thought experiments life of

8/2/2019 Thought Experiments Life Of

1/8

Do Thought Experiments Have a Life of Their Own? Comments on James Brown, NancyNersessian and David Gooding

Author(s): Ian HackingReviewed work(s):Source: PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association,Vol. 1992, Volume Two: Symposia and Invited Papers (1992), pp. 302-308Published by: The University of Chicago Press on behalf of the Philosophy of Science AssociationStable URL: http://www.jstor.org/stable/192844 .

Accessed: 02/04/2012 17:49

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of

content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms

of scholarship. For more information about JSTOR, please contact [email protected].

The University of Chicago Press and Philosophy of Science Association are collaborating with JSTOR to

digitize, preserve and extend access to PSA: Proceedings of the Biennial Meeting of the Philosophy of ScienceAssociation.

http://www.jstor.org
http://www.jstor.org/action/showPublisher?publisherCode=ucpresshttp://www.jstor.org/action/showPublisher?publisherCode=psahttp://www.jstor.org/stable/192844?origin=JSTOR-pdfhttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/stable/192844?origin=JSTOR-pdfhttp://www.jstor.org/action/showPublisher?publisherCode=psahttp://www.jstor.org/action/showPublisher?publisherCode=ucpress


2/8

Do Thought Experiments have a Life of Their Own?Comments on James Brown, Nancy Nersessian and David GoodingIan Hacking

University of TorontoI believe thatthoughtexperimentsdo not have a life of theirown. The tide of opin-

ion is going the otherway. That is evident fromthe amplecitationsprovided by ourthreespeakers.They include for example the outstandingcollection of papers,editedby TamaraHorowitz andGeraldMassey (1991)with its wonderful editorial introduc-tion thatreally does say what is in each paper.There are David Gooding's chapters(1990, chs. 8-10) on the relationshipsbetweenFaraday's hought experimentsand realexperiments.There is JamesBrown's book (1991). It is sadthat John Norton had to re-turn on family business to Australia,and was unable to chairourmeeting, because hisviews arein such absoluteopposition to Brown. The "empiricism"of Brown's titletoday is none other than that of John Norton(1991). There is also the forthcomingbook by Roy Sorensen, for which he has writtenan excellent popularprolegomenon(1991). In addition there are numerousforthcomingpapersthatI have not seen, andwhich arecited by our speakers. Finally this session itself confirms thatthe discussionof thought experiments is going througha substantialboom.

This enthusiasm for thoughtexperimentsis all the more remarkablebecause ourforbears have not written a great deal on the topic. As Nersessian remindsus, ourphilosophical predecessors-as opposed to scientists who used thought experimentsin their argumentationor reflection-are Mach andKoyr6.We might add a mentionof KarlPopper,whose comments in Logic of Scientific Discovery (1959, appendix XI)were singled out for praise by Koyre. And of course we have Kuhn's famous paper(1977 [1964]). He tells us thathe almost called it "The"Function forThought experi-ments-not just "A"Function as the final version of the title has it.We have heard threevery distinctapproaches oday.People have been struckbyBrown'splatonism.He uses a lower case "p",andhis platonism s fallibilist.Forme theimportantmessage in his papermay be that there aredifferentkindsof thoughtexperi-ments,a case arguedat greater engthin chapter2 of his book. SometimesBrown andIdifferin point of nomenclature. resistcalling Maxwell's demonpartof a thoughtexper-iment. It is partof a fantasy.HereI agreewith Nersessian,who hasremarked o me thatit is hardto see what is experimentalaboutthe demon.Perhaps t is only a rhetoricalde-vice to reinforceMaxwell's statisticalanalysis.The demondoes not, for me, prove eventhepossibility of anything.Perhaps t shows an aspectof a scene in statisticalmechanics.

PSA 1992, Volume 2, pp. 302-308Copyright? 1993 by the Philosophy of Science Association


3/8

303Browntells me thatpeople readilycall the demon a thoughtexperiment.Thatcertainlyhelps his case. He claims that Norton'sanalysis is inadequate,andI certainlydon't seethe demonfitting into Norton'sframework.But is it a thoughtexperiment?Nersessiansays thatthoughtexperimentscharacteristicallyareso compellingthatthere is no need toconduct theexperiment.The problemwith Maxwell's demonas anexperiment s thatyou can't conduct it at all, no matterhow muchfantasyand idealizationyou allow your-self. Nor can you conduct another antasynoticed by Gooding-that of Faraday onsid-eringwhether the electricalchargesfelt by mites on a spherearepositively ornegativelycharged.(Werethe mites ancestorsof Maxwell's demon?)PerhapsGoodingwould saythatMaxwell's demonis notpartof a thoughtexperimentbecauseyou cannotput your-self in theplace of thatdemon,or of Faraday'smites either.I do not deny,of course,thatthere s somethingof a continuumhere and understandBrown's wish to count the demonas partof the litany of thought experiments.

At any rate Brown's position must be seen in partas a contrastwith thatof the ab-sent Norton, who maintains thatevery thought experimentis sound insofar as it canbe cast into a valid deductive form (Norton 1991). Brown grantsthis of some, butnotof all thoughtexperiments. In particularhe wants to insist on thoughtexperimentsthatresemble intuitive insight in mathematics,rather than the combinatorialchecking ofproof. In my opinion, he grantstoo much to Norton by ceding to him the word "argu-ment". There aremany kinds of argumentthan areregimentedby symbolic logic.Norton claims that all thought experiments can be cast into the form of deductions.The ideal is thateach step is an instance of a rule of inference. That is how we havecome to define the word "argument"n elementary logic. An argument s a sequenceof sentences; it is valid if every member of the sequence is a premise or follows frompreceding sentences by one application of a rule of inference. It is sound if thepremises are all true. Thatis a powerful logistical idea. It capturesone of the strandsin mathematicalthinking. That strandis, to put it too crudely,algorithmicand Arabic.Brown's favoured examples are geometrical and Greek.There is no reason for him tograntthat all argumentsusing imaginaryexperiments, if they deserve to be com-pelling, must be capable of being turned into valid proofs. At most he needs to grantthat if they cannot be turned into valid proofs, they have shown themselves to be falli-ble. And Brown is an avowedly fallibilist platonist.

Nancy Nersessian's and David Gooding's approachesat first seem as differentfrom each other as from Brown's, yet all three arecompatible-and anti-Nortonian.All look at thoughtexperiments as something other thanconcealed derivations.Nersessian's approachis from the mental modelling wing of cognitive science."Simulated model based reasoning"is her catchphrase.David Gooding's languageonly looks different from hers. His keyword is "embodiment."We could seeNersessian and Gooding combined after a reading of MarkJohnson's recent bookTheBody in the Mind (1987). Nersessian has confirmed in correspondencethatshetakes embodiment to be a central feature of her analysis. I notice that both speakershave today favouredthe word "narrative",and Brown has used it too. All three havean essentially non-propositionalattitude to thought experiments.Stories come in sen-tences, but for these philosophers, we tell a story so that we can put ourselves into animagined situation. All three might reply to Norton thatto turna thoughtexperimentinto a derivation is to transformit from an experiment into something else. On this ac-count, Norton does not have a theory of thought experimentsat all! He has a theoryabout what can be constructed out of thought experiments.That in turnmay provide acriterion of correctness for thought experiments, but is not the experimentitself.

Thus all threespeakersfollow routesimportantlydifferent romNorton's.It maybetrue that thereis a formal derivationof every conclusionreached n a soundthoughtex-


4/8

304periment.CertainlyNorton'sown workis a marvellousway of unpacking he deductionsin Einstein'sstories.And we haveto regretonce againthat Nortonis not here to replyagainstthis commonfront thatI havejustreadinto the threepaperswe havejust heard.

Letus go backto first,or at anyrateearly,principles.What is theproblemaboutthoughtexperiments?Mach,who seems to have inventedthe termGedankenexperiment,seems to have seen no problemat all. Koyre,who took over the idea underthe name ofexpdrience maginaire,was equallyuntroubled.Koyr6thoughtthatimaginaryexperi-ments were ideal abstractions,n which he revelled. Machbelieved in contrast hat thereis somethingof a continuumbetweenrealexperimentsandthoughtexperiments.DavidGoodingis a Machian.Indeed he shows us acontinuum n Faraday'swork, a chain ofexperimentsendingwith an impossibleexperiment,a thoughtexperiment.But we stillcan see aproblemaboutthoughtexperiments.Kuhnstated t with characteristic6clat.If we have to do with a real thought experiment, the empirical dataupon whichit rests must have been both well-known and generally accepted before the ex-perimentwas even conceived. How then, relying exclusively upon familiardata,can a thought experimentlead to new knowledge or to new understandingof nature?(1977 [1964], p. 241).

This statement has become almost classical, quoted for instance at the beginning ofMark Wilson's paperon thought experiments (1991). And yet there is something notquite right aboutKuhn's statement.How on earth,he makes us wonder, could we findout something about the world "justby thinking"?But pause a moment. We do it allthe time! What,afterall, is appliedmathematics,but the deduction of what we don'tknow from what we know? Thereis of course an ancient traditionthat says deductionmerely makes us awareof what was containedin our premises. Verywell, but it stillleads us to new awareness,new understandingof the world. I shall returnto Kuhnbe-cause I do thinkhe has left a permanentmarkon our topic. Thoughtexperiments arevaluable when they bringout, in a succinct way, a conceptual tension between twoways of thinking,and force us to come to grips with it. Thought experiments are notsimply a matterof probingconceptual confusion. Ratherit is a tension between twoways of describingthe world, both internallyconsistent, but one in some way simplerthanthe other.The simplerway is forced to yield to the more rigorous. (Wilson has anexample of the more rigorous yielding to the simple).

Withoutin any way calling in question thataspect of Kuhn's insight, I would liketo add a ratherpedestrianway of understanding he quotationfrom Kuhn. It drives usalong Brown's path, and leads us to comparethought experiments to intuitive mathe-maticalreasoning. What is the philosophical probleminvolved? There is a famousproblem whose most eloquent statementhas been passed down to us for almost 2500years. That is the problemaboutthe slave boy in Meno, asked to construct a squaredouble the size of a given square.Is thatmathematics?It might be better to call it ra-tional geometry, by analogy with rationalmechanics. Plato was developing the idea ofmathematicalknowledge as pertainingto a higherplane thanknowledge of materialthings, but his picturewas not our simplistic one of an analytic-synthetic distinction,used in the twentiethcentury by logical positivists and others to combat platonism.Let us take Plato in his own terms,without forcing on him our distinctions betweenthe mathematicaland the empirical.When we do that,it is far more naturalto call Plato's story a thought experimentthan to call Maxwell's story a thought experiment.The fact thatwe call it mathemat-ics does not put me off. The argumentpresentedin Meno conforms to Nersessian'slist of five characteristic eaturesof thought experiments:


5/8

3051. It is a narrative-and what a narrative!2. We follow it throughas in the real world.3. The experiment works. Both Gooding and Nersessian note that we tend notto be shown the tinkeringthat led up to a polished thought experiment.Plato shows us the tinkering,and the initial failures.4. The thought experimentworks by abstractions.At one junctureSocrates ismade to say thatthe figures drawn on sandcould be largeror smaller.5. The thoughtexperimentis so compelling thatthere is no need actually toperformthe experiment.Indeedsomeone who startedmeasuring,after see-ing the boy's final proof, would not have understoodthe argument or wouldbe in the wrong millennium,wonderingaboutnon-Euclideangeometry).

The example given in Meno is partof a philosophical argument.It led Socrates to themost astonishing philosophical theory.The boy remembers from a pre-existing soul,and is helped by the process of dialectic. The theoryof anamnesis is a powerful re-minderof how deeply a profoundthinkercan be moved by the experience ofproof-the very experience to which Brown's platonism constantly directs us.But if that is the problem ("thehardnessof the logical must")then it is not a prob-lem peculiar to thoughtexperiments.It is a problemabout mathematicalknowledge.It is hardlysurprisingthat Brown gives us a picture for graspingan arithmetical den-tity, and calls it a thoughtexperiment.Note that his arithmeticalpicture-exampledoesnot obviously satisfy Nersessian's fifth feature.I don't well understandwhatperform-

ing the experiment would be, for the generalproposition.I can perhapssee thatitworks for any numeral but I don't see thatit works for every numeral.But I have nodoubt about the cogency of Brown's general aim. It is always an essential task, to re-mind us how strangemathematicsis.I neverthelessfind a problemwith Brown thatI find with the other two speakersaswell. The explanations of how thoughtexperimentswork,thatareofferedby our threespeakers,do extraordinarilyittle for me. We have been inoculatedagainstBrown's va-riety of platonismfor too long. Most of us see at once that his idea, of immediateac-quaintancewith universals,abstractobjects,just does not do any explanatorywork forus. But oddly the same is true for the ideas presentedby our otherspeakerstoo.Before urging thatclaim, I should acknowledge thatGooding's talk of embodi-ment is deeply important.If we look at the pictureof Stevinus's inclined plane (drawnby Brown), and think throughthe experiment,we want not just one figure, as inBrown's talk. We need at least a before and afterpictureof the plane and necklace,and we need someone snipping the necklace-simultaneously-at both ends of theblock of wood. We see ourselves getting in there andcutting it. In real life, we wouldhave a terrible time making the two cuts simultaneously-I might in fact go for thelowest point of the necklace and makejust one cut. But at any rate thereis embodiedaction. Just as pictures of real life experimentsusually omit the experimentersandwhat they literally do, so we too often forget thatit is the bodily feel of the thoughtexperimentthatconvinces us. Norton is right to produceformal derivationsarisingfromEinstein's thought experiments.But Gooding is right to emphasize the man inthe elevator. On his view, I believe, it is thatman who makes the storyinto a thoughtexperimentrather than a proof; even the man's being drugged,as in one version of theexperiment, is not irrelevant.


6/8

306Having spoken up for Gooding's idea of embodiment,I mustrepeatthatI find thatall threespeakersdo little for me when I look at a thoughtexperiment.I will take asan example a particularlyeasy one. In March 1686 Leibniz publisheda little anti-Cartesianbroadsidein the Acta Eruditorum.It had a trenchant itle:"A BriefDemonstration of a Notable Error of Descartes and OthersConcerninga NaturalLaw." It was concerned with perplexing concepts like momentum,kinetic energy andwork.This was a Kuhnianscene par excellence, in which conceptualclarification wasat issue. Thereis a certainresemblance of Leibniz's argument o the famous objects infree fall that Galileo tied together.But the reasoning is partof a process of sophistica-tion leading up to Leibniz's Essaie de dynamique(1693) andSpecimen Dynamicum(1695). But ratherthanconsidering Leibniz's first experiment,let us consider a sup-plement to it, dating from a laterperiod. that is connected with the idea of work. Theprojectwas to establish something we almost take for granted."Itis to be shown thatthe power (potentiae) requiredto lift 1 pound 2 feet is the same as thepower requiredto lift 2 pounds 1 foot." (Leibniz 1969, p. 298)."Ipostulate,"wrote Leibniz, "that I am permittedto assume various connectionsof the heavy bodies with each either,and their separationagain,andto introduceanyotherchanges which do not make a change of force. I also make us of threads, axes,levers and othermechanisms lacking in weight and resistance."Some abstraction.Leibnizthenprovedthat a body of one pound, falling through wo feet, will have ex-actlythe "power" potentiaewhich the translatorhere renders"force",althougha pageearlieras "power")required o lift a two poundbody throughone foot. And he does thisby dividingthe two-pounder nto two, and thendrawingupeachhalf successively by let-ting the one-poundweight fall one foot in succession, for a total of two feet.That,I suggest, is a paragonof a thought experiment.As I contemplateit, none ofourspeakersdoes anything for me. Yes, there is narrative.Yes, there is embodiment.Simulated model based reasoning? It seems easier to thinkaboutthe experiment,perhapsdrawa figure, thanto take me down to the level of mentalmodels. And as forBrown's universals with which we are acquainted,once again that does no work forme. This is a very unphilosophical type of objection to my colleagues, but it must beone thatthe lay person feels quite strongly.It certainly is at the core of the generalre-sistance to Brown's platonism. And if cognitive science were not so fashionable, Ithinkmany of us would feel the same resistance to Nersessian's conceptualmodels.They arepresentedas if they were explanatory,but in fact explain nothing.I am in fact more moved by reading Wittgenstein, for example one remark n thelast partof his remarkson mathematics. He draws a pictureof a mechanism,a sort ofpiston in which a wheel is driven roundby a rod fixed to it at point A, andpassingthrougha slide at point B.While the point A describes a circle, B describes a figureeight. Now we writethis down as a proposition of kinematics.When I work the mechanism its movement proves the propositionto me; aswould a constructionon paper. The proposition correspondse.g. to a pictureofthe mechanism with the paths of the points A and B drawn n. Thus it is in acertainrespect a picture of thatmovement. It holds fast what theproof shewsme. Or-what it persuadesme of. (Wittgenstein 1978, p. 434.)

Wittgenstein'sword "picture"does more for me than the fashionable word "narra-tive". It is essential to recall his much earlierthought:


7/8

307The experimentalcharacterdisappearswhen one looks at the process as amemorablepicture (p. 68).

I do not deny that a mentalmodeller may in some distantday provide an account ofhow Wittgenstein'spictureswork. In the meanwhile, you will find many aphoristicinsights aboutpicturesandexperimentsin the Remarks on the Foundations ofMathematics. I commend consulting the index to the third edition (1978).Wittgensteinis among otherthings a greatcomic author, n a certain Austriantra-dition that we dowdy philosophers of science tend to ignore. And when we look atthoughtexperiments, especially the ones that are shown us by Brown, we may experi-ence a touch of iconoclasm. Whatdo these picturesremindme of? They remindme ofjokes, thatis, very trenchantwell-worked out items with a punch line. They also re-mind me of optical illusions. That is, they areexperiences thatit is hard or impossibleto exorcise. They are snappy and, like optical illusions or probabilityparadoxes, they

will get you every time, no matter how well preparedyou are.Over a decade ago I wrote thatexperimentshave a life of their own. I intendedpartlyto convey the fact thatexperiments are organic, develop, change, and yet retaina certainlong-term development which makes us talk aboutrepeatingandreplicatingexperiments-see Hans Radder'spaperat this conference (See PSA 1992, Vol. 1). Ithink of experimentsas having a life: maturing,evolving, adapting,being not only re-cycled but also, quite literally,being retooled. But thoughtexperiments are ratherfixed, largely immutable. That is yet anotherrespect thatthey are like mathematicalproofs, but good proofs have proof ideas that can be used over and over in new con-texts-which is not, in general the case with thought experiments. They have just onetension to expose. Of course there are false starts,and the exposition gets neater over

time. And here the prescience of Kuhn's papercomes to the fore. The reason thatpeo-ple wrestle with thoughtexperiments,use them for exposition andput-down argu-ment, is thatthey can reveal tensions between one vision of the world and another.They can dislodge a person from a certainway of describing the world. They can re-place one picture by another.That is theirjob, their once and futurejob.Kuhn, in the discussion following our presentations, urged against me that thoughtexperiments do have a life of their own. But he was making a point that is consistentwith mine. He was urging thata thoughtexperimentis constantly repeated,and notinterfered with. People do rethinkthroughthe thoughtexperiment, even from genera-tion to generation.But what they think is what was once thought, and continues to usethe very same diagramsand dialogue-just as Plato's account of Socrates and theslave boy leads to us to drawthe same diagrams,from generationto generation, onceupon a time in the sand, now on paper or a transparency.But is this life? Is it not likeacting a partin a play? Olivier broughtnew life to Othello, but no matter how pro-found the power of Othello or Lear, those charactersdo not have a life of theirown,Tom Stoppardnotwithstanding.Once the thoughtexperimentis writtenout in perfec-tion it is an icon. Icons, to reiterate,do not have a life of their own.

ReferencesBrown, J. R. (1991), TheLaboratoryof the Mind: ThoughtExperimentsin NaturalScience. London and New York:Routledge.Gooding, D. (1990), Experimentand the Making of Meaning. Dordrecht:Kluwer.


8/8

308Horowitz, T. and Massey G.J. (1991), ThoughtExperiments n Science andPhilosophy. Savage, Md.: Rowman & Littlefield.Johnson,M. (1987), TheBody in theMind: theBodily Basis of Meaning,Imagination, and Reason. Chicago:University of Chicago Press.Kuhn,T.S. (1977 [1964]), "A Functionfor ThoughtExperiments",reprinted n TheEssential Tension: Selected Studies in Scientific Traditionand Change.Chicago: Chicago University Press, 1977, 240-265.Leibniz, G.W. (1969), in Leibniz:Philosophical Papers and Letters,L. E. Loemker(trans.anded.). Dordrecht:Reidel.Norton, J. (1991), "ThoughtExperimentsin Einstein's Work" n Horowitz andMassey, 129-148.Popper,K.R. (1959), TheLogic of Scientific Discovery. London:Hutchinson.Sorensen, R. (1991), "ThoughtExperiments",American Scientist 79, 149-63.Wilson, M. (1991), "Reflections on Strings",in Horowitz andMassey, 193-207.Wittgenstein,L. (1978), Remarkson the Foundations of Mathematics.3rdedition,Oxford: Blackwell.

thought experiments life of

Documents