's view that the computational structure of the brain neatly mirrors the descriptive structure of propositional attitude ascriptions: Fodor, Clark says, is guilty of conflating the two kinds of cognitive science in that he adheres to an overstrict model of causation, buttressed by "a fear that beliefs and desires can only be causes if they turn up in formal guise as part of the physical story behind intelligent behavior" (1989a, p. 160; cf 1988a, p.277, 1988b, p.609). Clark thinks this fear is groundless: AU that we need is that there should be some physical, causal story, and that talk of beliefs and desires should make sense of behavior. Such ill l)t,(;ll( )NANI)llvl ll;()l lxl,l ANAII()NlN(;()NNl Oll()Nll;M llbll rtrakirrg scrrsc tlocs irrvolvc ir noliorr ol clusc, sirtcc bclicls tlo cattsc actiotls' lJut uuless wc bclicvc that thcrc is onty onc noclcl oI causation, the rrhysical, this Ilccdn't causc ally discornfort. (Clark 1989a' p' 160) What kind of nonphysical model of causation, then, does Clark offer instead? How can truly causal explanation be exempted from the condition of causal efficacy? Abandoning an earlier discussion involving analogies to other allegedly nonphysical cases of causation (1988a; criticized by Tienson, 1990), Clark offers a principled defense of nonphysical causation drawn from Frank Jackson and Philip Pettit (1988; Clark, 1989a, pp. 196-198). Clark applies to connectionist explanation their defense of the view that "features that causally explain need not cause" (Jackson & Pettit, 1988, p.392). This involves a distinction between "causal process explanations" and "causal programme explanations" (1988, pp' 388-399)'giiefly , a causal process explanation satisfies the condition of causal efficacy in that it cites the actual causally productive features that are efficacious in a particular or given range of cases. These process explanations will be those given by clark's engineering project. A causal program explanation, on the other hand, cites a feature or property, common across a range of cases, which causally programs the result "without actually figuring in the causal chain leading to an individual action or instance" (Clark, 1989a, p. 198). In explaining a glass vessel's breaking either by its fragility or by an increase in the temperature of the gas inside it (to take two of Jackson and Pettit's examples), we are not citing the particular causes of the shattering, which might be, respectively, the categorical basis of the glass's structure or the impact of a number of molecules on the walls of the vessels (or indeed uny of a multitude of ways that fragility or increase in temperature might have been realized) (Jackson & Pettit, 1988, p. 395). Although neither fragility nor increase in temperature causes the breaking, say Jackson and Pettit, they can be said to program the breaking, and thus explain it' Clark's borrowing from Jackson and Pettit is intended to account for the value of high-level explanations of connectionist networks. The point for Clark is that such program explanations buy us an increase in generality "at the cost of sacrificing the citation of the actual entity implicated in the particular causal chain in question" (1989a, p.197). It justifies the cluster analytic level of connectionist explanation, which cites global partitions in activation space as its constructs. Networks with distributed representations, when set running with different random distributions of hidden unit connection weights, may turn out to have identical cluster analyses even when embodying entirely different arrangements of individual connection weights 1f .VI. bhurchland, 1989a, Section 5). One cluster analysis, in other words, is multiply realizable at the lower level of numerical specification of dynamic connectivity patterns. Churchland's point that the causal laws of cognitive evolution operate at the level of individual weights rather than at the level q .tlr,l r;t,rloN ()l lltc l)artitiolls irt activ;ttiort sl)aco (idcn(ilicrl by cluslct lulirlysis) ciln l)c tlcl'lcctcd, Clark thinks, by a goocl libcralisnr about causation. 'l hc clrrstcl analysis "causally programmes the system's successl'ul lrcrl'ormancc, b1l il is not part of any process explanation" (1989a, p. 199). clark goes on to give similar justifications for the explanation ol'cclnncctionist networks at the yet higher levels of both symbolic AI and conrron sense psychological explanation. Equivalence class groupings at thesc highcr levels may unite what would be otherwise apparently disparate cognitivc mechanisms. They are not "mere approximations to the conncctionisl cognitive truth," but capture constructs which, though themselves car.rsally inefficacious, highlight important facts about an important range ol' "cognitive constitutions" (1989a, p.200-201).In other words, the interest ol. utility of a level of explanation is sufficient for its legitimacy. Again, it is not the value of these explanations which I necessarily wanl to question, but rather the explanation of their value. I can agree with Clark that "explanation is a manyJeveled thing," and that it is important in cognitivc as in other sciences to subsume a single phenomenon "under a panoply ol increasingly general explanatory schemas" (1990a, p. 196). In some cascs, important higher level similarities between systems realized in different substructs might be invisible at the lower level. In connectionist explanation, in particular, explanations at the level of dynamic activation equations may obscure interesting cognitive similarities which are apparent at a higher levcl of generalization (Clark, 1989a, pp. 181-182, 197). But, I maintain, Clark's reliance on utility alone as a measure of the legitimacy of a high-level explanation leaves out important detail. The concomitant rejection of thc condition of causal efficacy as a necessary condition on a level's legitimacy stems, I propose, from an overstrict idea of the kind of reductionism suclr a condition entails. To carry this point, I need to step back for a moment from the specific problems of connectionist explanation, and give a positive account of the relations between explanation, reduction, and causation r.vhich will elucidatc an acceptably weak constraint of reducibility on a level of explanation. This will then not only justify some high-level explanations, as Clark wants, but give an account of why they work, of the relations between levels of explanation in a way that his causal liberalism cannot. This is inevitably a sketchy treatment of controversial issues in the philosophy of science, detailed treatment of which I pursue elsewhere (Sutton, 1993).1can only plead that the sketchiness is justified by the urgent relevance of these debates to connectionism. They are inspired to some extent by the work on reduction in the psychological context of Richardson(1979), Hooker (1981), Enc (1983), and the Churchlands (P.M. Churchland 1979, 1985; p.S. Churchland 1986, 1988). Most notably, I am going to assume that the supervenience of one level on another entails the reducibility of the supervening level. A number of philosophers have recently argued for this (for example Rosenberg 1985, ill t)r,(;ltoN ANI) ltvt lr;ol Ixl,l ANAlloN lN (joNNl ()ll()Nll;M lJblr llacon l9tl(r; lrollr criticisctl by l(irrcairl, l9t{7). Nol only ckrcs tltc casc lrolcl tup, but thc rcsulting conccption ol'rcclucibility is attractivcly wcakcr than that accepted in the traditional positivists'arguments for the reductive unification of science. All that is important for my argument against Clark is that something like my picture of reduction and causal explanation both is plausible and promises a robust justification of the legitimacy of high-level explanations. REDUCTION AND LEVELS OF EXPLANATION Outline of a Theory of Reduction and Levels of Explanation Theories and levels are open to reduction if their contents are. (From here, for convenience, I will talk of intertheoretic reduction, but the account applies equally to interlevel reduction). Reduction as an intertheoretic relation is dependent on an ontological relation between theory-contents, the actual things in the world that true theories quantify over, the actual entities, properties, and relations. I'll be talking about property reduction rather than event reduction or any other sort, but the metaphysics is adaptable to most preferred ontologies. There are two methods of property reduction. First is plain identity. An identity theory simply identifies F-ness, for instance, with G-ness: There are not two properties, but one. Suppose that an identity theory is successful. Then the content of the theories involved, the properties cited in the theories' explanations, turn out to be literally identical. The theories reduce, the two theories are really one, and the two properties are really one. Of course, the relationship between the properties in question might be more complex than plain identity. The other tool of reduction, besides identity, is supervenience. One difference between them is this: if one property supervenes on another, there remain two properties, whereas if one property is identical with another, to say there are two properties is strictly false. Another, and perhaps the defining, difference, between identity and supervenience is that they bear different modalities. If F is identical with G, it is impossible that F be identical to H and not to G as well. Whereas, if F supervenes on G, it remains possible that F might supervene on H, and not on G as well. To put the same point differently, F can supervene on many properties, though it is identical with only one. We can draw out the modality of this point by using a possible worlds analysis. If F is identical to G, it is so in all possible worlds. But if F supervenes on G, it does so in merely some world(s). In a world where F supervenes on G and on no other property, there would be no actual difference between the state of affairs in which F is identical with G, and that in which F supervenes on G. The difference between these two cases is a matter not of actuality, but I Ili( i ol'accoll)l)itllyillg llloclitli(y. lior'ltlirclical l)utl)oscs, llris llccrrliirlly metaphysical, transworld, dill'crcncc might not bc worth worr.yipg 1ll.,rrl.If you restricted enough the domain of a theory il', I'or. insta'cc, yorr restricted it to this world, or to a set o1'temporal or spatial parts ol'this wor.lrl it would turn out that the class of identical properties ancl thc cl'ss .l supervenient properties was co-extensive. of course, the intcresting cascs arc those in which, even within restricted domains, a property ,up"rJ"n., un ,, whole range of other properties. In these cases, restricting the domain to ollc particular realization of a supervenient property will often be a pointlessly tedious task: Here the difference between supervenience and identity remai.s, for practical purposes, marked. But whatever the details of particular cases, it remains true that propertics that supervene can be taxonomized separately, in virtue of the faci that thcy belong to different transworld classes. This metaphysical analysis gives us a clarification of the notion of levels. one level is different fiom anothcr not in actuality, but merely in possibility. Levels are distinguished not by the properties they have intrinsically but by the relational properties they bear to other possible worlds. This fact partly serves to explain why levels arc so odd. compare the case of property reduction by identity. Talk of levels, it seems, is particularly inappropriate. For, if the upper level is identical to the lower level, it is strictly a misnomer to talk of two levels. The only credence the notion of levels can be given in the identity case is a linguistic credence. Identical things can go by different names. For reduction by identity, then, levels are individuated merely linguistically; whereas, for reduction by supervenience, levels are related by their relational properties to other possible worlds. How then does this metaphysical account of reduction and levels relate to specific problems of explanation in the philosophy of psychology? I suggest that it gives us the materials of a positive alternative to crark's causal liberalism. His suggestion that the explanatory interest or utirity of any equivalence class of systems is sufficient for its legitimacy left puzzies about how high-level explanations can explain, especially.uuruily."pluin, without citing physical causes. we are owed an account of the relation, whether it is interesting and useful or not, between higher levels of explanation and lower levels. I suggest, then, that the interest or utility of a level of explanation is neither necessary nor sufficient for the level's legitimacy. Instead, a level of explanation is legitimate if and only if it both: 1. cites real, causally active entities and properties, and 2. is reducible, in the ways specified, to another level of explanation. Now, clark's legitimate levels (on some readings at least) do meet the first ilt t)r,(;il()N ANI) llvt ll;()t txt't ANAil()N tN (;()NNl Oll()Nl[;M {}5/ clilclitlrr, sirrcc lrc clairrrs llrat crrtilics ancl plopcrlics don't lravc to bc physically causllly acl ivc to bc causally activc, but I have suggested that more dctail ncccls to bc adclccl. Thal detail is to be found, I suggest, in the spelling out ol lhc' second criterion. I expand first on my negative appraisal of pragmatics in psychological explanation, and discuss the kind of legitimacy that is at issue here, and secondly on how my kind of reductionism should be weak enough to deflate much traditional anti-reductionist criticism. It will then help to reinstate the condition of causal efficacy on explanation, and thus provide robust legitimation for genuine high level explanations. Reduction, Legitimacy, and Explanatory Utility The kind of legitimacy for which I have suggested criteria is ontological legitimacy. Explanation, the idea is, cannot just be free-floating: It is, among other things, ontologically committing. This is not intended to be anything like a full-scale theory of explanation. Explanation does a lot of things besides make ontological commitments. In particular, pragmatic considerations will often be of central importance: as van Fraassen suggests (1980), whatever reduces someone's puzzlement can count as an explanation. I am not denying the importance of utility, merely claiming that it isn't all there is to a theory of explanation, deliberately divorcing it from that part of the theory of explanation which arises from taking ontological commitments seriously. Note first that what is excluded here is the purely epistemological point about what it is for an explanation to be interesting. Many levels of explanation which are legitimate on my criteria will be tedious in the extreme. These will most notably include cases where a higher level explanation cites entities or properties which are defined only functionally, or which are highly disjunctive. Such entities or properties will normally be reducible to a huge disjunction of entities or properties at a lower (micro)level. Examples of this kind would be watches (or the functional state of being a watch), airfoils, crumpled shirts, games, friendships, haircuts, and home runs. In these cases, no pragmatic benefit at all will come from focusing on the lower, reducing, level explanations: Indeed in these cases, as soon as you start to do any reducing at all, there is a likelihood of missing similarities, important to us, which can be understood at higher levels such as the level of description of gross behavior. These things just are realized too variously for the reductive stories to have any pragmatic value. But what you do get from knowledge of the low-level disjunct, or, more commonly, from knowledge that there is a low-level disjunct, is ontological sanction for the high-level construct cited in the explanation. I see no reason why this shouldn't be true even in the large number of cases where the lower level disjunct is open-endedly large (which has been suggested by Fodor (1986a, p.l9), and Kincaid (1987, pp.344-347), as a problem for reductionism). Inductive confidence that there ,q 3ltu l;t,l l()N is a rccluction ltossiblc irr any palticrrlll lculizing citsc is itll tlritl llrc rctlrrcilrilitv critcrion rcquircs. Thcrc lcally arc cnurrplccl sltirts, rlcut()lls, rvltlt'ltr's, molecules, haircuts and home runs in a way that tltcrc lcally atcll'l wilclrcs, Ptolemaic epicycles, animal spirits, sunriscs, gods, and ghosts. IJttl it sccrtts an advantage of the present account that it leaves entircly opcn tlrc clttcsliott of which, of those things which there really are, will bc redttciblc to ilrr explanatorily interesting lower level. All that this shows is that interest is not necessary for legitimacy: n()t vcr y controversial. We are all too familiar with the fact that our limited cpistcttto' logical horizons can't cope with the vast amount of genuine things in tlrc' world which could feature in (ontologically) Iegitimate explanations. 'l'o dispute Clark's claim that interest is sufficient for legitimacy, I have to sllow that the converse point holds, too, in other words, that a level of explanatitlrr can be interesting and useful without being legitimate. Sincere theology and parapsychology, eighteenth century phlogiston thcoly and animal spirits theory, Cartesian dualism, and the like, are all fascinating discourses. But while the levels of explanation employed in such disciplincs may be interesting and useful, they are not (ontologically) legitimate, for thcy posit entities, properties, and processes which are not reducible in my wcak sense, do not exist, and have no causal powers. This is not just a cheap appcal to discredited ontologies: The possibility envisaged by eliminativists (error' theorists) about any particular level is exactly that the constructs of that levcl are of the same status as the "denizens of [these] discredited ontologies" (Dennett, 1988, p. 538) were before they were discredited, that we are now on some critical cusp of conceptual change. The point is that it is impossiblc to know in advance whether any particular level of explanation is analogous to these eliminated levels or to levels, such as macroscopic levels of geological or meteorological explanation, which are reducible and thus legitimate. Yotl can only find out which of these possibilities holds by looking for possible reductions in the particular case. It can not, in other words, just be assumed that the psychological constructs cited in the higher levels of psychological explanation have the real causal powers they are thought to. Many of them, or things very like thern, probably do exist, and probably do have causal powers pretty much like those which people think they do, but if this is so, it is because they are reducible in my sense to the constructs cited in lower level explanations. Only by way of this reducibility can we understand why such explanations work. If they are legitimate, they are so because of their reducibility, not in spite of their irreducibility. The interest of a level of explanation, then, has little or nothing to do with that level's legitimacy. Varieties of Reductionism A common challenge to this kind of reducibility constraint on explanation is that it is, or leads automatically to, a much stronger reductionism. This lll l)t,(,ll()r.J Al{l) llvl lii()l lxl,l nl'ln ll()N lN (;(,NNl ( jll()Nl:iM itlrl} s(t()nl.lct rcrlrrtliorrisrn is ollcn rlesr'rilrctl irs rr kirrtl ol it ptioti vierv llritt lxtsic [r',v-levcl ltlrysies is llrc rlrrly sclitlrts tltcoty. ltctlttclitltr, llrc corttlllailtl gocs, woulcl cntail thc clinrination ol thc lcclucccl lcvcls ancl tltcir lcltlacclucnt by thc <lnly lcgitinratc lcvcl: Only thc entitics dcscribcd in basic physics are "really real." On this vicw of reduction, there are no "flow" relations between levels there is only one level. Clark seems to share this vision of what reductionism amounts to, for he claims that his causal liberalism will allay the fear of "the specter of reduction" (1989a, p. 181). But this picture is misguided. Reduction in the sense outlined above does not entail elimination. This is impossible to stress too strongly, for the assumption that it does that successful reduction spells the end of the legitimacy of the constructs of the reduced level or theory still motivates much hostility toward what are actually weaker versions of a reducibility constraint (this misconstrual runs, for example, through the recent critique of physicalism by Crane & Mellor, 1990; see their criticism of Fodor & Field on p.193 for one instance). But in fact reduction specifically rules out elimination: Successful, smooth reduction, on the contrary, actually guarantees the reality and the legitimacy of the higher reduced level' Finding out what the higher level constructs are identical with or supervenient on tells you what they are, not that they do not exist. Of course some attempts to reduce may fail: Nothing like a smooth reduction may be possible. In these cases, reductionism may lead to complete displacement of the higher level, but this will be precisely because reduction has failed (P.M. Churchland, 1979, Section 1l; P.S. Churchland, 1986, pp.278-295; Duran, 1988, pp.296-299). The reducibility constraint allows for a range of reductive possibilities ranging from smooth, retentive reductions, most notably identities, which "preserve ontology" (Hooker, 1981, p. 201) by guaranteeing the legitimacy of the reduced level or theory, to the opposite extreme where there is nothing to which the higher level reduces, and elimination becomes a live option. So the reducibility constraint is intended to be sufficiently weak to escape criticism of the a priori scientism of that implausibly strong reductionism just sketched. All that it should include is what is naturalistically explicable within the constraints of physicalist monism. All that it should unproblematically exclude are theories which postulate gods, nonphysical minds, "queer" moral values (in Mackie's (1917) sense), phlogiston, animal spirits, and the like. But the ontological legitimizing of psychological explanation doesn't just mean drawing out the implications of materialism. It must also explain the success of psychological explanation. For understanding of how and why it works, there must be demonstrable confidence that the entities and properties it cites are reducible in the specified sense. It may, however, still be thought that my reducibility constraint is too strong to do the job I want. I was looking for a principled defence of the condition of causal efficacy against Clark's causal liberalism (and other q 360 sUIl()N versions, like Dcppctt's, ol'"snrirll-r'" r'calisrtr). My clclcltsc is tltcitttl ltl lt;l;tcitl at least to both Foclorian intcntiorral big-l{ rcalists aucl to clirrtirtativists, lltc two groups Clark sees as "united in [the] mutual c1ror" ol'thc coltclitiolt of causal efficacy. But then, it may be objected, isn't Iiodor in partictrlitr notorious for his view that psychology is irreducible to, say, neuropltysitllogv (Fodor, l931)? And don't functionalists in general support a thesis ol'tltc autonomy of psychology from lower level sciences? Haven't I, in other worcls, misclassified the relevant options on these questions of reduction, causatiorl, and explanation? I don't think so2. That the multiple realizability of psychological statcs is no bar to a sufficiently weakened reducibility constraint has been argttccl by a number of reductionists (P.M. Churchland, 1979, p.1 12, 1988a; cf . Enc, 1983, Hooker, 1981, Richardson 1979)' And here is Fodor itt Psychosemantics: "It's hard to see ... how one can be a Realist abotrt intentionality without also being, to some extent or other, a Reductionist" (Fodor, 1987, p. 97). The extent to which a Fodorian intentional realist mig[( be a reductionist is captured, I claim, by something very like the weak requirement of reducibility on explanation which I have suggested. That Fodor could be sympathetic to the kind of account I've sketched is confirmed by his immediately subsequent remarks: If the semantic and the intentional are real properties of things, it must be in virtue of their identity with (or maybe of their supervenience on?) properties that are themselves neither intentional nor semantic. Il aboutness is real, it must be really something else. (Fodor, 1987, p.9'7) This is a pretty good statement of the reducibility requirement, and it fits neatly with Fodor's Representational Theory of Mind (RTM; see for example Fodor, 1986b). RTM's vindication of common sense psychology requires "a respectable science whose ontology explicitly acknowledges states that exhibit the sorts of properties that common sense attributes to the [propositional] attitudes" (1987, p. l0). This amounts to a commitment to finding "in-thehead reductive correlates of propositional attitudes," such that the correlates can be straightforwardly implicated in the physical causal chain (Clark, 1988a, p.268; cf. 1988b). Fodor, then, specifically accepts the condition of causal efficacy on explanation which Clark rejects3. For Clark, like Dennett and Wilkes but 2 See also Duran (1988, p. 298), discussing extremes of cognitive theories lrom Pylyshyn to Patricia Churchland: "Virtually no one, so far as I can see, is against the possibility of reduction." 3 For more on Fodor and Pylyshyn's (weak) reductionism (of classical symbol structures to physical structures in the brain) see Fodor and Pylyshyn (1988), pp. l3-14 and note 9. This is why Dennett has described an "imaginary vindication of the language of thought hypothesis" as "a triumphant cascade through Marr's three levels" (Dennett, 1 987, p. 22'l ; cf . Clark, 1990a, pp. 200-203) ilr r)U(;il()N ANI) llvl ll;()l I xl'l ANn ll()N lN o()NNl (;ll()Nll;M :l{il rrrrlike Iiotlor itrrtl llrc ('lrrrlclrlirrrtls, lellitirtutlc cxplltttitlitllts tlott'l ltitve lo cilc prol)0rtics wlrich alc iclcntical rvith <ll srrpclvctticttt utt tttltct';tto1'rcllics, tlt' w|ich arc clircctly imlrlicalccl irt thc citttsal ploccss lcading to bcltaviour. What unusually unites thc climinativists with Fodor here is a parallel vicw about the factors relevant to explanatory legitimacy. They agree that legitimate explanation must cite real, causally active entities and properties, and that these must be reducible, in our weak sense, to another level. Of course the Churchlands make much of the radical consequences of reductionism, because their hunch, contrary to Fodor's, is that the search for reductive correlates of propositional attitudes will generally fail. But it is worth noting again that eliminativism will only be an option to the extent that the reductive enterprise fails. THE VALUE OF HIGH LEVEL EXPLANATIONS But what is persuasive about the condition of causal efficacy, even when it is spelled out in these weakly reductive fashions? Are there any considerations which make the requirement of reducibility attractive, other than the odd bedfellows it brings together in Fodor and the Churchlands? I think there are. I'll discuss them first in the specific context of cluster analyses of connectionist systems, then draw some general conclusions about Clark's liberalism, and finally look at the consequences of my approach for the debate over the status of common sense psychological explanation. Cluster Analysis Remember the way Clark defended the value of high-level explanations of connectionist networks. The partitions given in a cluster analysis, for instance, play no part in a genuine (low-level) causal process explanation (which would be in terms of connectivity weights alone). But this is no bar to, nor is it relevant to, the legitimacy of cluster analytic explanation, for which, says Clark, its utility is sufficient. In particular, it is a level of explanatory generalization at which are grouped only those systems capable of carrying out a particular task, of satisfying a function in extension. It brings together at a useful level of abstraction all and only the range of networks which can "negotiate that cognitive domain" (Clark, 1989a, p. 199). Two responses to this are possible in the light of my metaphysical digression. The first is the radical one (originally by Paul Churchland), that genuinely causal explanation will be only at the numerical level, because only such an explanation will account for the specific characteristics of the actual network with respect to learning, generalization, performance on degraded input, and the like (P.M. Churchland, 1989a, Section 5; Clark, 1989a, pp. 193-194). The point would be that only a causal process explanation, and ''w 302 SUI ION not a program explanation, will do justicc to thc lrltcttoluctta. But I don't find this eliminativist response satisfying, in thc citsc ctl'clttslcl analysis anyway.a My hunch is that cluster analysis can givc tts gcrtttittc causal process explanations of the results of processing. This second responsc accepts the value of cluster analytic explanation, but, in contrast to Clark, also has an account of why they are valuable, why they work. They arc valuable because they are causal explanations in the strongest, process sensc of causal explanation: because they both (a) cite real, causally active entities, properties, and processes; and (b) are (weakly) reducible, in fact supervenc on, lower levels. "Causal program" explanations in general must be identical with or supervenient on lower level (process) explanations to be legitimate. The reducibility explains why they are valuable: because the partitions in state space which they cite, for instance, are really there in state space, constituted by, realized by, and supervenient on the (numerical level) connectivity weights in the particular case. Causal Liberalism Revisited This idea has already been applied more generally by N{ark Rowlands to the examples given by Jackson and Pettit (Rowlands, 1989). While agreeing that functional and disjunctive properties can play a role in true causal explanations, Rowlands notes that this is because they bear some relation to properties which are causally productive (in the "process" sense). Such a property must be realized in an actual case by a lower level property which is causally productive (Rowlands, 1989, p. 272). "The explanatory capacity of the supervenient disjunctive or functional property rides on the causally productive capacity of the property which realizes it" (p. 2'73). Only the actual realization of an increase in temperature or of a glass's fragility is causally productive in a particular case. I would add to Rowlands'reading that it is only because of such causally active specific realizations that we say the increase in temperature or the fragility causes (and causally explains) the breaking. In each case, increase in temperature or fragility does cause, because both are supervenient on the particular realization in that case. The point could be extended to bring into question the utility of the causal program,/causal process distinction (on which Clark's causal liberalism is a Indeed Churchland later accepts that whethcr we look to the specific point in weight-space or to partitions in activation space depends, centrally, on what we're doing. Hc thinks that "while the weights are of essential importance for understanding long-term learning and fundamental conceptual change, the partitions across the activation space, and thc prototypical hot-spots they harbor, are much more useful in reckoning the cognitive and behavioral similarities across individuals in the short-term. People react to the world in similar ways not because their underlying weight configurations are closely similar on a synapse-by-synapse comparison, but because their activation spaces are similarly partitioned" (1989b, pp. 234). Utility comes in only here, after the reductive effort, when we already have sophisticated means of relating high level to low level. Itl l)tl(jll()NANI)llvl l:;()l lxl'l ANAII()NlNt;t tNNl (;ll()Nll;M il(ilt lllsccl) itsell. lior trrrtilctlrrcliortists likc.litcksott, l)ctlil, irrrtl ('lark, scckirrg to cr.6clc Ical ol "tIc spcctcr ol'r'ccluctiorr," tllc "r'citl" cattsal proccsscs cilll surcly only occul at the basc levcl ol'micropltysics. lior even the numerical levels of connection weights or of synaptic biochemistry are multiply realizable in different configurations of constituent parts: The numerical level stands in the same kind of orie many relation to the subatomic level as does the cluster analytic level to the numerical level itself. To accept the views of causation and explanation espoused by reductionism as the antireductionists construe it would, it seems, lead necessarily to the denial of true causal process explanation at any level above subatomic physics. This, surely, is a reductio a(l absurdum of their misconstrual of reductionism rather than an accurate picture of a serious view. According to the reductionism they attribute to reductionists, neurophysiological constructs like columnar processing and cell assemblies, or even neurons and their biochemical interactions could not figure in genuine causal process explanation. In Jackson and Pettit's examples, high-level explanation of the glass's breaking in terms of the increase in temperature of the gas inside, or of the glass's fragility, is not in principle any different in causal status to explanation in terms of the impact of molecules on the walls of the vessel (which realizes the increase in temperature), or of the categorical basis of the glass (which realizes its fragility)5. Rowlands makes similar observations on the program/process distinction. He thinks that ,,in itself, the distinction is fundamentally sound:" but this is odd, for he recognizes that "the use of program explanations in science is very widespread indeed ... natural science must deal almost exclusively in program explanations." If this is so, how can the distinction be sound, when one side of it is all but empty? "[A]lmost all the explanatory properties invoked by even a foundational science such as physics" are cited only in program explanations (Rowlands, 1989, p.271). But apart from showing the emptiness ol the distinction he claims to support, Rowlands also reiterates the traditional view that multiple realizability debars reduction: To deny the ubiquity of program explanations even in science is to fall victim to what Blackburn calls the Tractarian View of physical properties: The mistake of supposing that for any physical property there should be a story, in terms of the configuration of some constituent things, saying what it is. (Blackburn, 1991, pp.206-208). 5 One response here would be to refer to thc view of David Braddon-Mitchell and John Fitzpatrick that true causation cloes occur only at the microstructural level. Only the actual miciostructural instantiation of a high level does the causing, and high level regularities merely explain (Braddon-Mitchell and Fitzpatrick, 1990, section 4). I, in contrast, want to maintain true high level causation where suitable reductive relations bctween levels hold. Braddon-Mitchell and Fiizpatrick ten4 towards the neo-behaviourist position that the implication of mental states in the causation of action is an unnecessary requirement. Whatever the merits of their view, it won't help Clark, for he is committed to genuine causation at the higher levels of explanation. I i q 364 t;tJ I loN The casc is supportcd by thc notolious cxanrplc ol'lhc rnulliltlc lculiztrltililv of temperature across difl'erent physical bascs lbr solicls, gascs, ;tllsnxr, lrntl vacua. But the disjunctive nature of the physical bascs is no clbjcction to particular (domain-restricted) reductions. The Tractarian View can bc usclirlly modified and, contra Blackburn, supported by adding the (latcr) Wittgensteinian point that a family resemblance is all that is required among reductive realizations. There are many possible but somehow related storics, in terms of configurations of constituents, to be told about your averagc high-level physical property (for weakly reductionist treatments of thc temperature case, see Hooker, 1981, pp. 47-49; P.M. Churchland, 1988a, ch.2). Common Sense Psychology and the Condition of Causal Efficacy Folk-psychological explanation is, for Clark, "just one more layer in rings of ever-more explanatory virtue" (1989a, p.200). Like other high-level explanations, it groups "apparently disparate physical mechanisms into classes that reflect our particular interests" (p. 201). But the utility of common sense ascriptions of mental states on the basis of behavior, for Clark, implies nothing about in-head processing, about the nature of the "engineering" account ofthe causes ofthat behavior (1988a, 1988b). Folk psychology works as a descriptive model which fixes on important regularities in behavior, not because there are any reductive correlates of its explanatory constructs in the head. But such a defense of common sense psychology against the advance of neurophilosophy is unilluminating: it simply leaves unexplained the relation between the descriptive account and the engineering account. Clark does assert that his criterion of interest for the legitimacy of a level should not be taken to allow that "anything goes" (1989a, p.201), but he still gives no principled grounds other than interest for distinguishing legitimate from illegitimate explanations. This defect is perhaps partly to be remedied by his commitment to mixed models of cognition, where connectionist and language of thought systems are working in tandem (1989a, Ch.7; 1989c, 1990b). In these cases some aspects of the descriptive project will, presumably, be backed by an engineering, in-the-head story which shares its form. But to the extent that Clark does support the connectionist rejection of causal processing descriptions which mirror the form of natural language semantics, his overview of causal explanation doesn't tell us why common sense explanation works as well as it does. Even with functional concepts, defined in terms of a causal,/functional role and not in terms of the occupant(s) of the role, we still need an account of how all the particular occupants come to fill the role, of what it is that makes them the sorts of things which can fill the role. If you're a functionalist about watches, or about beliefs, you still need a story of how specific ilt l)tl(;lloNANl)llvl ll;ol Ixl'l ANAlloNlNooNNl (llloNl:;M l]6{r ltltysicul corrligrrlirliorrs ol'lowcl lcvcl cnlilics colltc lo ltlay tltc t'olc watcltcs ol bclicl's play. IIr (hc watclt citsc, thc rcclttctivc story will bc very tcdiously disjunctive. Watches arc so variously realized that, given our inductive confidence that reducibility could go through in any particular case, we won't tend to do it because it won't be very interesting. But we just don't know yet what reductive stories about mental states and processes would be like, where on the continuous spectrum of reductive possibilities they would fit. Would they reduce smoothly, to the retentive extreme (as suggested by both Fodor's Representational Theory of Mind and O'Brien's connectionist vindication of folk psychology)? Would they prove entirely irreducible, as forecast on the Churchlands' eliminative extreme? Or would they fall somewhere in between (as suggested, for example, by Smolensky's (1988, pp.59-61) limitivism, which sees high-level cognitive explanation as approximately correct, as falling in the middle of the range of reductive possibilities for high-level explanation). Tienson, criticizing Clark, has noted that even if mental states are individuated functionally or conceptually, the fact that something satisfies this conceptual demand on a kind of ascription is still an empirical, not a conceptual fact, and requires empirical explanation. The conceptual/ functional demand does not, as Tienson puts it, explain its own satisfaction. "Quite the opposite. A satisfied a priori demand requires an empirical explanation. That it be to a considerable degree liquid is a conceptual demand on calling something soup. But its being liquid is an empirical fact, subject to empirical explanation" (Tienson, 1990, p. 160). Without a theory relating causal explanation at different levels to the reductive relations between those levels there seems little prospect of such empirical explanation. Because Clark is wary of the ontological commitment of explanation, he cannot account for the way genuine true explanations latch onto the world. Explanations must, mostly at least, cite real entities, properties, and processes' If they did not, they wouldn't tend to work as often. But Clark, by exempting high-level constructs from the condition of causal efficacy and the need for (weak) reducibility, leaves them ontologically loose and free-floating. For him, the explanatory utility of high-level explanation is all that is required' But this is to close off a priori the discovery of error and of any possibility of revision of the high level. This attitude is perhaps clearest where Clark is discussing Fodor's attempt to find computational structure in the brain which reductively mirrors the structure of propositional attitudes: Fodor's approach is dangerous. By accepting the bogus challenge to produce syntactic brain analogues to linguistic ascriptions of belief contents, he opens the Pandora's box of eliminative materialism. For if such analogues are not found, he must conclude that there are no beliefs and desires. The mere possibility of such a conclusion is surely an elfective reductio ad absurdum of any theory that gives it house space. (Clark, 1989a, p. 160) +t 366 sLJ I toN This is a transcenclcntal argumcnt agaittst clitninativisnr. Il' alty llte<lly allows the possibility that its own falsity would cntail climinativism's tnrth, that theory must be ruled out a priori as absurd. This is a strangc argunrcnt, to say the least. Transcendental arguments against eliminativism arc at bcst fairly pointless, since they have no prospect of ever convincing those against whom they are aimed, and at worst seriously misguided. Eliminativism may be implausible, but it is not incoherent (see Devitt's 1990 critique ol' Boghossian, 1990, for criticism of another such transcendental argument). The spectrum between "big-R" realism and eliminativism about the theoretical analogs of the concepts of common sense psychology is an exhaustive one. 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