role functions

8/3/2019 Role Functions

1/23

Role Functions, Mechanisms, and HierarchyAuthor(s): Carl F. Craver

Source: Philosophy of Science, Vol. 68, No. 1 (Mar., 2001), pp. 53-74Published by: The University of Chicago Press on behalf of the Philosophy of Science AssociationStable URL: http://www.jstor.org/stable/3081024 .

Accessed: 07/10/2011 12:46

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 Philosophy of Science.

http://www.jstor.org
http://www.jstor.org/action/showPublisher?publisherCode=ucpresshttp://www.jstor.org/action/showPublisher?publisherCode=psahttp://www.jstor.org/stable/3081024?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/3081024?origin=JSTOR-pdfhttp://www.jstor.org/action/showPublisher?publisherCode=psahttp://www.jstor.org/action/showPublisher?publisherCode=ucpress


2/23

Role Functions,Mechanisms,andHierarchy*Carl F. CraverttDepartmentf PhilosophyFlorida nternational niversity

Many areas of sciencedevelop by discoveringmechanismsand role functions.Cum-mins' (1975) analysisof role functions-according to which an item's role function isa capacityof that item thatappears n an analyticexplanationof the capacityof somecontaining system-captures one importantsense of "function" n the biological sci-ences and elsewhere.Here I synthesizeCummins'account with recent work on mech-anisms and causal/mechanicalexplanation. The synthesis produces an analysis ofspecificallymechanisticrole functions, one that uses the characteristicactive, spatial,temporal,and hierarchicalorganizationof mechanismsto add precisionand contentto Cummins'originalsuggestion.This synthesisalso shows why the discoveryof rolefunctions is a scientificachievement.Discoveringa role function(i) contributes o theinterlevel ntegrationof multilevelmechanisms,and (ii) providesa unique,contextualvarietyof causal/mechanical xplanation.

*ReceivedJune2000;revised October 2000.tSend requests or reprints o the author,Departmentof Philosophy,FloridaInterna-tional University,3000 Northeast 151Street,North Miami,FL 33181-3000;[email protected].$Thanks o Ron Amundson,RobertCummins,LindleyDarden,MichaelDevitt,StuartGlennan, Peter Machamer,KarenNeander, Greg Morgan, PierrePoirier,GualtieroPiccinini,WesleySalmon,Ken Schaffner,StevenSmall,WendyStuart,NathanUrban,MarcelWeber,WilliamWimsatt,and KirstenWood for commentson earlierdrafts ofthis much reworkedpaper.Thanks also to the students n Darden'sgraduateseminar(Fall 1998)in the Philosophyof Biology for usefulfeedbackon multipledrafts.Manyof these friends and colleagueswill not recognizethis as the paperthat they so gener-ously read. Any mistakes are mine and mine alone. This work was supportedby theNational Science Foundation undergrantnumberSBR-9817942.Any opinions,find-ings and conclusions or recommendations xpressed n this material are those of theauthor and do not necessarilyreflect hose of the National Science Foundation.Philosophy of Science, 68 (March 2001) pp. 53-74. 0031-8248/2001/6801-0004$2.00Copyright 2001 by the Philosophy of Science Association. All rights reserved.

53


3/23

CARL F. CRAVER1. Introduction.Manyareasof science(andespecially he biologicalsci-ences)developby discoveringmechanisms nd the rolefunctionsof theircomponents.Promisesof understandinghemechanisms f, forexample,development, isease,andcognitionarecoupledwithclaims o havefoundthe roles of variousgenes,cell types,and brainregions.Yetphilosophershavesaidsurprisinglyittle about howone discoversan item'srole,aboutwhysucha discoverys a major cientific chievement, rabouthowthesemechanistic nd functionaldescriptions rerelated.Most of what has beensaid about rolefunctions n thephilosophyofsciencehas beensaidby Cummins 1975,1983),whosepositionis oftenrepeatedandrarelyrevised.Cummins' ccounthighlights heconceptualinterdependencef role functionsandwhathe calls an "analytic xplan-atory strategy"of understandinghe capacitiesof systemsby analyzingthem into thecapacitiesof theircomponents.HereI explore hepossibil-itiesandconsequences f synthesizingCummins' ccountof rolefunctionswith recentwork on the natureof mechanisms nd causal/mechanicalx-planation.Thesynthesis ieldsa moredetailedanalysisof properlymecha-nisticrole functionsandof theempirical riteriabywhichmechanisticoleascriptions re evaluated.Thissynthesisalso showswhythediscoveryofa mechanistic oleis a majorscientific chievement.t is an achievement,first,becausediscoveringan item's mechanistic ole is one way of inte-grating t into a multilevelmechanismand, second,because ntegrationinto a higher-levelmechanism onstitutesa unique,contextual arietyofcausal/mechanicalxplanation.Theargumentativetructure f thepaper s as follows.In Section2, IreviewCummins'1975)"analytic ccount"of rolefunctions, mphasizingas he did the connectionbetween unctionalascriptionand analyticex-planation.In Section3, I discuss he character f mechanisms nd,mostimportantly, heiractive,spatial,and temporalorganization. then usetheseaspectsof mechanistic rganizationo specify-more preciselyhanCummins ould-what one assertsof an item in ascribingt a mechanisticrole function.Thisunderstandingf the contentof mechanistic ole as-criptionshighlights he diversekinds of evidenceby which mechanisticroleascriptions reevaluated. n Section4, I distinguish ontextual, so-lated, and constitutivedescriptionsof an item'sactivityin a multilevelmechanism. t is by elaborating ndaligning hesedifferentkinds of de-scription, suggest, hatthelevels n multilevelhierarchies reintegratedtogether. n Section5, I introduce hepossibilityof contextualmechanisticexplanation s a uniquevarietyof causal/mechanicalxplanation,and Irespond o some criticisms f thissuggestion.2. CumminsnAnalyticExplanationndRoleFunctions.Cummins'1975)analysisof function-ascribingtatements s now the canonical account

54


4/23

ROLEFUNCTIONS, MECHANISMS,AND HIERARCHYof role functions.Cummins'"regimented econstruction" f function-ascribing tatementss as follows:

X functionsas a 4) n S (or the functionof X in S is to >)relative oan analyticaccountA of S'scapacity o Wustin caseX is capableof4


5/23

CARL F. CRAVER

a)

/F

b)I"_..uiIE

Figure1. Relationbetween ystemS'sx-ing and the4>-ingf Xs, representedbstractlyn(a) andapplied n the case of thecirculatoryystem n (b).

to regulate he directionof blood flow (>4))and linkingthose partsto-gether n theprogrammed -ingof the circulatory ystem.Cummins mphasizeshat not all analyticexplanations reinteresting.An analyticaccount of S's x-ing is explanatorilynterestingonly to theextentthat:(C1) The analyzing capacities {1>, >, ... , n} are "less sophisti-cated"than the analyzedcapacity v);(C2) The analyzing capacities {4), 2,. ., 4)n}are "differentin type"fromthe analyzedcapacity W); nd

56


6/23

ROLEFUNCTIONS,MECHANISMS,AND HIERARCHY(C3) The analyzing components {XI, X2,... , Xm}and {i1, 42, ? ? ?,4n)}exhibit a complex organization such that together they v(see 1975, 191).

The analytic explanation of the circulatory system's working arguablysatisfies these criteria, although Cummins does not show that this is so orsay what it means for two items to be "different in type" or "less sophis-ticated." The parts are (perhaps simply by virtue of being parts) less so-phisticated and different in type than the whole system; and the activitiesof hearts, arteries, kidneys, and valves have to be organized properly ifeach is to play its role. So an analytic account of the circulatory systemwould arguably be explanatorily interesting. But what does it mean tobe "different in type," "less sophisticated," and "organized" in this con-text?Here one begins to sense that Cummins hasn't said enough about sys-tems, about analytic accounts, or about how components in analytic ac-counts are "organized" to distinguish interesting from uninteresting an-alytic explanations or to distinguish cases that warrant the ascription ofrole functions from those that do not. Criteria C1-C3 do point to impor-tant symptomsof interesting analytic explanations, but they do not in anyway characterize what makes those explanations interesting.And withoutsuch detail about the character of causal/mechanical explanations, one isleft with an underspecified notion of a role function. By filling in theseabstract placeholders in Cummins' account one can produce a richer andmore precise image of properly mechanistic role functions. The charac-teristic active, spatial, temporal, and hierarchical organization of mecha-nisms can be used to specify-more precisely than Cummins has-whatis assertedby an analytic account, and hence by the ascription of a mecha-nistic role.

3. Mechanisms and TheirOrganization.There are many different kinds ofsystems (e.g., formal systems, procedural systems, and representationalsystems), and Cummins does not specify which he intends. In this section,I suggest that one restrict the idea of a "system" in Cummins' analysis tomechanisms (described in Section 3.1). I then use the characteristicactive,spatial, and temporal organization of mechanisms (described in Section3.2) to add necessary precision to the idea of a mechanistic role functionand, further, to make sense of the empirical criteria by which mechanisticrole ascriptions are evaluated (discussed in Section 3.3). There may beother interestingkinds of organization than mechanistic organization, butthe content of a functional attribution will only be as precise and mean-ingfully specified as is the accompanying notion of "organization."

57


7/23

CARL F. CRAVER3.1 Mechanisms. Mechanisms are collections of entities and activitiesorganized in the production of regular changes from start or set up con-ditions to finish or termination conditions (Machamer, Darden, Craver2000; cf. Glennan 1996, 52; cf. Bechtel and Richardson 1993, 17). Theentities in mechanisms can be taken to correspond to Cummins' {X1,X2,... , Xm}; they are the physical parts of the mechanism (e.g., the hearts,kidneys, and veins). Activities are the things that these entities do, eitherby themselves or in concert with other entities. The activities in mecha-nisms can be representedas the {(2, 2, ..., ,)} in Cummins' account.23.2 Mechanistic Organization.Descriptions of mechanismscharacterizehow entities (Xs) and activities (b4s)are organized to do something (W).Cummins' account of analytic explanations appeals, without explication,to the "organized" or "programmatic"exercise of capacities. And crite-rion C3 explicitly uses "organization" to distinguish interesting from un-interesting analytic explanations. What is the characteristic organizationof mechanisms?It is perhaps too often claimed that the whole is greater than the sumof its parts. However, Wimsatt (1986, 1997) has revitalized this cliche byfocusing on conditions under which the whole is nothing but the sum of

its parts-conditions of mere aggregativity. (For a related treatment, seeHaugeland's 1998 discussion of morphological and systematicexplanationin Ch. 1 and of decomposition in Ch. 9).3 Suppose that a property yvofthe whole S is a function of the properties {4(, 42, . .., ,(} of the parts{XI, X2, ... , Xm}.4 Then a Wproperty of S is an aggregate of the )properties of Xs when:2. Callingthem "activities"rather than "capacities" as is Cummins'preference) m-phasizesthe way that mechanismswork as opposed to the way that a set of partshasthe capacityto work or is disposedto work.This shift of gestaltis recommended nddefendedby Machamer,Darden,and Craver(2000). (See Footnote 4 below for a dis-cussion of my shiftingontology for ?ysand 4s).3. Thanksto Cumminsand Poirier(personalcommunication) or remindingme of thediscussion n Haugeland1998,Ch. 1.4. I use the terms "capacity,""activity,""property,"and "event"to characterize4and v in differentpartsof thisessay.This is mostlyan effort to stresspointsof contactacross these terminologicaldifferences.The following remarkswill perhapsassuageworries that I am ignoring importantdifferences.I use "capacity"when explicatingCummins. I prefertalk of activities, and I take the notion of a "capacity"to be asubstantivalistway of expressingthe fact that there are certainpropertiesof entitieswhich allow them to engage in activities(see Footnote 2). I use the term "event" todescribe4 andW n Section 5 in an effortto highlightconnectionswithSalmon's 1984)imageof causal/mechanical xplanation;all activitiesareevents,but not all events areactivities.In the presentsection, I use the term"property," ike "event,"to stand foreither hehavingof a propertyat a particular ime ina particularplaceor theoccurrence

58


8/23

ROLEFUNCTIONS, MECHANISMS,AND HIERARCHY(W1) xvis invariant under the rearrangementand intersubstitutionofXs;(W2)W emainsqualitativelyimilar if quantitative,differingonlyinvalue)withthe additionor subtraction f Xs;(W3) \v remains invariant under the disaggregationand reaggregationof Xs; and(W4)There are no cooperative r inhibitorynteractionsamongXsthat are relevant o W.

Wimsatt'scriteria(W1-W4) gestureat the sense of "organization" e-quired orexplicatingheconceptual nterdependencef mechanisms ndmechanistic ole functions,for getting beyondthe symptomatic riteriaexpressedn Cummins'C1-C3, and so for developinga mechanistic c-count of role functions.Comparethe circulatory ystem(S,) to a neat glass of gin (S2)and,likewise,hearts,kidneys,andarteries theXs in S1) o unit volumesof gin(the Xs in S2).The circulatory ystem(S,) deliversgoods to tissues(VW),and the glass of gin (S2)has a certain volume (W2). The parts (Xs) of thecirculatory ystem,such as the heart and kidney,cannot be intersubsti-tuted for one another(W1). Kidneysdo not pumpblood andheartsdonot filter t. Changing ven thespatialrelationsamongthecomponents fthe circulatory ystemwould(at least in manycases)completelydisruptthe behaviorof the wholesystem W2).In fact,onlyjudiciousremovalofpartsfrom(or additionof partsto) the circulatory ystem s compatiblewith its continuedworkingas a whole(W3).Finally,thereareexcitatoryandinhibitorynteractions etween hecomponents f thecirculatory ys-tem(W4),andthis is exactlywhyyou cannot tinkerwiththeentitiesandactivitiesof the circulatory ystemtoo capriciouslywithoutbreaking t.The volumeof gin,in contrast,staysthesamevolumeof gin anywayyoustir it. The total volumeonlyincreasesand decreases syoupourandsip,andunitvolumesdo not worktogether nanyinterestingense o producea total volume(evenif theysometimes eemto).Each of Wimsatt'scriteria W1-W4)pointsto the absenceof cooper-ative activityamong the parts in mereaggregates.The componentsofmechanisms, in contrast to those of mere aggregates, have an active or-ganization; they act and interact with one another in such a way that theW-ing f S is more thanjust the sum of 4 properties. n fact, the ( prop-erties of mechanismsare not reallymerepropertiesat all; they are theactivitiesof and amongthe entities n the mechanism.Typically,mecha-nismsarecomposedof differentkindsof entities likehearts,kidneys,andof some sort of temporallyextendedhappeningor change.The v propertiesof mereaggregatesareoften, but not always,events in the firstsense,while the v propertiesoforganizedmechanismsare typically,but not exclusively,eventsin the second sense.

59


9/23

CARL F. CRAVER

valves)engaging n differentkindsof activities likepumping, iltering,anddirecting) ndacting ncooperation rcompetitionwithspecific therentities n the mechanismW4).It matterswhichXs 4)withwhichothers,and it matterswhere,when,how much,and how often. This is why thepartsof mechanisms ften cannot be reorganized andomly W1),addedor subtractedat will (W2),or takenapartand put back togetheragain(W3)withoutdisturbingheirabilityto %V.s Wimsattnotes, it is oftenby doingtheseunhealthy hingsto a mechanism hat scientistsare able toteaseapartthe relevant eaturesof its organization.Not coincidentally,these are also the kinds of experiments one to discoverand specifyanitem'smechanistic ole.The activeorganization f mechanismss sustainedby theircharacter-istic spatialand temporalorganization.The same entitiesand activities,strung ogether n different patialandtemporal elations o oneanother,can yieldverydifferentmechanisms.One understands mechanismbydiscoveringtscomponent ntitiesandactivities, ndby learninghowtheiractivitiesare spatiallyand temporallyorganized(Craverand Darden,forthcoming); spectsof this organization re also amongthe evidentialcriteria or evaluatingascriptions f mechanistic ole functions.Startingwith the spatialorganization f mechanisms, he entities in

mechanismsoften havecrucialsizes,shapes,orientations,and locationsthat allow themto engagein certainactivities and hencecertainroles)and not in others.The heart'ssize is appropriatelyelated o that of thevena cava and that of the aorta such that it deliversblood in regularvolumesto distalregionsof the bodywithin a specificrangeof rate andpressure. t has a shapethat allowsit to act as a bellows, ts auriclesandventriclesalternately eceivingandexpellingblood. The heart s situatedbetween he venacavaandthepulmonary rtery ononeside)andbetweenthe pulmonaryvein and the aorta(on the other).Partof understandingtheorganization f thismechanism, sHarvey [1628]1963)exhibitsn thediverseobservations nd experiments f his Movement f the HeartandBlood in Animals: An Anatomical Essay, is understanding the spatial ar-rangementof its componentparts.The other part of understandingmechanism's rganization, lso featuredn Harvey's reatment,s under-standinghow the activitiesof these componententities are temporallyorganized.The activitiesof mechanismsxhibita temporal rganizationn the ac-tivity v of the mechanism s a whole S. Theorder,rate,and durationofsuccessive omponentactivities ks) iscrucial or S'sx-ing.Bloodenteringthe heartthrough he venacava is thenpumped hrough he pulmonaryarteryto the capillariesof the lungs,whereoxygenand carbon dioxideareexchanged.Bloodreturns o the heartvia thepulmonary ein beforebeing shippedoff to the rest of the body. Thereis a sequenceof stages

60


10/23

ROLEFUNCTIONS,MECHANISMS,AND HIERARCHYhere from beginning to end, and it would not be possible to change theirorder without gumming up the works (or making it a different mechanismentirely). The different stages also have characteristic rates and durationsthat are crucial to the working of the mechanism (as the diagnostic valueof heart rate and blood pressure attests). Timing is everything for mostmechanisms, and learning a mechanism's timing provides important cluesto how it works and to how it does not work.

Understanding how a mechanism works is just understandinghow oneactivity leads to the next through the spatial layout of the componentsand through their participation in a stereotyped temporal pattern of ac-tivities from beginning to end. This is what it means to say that a mech-anism is "organized" or, as Cummins sometimes says, "programmed"; tmeans that the components have active, spatial, and temporal relations toone another such that together they x. An analytic account for a mecha-nism is not just a list of entities and activities; it is a description of amechanism. And that description involves, in addition to a list of entitiesand activities, a description of how they are organized together actively,spatially, and temporally in S's V-ing. Specifying the mechanistic role ofsome component X, accordingly, involves describing how X is organizedwith the other entities in S such that it contributes to S's V-ing.3.3 Implications: Mechanistic Role Functions. Attributions of mecha-nistic role functions describean item in terms of the properties or activitiesby virtue of which it contributes to the working of a containing mecha-nism, and in terms of the mechanistic organization by which it makes thatcontribution.This insight about the connection between role ascriptions and mecha-nistic organization is lost if one abstracts role functions away from thedetails of how functions are instantiated in mechanisms. Cummins skirts

this difficulty when he allows that analytic accounts may be specified bya "flow diagram," and so specified "independent of whatever theory isrelevant to describing the details of the realization." (1983, 100) In oneversion, Cummins (1983) goes so far as to drop explicit reference to theparts {XI, X2,..., Xm}in analytic accounts, and speaks only of the anal-ysis of one capacity into the programmed exercise of sub capacities {


11/23

CARL F. CRAVERroles might be filled in that mechanism. Imprecise and abstract role de-scriptions are often used in discovery as rough-draft stand-ins for moredetailed accounts of how an item fits into this mechanistic organization.But the meaningfulness and precision of an item's role ascription shouldbe evaluated with reference to the precision with which one can detail theorganization of the system containing the item. It is by detailing how anitem fits into the spatial, temporal, and active organization of a mechanism(showing exactly how it contributes to S's W-ing)that one specifies itsmechanistic role. There may be other kinds of role functions that are notmechanistic role functions, but the point remains that spelling out thosealternatives will require an analysis of the intended sense of organizationin a manner comparable to that being explored here.The importance of mechanistic organization in the analysis of mecha-nistic role functions is reflectedin the kinds of evidence by which mecha-nistic role functions are attributed and evaluated. The active, spatial, andtemporal varieties of mechanistic organization are often used to determinewhether or not an item can play a given role. The item has to be in theright place at the right time, it cannot be spatially isolated from othercomponents, and it has to have the right size, shape, orientation (and otherrelevant properties) to interact with the other components of the mecha-nism. An activity that happens at the wrong time, that takes too long, orthat unfolds too slowly for a given role cannot fill that role. Role attri-butions are also tested by adding and removing parts, by moving partsaround, and by substituting them one for the other, as suggested by Wim-satt's criteria (W1-W4).The analysis of mechanistic role functions produced by melding Cum-mins' account with work on the organization of mechanisms clarifies whatis involved in characterizing an item's role in a mechanism and clarifieswhat would count as a complete description of an item's mechanistic role.This more precise analysis of mechanistic role functions also helps to showwhy the discovery of an item's role is a major scientific achievement. Thefirst reason (discussed in Section 4) is that describing an item's role is oneway to integrate that item into a multilevel hierarchyof mechanisms. Thesecond (discussed in Section 5) is that role ascriptions provide a contextualvariety of causal/mechanical explanation.4. Interlevel Integration: Contextual, Isolated, and Constitutive Perspec-tives. Many of the theories in the contemporary biological sciences (andelsewhere) have the mechanistic organizational structure that I've beenexplicating, and most that do also exhibit a hierarchical mechanistic or-ganization. That is, they describe nested networks of mechanisms withinmechanisms, in which higher-level activities (ys) of mechanisms (Ss) areinstantiated by the organized activities (4(s) of lower-level components

62


12/23

ROLEFUNCTIONS,MECHANISMS,AND HIERARCHY(Xs),andtheseare,in turn, nstantiated y the activities as) of still lowerlevelcomponents Ps).Therelationship etweenowerandhighermecha-nistic levelsis a mereologicalpart/whole elationshipwith the additionalrestrictionhat the lower-levelpartsarecomponentsof (andhenceorga-nizedwithin)the higher-levelmechanism.Lower-level ntities(e.g., theXs) areproperpartsof higher-levelntities S),andso the Xs areno largerthan,typicallysmaller han,andalwayswithinthe spatialboundariesofS. The activitiesof the lower-levelpartsarestepsor stages n the higher-level activities.Exactlyhowmanylevels thereare andhow theyareto beindividuatedare empiricalquestionsthat are often answereddifferentlyfor differentphenomena.Thecirculatory ystemhasa hierarchicalmechanistic rganization.TheactivitiesWof the circulatory ystem S are instantiatedby the heart'spumpingof blood, the kidney'sfiltrationof the blood, and the venousvalves'regulationof the directionof blood flow (the 4-ing of the com-ponentXs). And thesecomponentactivitiescan themselvesbe describedin terms of their underlyingmechanisms.The heart'spumpingcan beexplainedbyreferenceo thecontractionsa) of componentheartmuscles(Ps),and thekidney's iltration anbeexplainedbytheorganized ctivitiesof its componentglomeruli, ubules,pores,and ionicgradients.Thisde-scription ouldpotentially ontinueon to the activitiesof entitiesappear-ing in stilllower-leveldescriptions f mechanisms.Onegoal in describinghierarchically rganizedmechanismss to inte-grate these different evels togetherinto a descriptionof one coherentmechanism.Thisinterlevelntegration f mechanistic ierarchiesnvolveselaborating ndaligning ontextual + 1level), solated 0 level),and con-stitutive(-1 level)descriptionsof the -ing f some X. Each of thesedescriptionsdescribesX and its 4-ing from a differentperspectiven amultilevelmechanism.5

A contextual escriptionf someX's


13/23

CARL F. CRAVERiii. expels blood; andiv. contracts.

Descriptions i-iii) arecontextual n varyingdegrees; heyeach describethingsthat the heart could not do by itselfwithoutbeing organized o-getherwithother entitiesand/oractivities.The heartcannotexpelblood(iii)withoutblood,and the expulsionof bloodwillonlycirculate t (ii) ifthe veins and arteriesare appropriately rganized.Eventhen, the heartcannot distributeoxygenand calories(i) in the absenceof oxygenandcalories.A description f the X's mechanistic ole function s contextualto the extentthat it makesexplicitreference o objectsotherthan (andoutsideof) X itself.Referenceo objectsbeyondthe boundaries f theheart,notice, s notrequiredn describing he heart'scontraction iv). Contractings some-thingthat the heartdoesby itself;6n describinghe heartas contracting,one makesno commitments oncerninghe mechanistic ontext n whichthis activity s embedded. t is an isolateddescription.n formingan iso-lateddescription f someitem'sactivity,one drawsan idealizeddividinglineat the spatialboundaryof the item andrecognizesa limitednumberof crucial nterfacesacrossthatotherwise losedboundary.An interfacemayherebe understood s a reasonablywell-defined ndregularocusofcontactor interactionwithobjectsoutsideof thatboundary cf. Hauge-land1998,Ch.9 andalsoWimsatt1974; achties his discussion o Simon1969).Typically herearemanysuch interfacesbetweenan item and its en-vironment, ndoftenonlysomeof theinterfaces rerelevant o theitem'scontributionto a contextualmechanism.The heart makes glub-blubnoisesandgeneratesheat,butthese arenot relevant o the circulation fthe blood, evenif they maybe relevant o othercontainingmechanisms(suchas diagnosticor thermoregulatory echanisms).Therelevant nter-faces between he heartand the othercomponentsof thecirculatory ys-tem lie at thespatialboundaries f the heartwiththeincomingveins,theoutgoingarteries,andthe bloodcoursing hrough hem all. Describednisolation rom ts contextat thoseinterfaces,hecontribution f the heartto thecirculatory ystem s to contract;hisis what t does(alone) hat(intherightcontext)contributeso theexpulsionof blood,the circulation fblood, and the distribution f oxygenand calories.The distinctionbetween ontextualand isolateddescriptions ringsoutan ambiguityn Cummins'unregimentedenditionof hisanalysisof rolefunctions.He claims hat"to ascribea function o somethings to ascribe6. I am neglectingthe numerous nterfacesbetweenthe heart and the restof the bodythat are relevant o its contraction.As B-moviesattest, however, heheartcan continueto contract n isolationfrom that context.

64


14/23

ROLEFUNCTIONS, MECHANISMS,AND HIERARCHYa capacity o it that s singledoutbyits roleinananalysisof somecapacityof a containing ystem." Culmmins983,99)But this leaves t ambiguouswhetherthe functionis the capacity,described n isolationand simply"pickedout" by its contextualrole, or, instead,the contextualrole byvirtue of which the capacity s pickedout. A completedescription f anitem'srole woulddescribe ach of these.One canknow an item'scontex-tual role withoutknowingthe isolatedactivity by which the itemplaysthat role in a givencontext;andone can know an item's solatedactivitywithoutknowingwhatthat isolatedactivitycontributeso a higher-levelmechanism.There s a difference,afterall, betweenknowingthat sparkplugs producesparksand knowinghow that sparking s situatedwithinthe complexmechanisms f an engine.In the formercase, we knowthesparkplugs' solatedrole; n the latterwe describe heirrolecontextually.(Fora relateddiscussionof the contextualaspectsof capacityascription,see Glennan1997).Isolateddescriptionsof an X's 4-ing specifythe activityfor which alower-levelmechanismwill be soughtand so fix the active,spatial,andtemporalboundariesof that mechanism cf. Kauffman1971;Glennan1996). Isolated descriptions, hat is, frame constitutivedescriptions flower-levelmechanisms.Constitutivedescriptions haracterizehe orga-nizedactivities{a,, O,2..., ak} of entities{P,, P2, ..., Pj} thatinstantiatethe (-ing of X.So it is possibleto describean item'sactivityin three distinctways,dependingon how one looks at it withrespect o a hierarchy f mecha-nisms.Ignoring ts context,one can describex's 4-ing in isolation.Look-ing down to lower-levelmechanisms, he activityis describedconstitu-tively.Andlookingupto higher-levelmechanisms,heactivity sdescribedcontextually.Bechtel 1986)has sketched similarpictureof these nterlevel elations.HecriticizesCummins orfocusing"ononlyasingleevel n nature"nsteadof three:one(B1)for"components";ne(B2)for the"systemunctioningas an organizedwhole";and one (B3)for "theenvironmentn whichthesystemis eitheradaptiveor nonadaptive"1986, 39; emphasisadded).7Bechtelclaims hatthe third evel, B3,"allowsone to integrateknowledgefrom several evels andthusprovidesa neededperspectiven developingexplanationsn thesciencesdealingwithsuchsystems"1986,42).My pictureof theserelationships iffers rom Bechtel's B1-B3)in that(i) I do not reifythese threedescriptions s "levelsof nature"but as de-scriptiveperspectives n a multilevelhierarchy,and (ii) I do not tie myanalysis o the "adaptiveness"f higher-level ctivities.7. The way that I read both Cumminsand Bechtel,Cummins'account includesbothB1 (X's 4-ing) and B2 (S's V-ing).

65


15/23

CARL F. CRAVERBechtel's mageblursthe distinctionbetweendescriptiveperspectivesand mechanisticevels.Contextual, solated,andconstitutivedescriptionsof an item'sactivityare threedifferent erspectives n thatitem'sactivityin a hierarchically rganizedmechanism;heyarenot levelsofnature.Myperspectival pproachs represented raphicallyn Figure2 as a relation-

Figure2. Contextual,solated,and constitutive erspectivesn an X's O-ing.The contex-tual perspective evealshow X's 4-ing fits into a higher evelmechanismS's W-ing.Theisolatedperspective escribesX's 4-ing in termsof input-output elationshipscross nter-facesbetweenX andthe othercomponents fS.Finally, heconstitutive erspectiverovidesa description f the mechanismor X's


16/23

ROLEFUNCTIONS,MECHANISMS,AND HIERARCHYship amongthreemechanisticevels:one for S'sw-ing,one for X's isolated4


17/23

CARL F. CRAVERnism when (i) the activity has been fit into the organization of a higher(+1) level mechanism, (ii) the isolated (0-level) activity has been ade-quately described, and (iii) the activity has been explained in terms of itslower (-1) level mechanism. One understands how the heart or kidneyfits into the circulatory system to the extent that one knows how it isorganized into the circulatory system, one has characterized its activity inisolation, and one knows the constitutive mechanism of that isolated ac-tivity. Integrating an item into a hierarchical mechanism involves elabo-rating and aligning these three kinds of description. In the process, oneshows how the organizedac-ingof Ps could constitute the isolated behaviorthat is X's 4-ing, and shows how X's 4-ing is organized together withother components in S such that S can W.One might expressa quite generalintegrative strategy in the discovery of mechanisms in a slogan: "Up forroles, down for mechanisms" (This strategy might be added to those dis-cussed in Darden 1991, Chs. 2 and 15).So one reason that it is a scientific achievement to describe the mecha-nistic role of an item in a mechanism is that doing so is one way of inte-grating that component into a multilevel mechanistic hierarchy.5. Contextual Explanation: A Third Aspect of CausallMechanicalExpla-nation. A second reason that discovering a role is a scientific achievementis that contextual role descriptions are explanatory. Consider Salmon'sgeneral sketch of the nature of causalmechanical explanation (1984).Salmon distinguishes his causalmechanical account of explanation fromHempel's (1965) covering law model by noting that whereas Hempel'sexplanations explain an event by showing that it fits into a nomic nexus,causal mechanical explanations explain by showing how an event fits intoa causal nexus. Salmon's causal nexus is composed of causal processesinteracting with one another. On Salmon's causal/mechanical model, ex-plaining an event (or type of event8) is a matter of situating that eventwithin this geometrical network of causal processes and interactions (per-haps supplemented by statistical relevance relations linking causal pro-cesses and interactions to the explanandum event).9

8. Salmonintends this imageto workfor both singularevents andgeneralregularities,except that in the case of regularities,"we apply preciselythe same considerations oany volume of space-time hat is similar n relevantrespects.The relevantsimilaritiesare givenby the natureof the regularity hat we are tryingto explain."(1984, 275) Asimilarmove can accommodategeneralcontextualexplanationsas well.9. Hitchcock(1995, 1996)has chargedthat Salmon's renditionof the causal nexus istoo austere to providean adequate pictureof causal/mechanical xplanation.Salmon(1997)recommendsusing statisticalrelevancerelations o augment he barrengeomet-rical structureof the causal nexus. This move is explored n Craver(1998, Ch. 4).

68


18/23

ROLE FUNCTIONS, MECHANISMS, AND HIERARCHYSalmonrecognizeswowaysof situatingan eventwithina causalnexus,and he refers o these as two distinct"aspects"of causalmechanical x-planation:an etiologicalaspectand a constitutiveaspect.These are de-pictedin Figure 3(a),modeledupon Salmon'sdiagram 1984, 275).Theetiological aspect of an explanation for some event "fill[s]in the causallyrelevantprocessesand interactionshatoccupy hepast ightcone"of thatevent.(Salmon1984,275) Etiological xplanations re backwardooking,revealing he relevantportionsof the causalnexus n theevent'spast.Weprovidean etiologicalexplanation f whyJohn s a victimof heartdiseasewhen we blame his smokingand diet and, perhaps, he mechanismsby

a)

PastLightConeb)

I 1

Figure 3. Constitutive, etiological, and contextual explanations. Figure 3(a) is modeledupon Salmon (1984, 275) and exhibits the etiological and constitutive aspects mechanisticexplanation. Figure 3(b) superimposes the contextual aspect onto Salmon's diagram.

69

0*


19/23

CARL F. CRAVERwhich smoking and diet produce heart disease. The explanandum is anevent or a type of event and the explanans reveals the antecedent mech-anisms by which the event occurred/occurs.The constitutiveaspect of an explanation for some event, "lays bare thecausal structure" of the event by revealing "the internal causal mecha-nisms" that account for the event's "nature"(Salmon 1984, 275). Consti-tutive explanations describe, in the language of Section 4, the lower-levelmechanisms of X's (-ing. Constitutive explanations are inward lookingand downward looking, looking within the boundaries of X to determinethe lower level mechanisms by which it can (. The explanandum of aconstitutive explanation is the


20/23

ROLEFUNCTIONS,MECHANISMS,AND HIERARCHYdifferent ypesof causalmechanical xplanations orrespondingo threedistinctwaysof situatingan iteminto a nexusof mechanisms:tiological,constitutive,and contextual.Sinceany givenX or4 maycontributeo thediverseV-ings f anynum-ber of Ss, contextualexplanationsareineliminably erspectival.Thatis,theyrelyuponsharedbackground ssumptionshat S canwor that they-ing of S is important, ignificant, r relevant.Thisperspectivaleatureofrolefunctionshasbeentakenas a signofweakness.Learninghatthe heartpumpsblood seemsto explainwhat theheart s for in a waythatlearningthattheheartmakesglub-blubnoises does not. But thisweakness eflectsinsteada breakdownn the sharedbackground ssumptions equiredorcontextual xplanationo gethold.Eitherwe find t difficult o conjurehex-ing of anS to which heglub-blub oisescontribute, r we find t difficultto conjure uchan Swhosew-ing s important, ignificant, r relevant.Theidea that theheart s formakingglub-blubnoisesonlyseemsabsurduntilone is ableto conjure oftenwith somecontortion)a suitablemechanisticcontext or thoseglub-blubnoises(likea diagnosticmechanism rperhapsthe mechanisms f fetalauditorydevelopment;ee AmundsonandLauder1994).Thisperspectivaleature houldnot besurprising;t is notunique ocontextualcausal/mechanicalxplanations,but can also be foundin con-stitutiveexplanations as emphasizedby Kauffman1971)andetiologicalexplanationsasarguedbyvan Fraassen1980).Walsh andAriew(1996)proposea taxonomyof sensesof "function"based on the different"causal,""etiological," nd"teleological" xplan-atoryuses to whichthat notionis put. The firsttwo of these(causalandetiological)correspond, espectively,o constitutive ndetiological xpla-nations.The last, teleologicalexplanations,"explain he prevalence nd(or)persistence f traittypesby citingtheircausalcontributiono averagefitnessof individuals"513).Like Bechtel 1986),WalshandAriewdo notrecognizethe possibilityof a contextualvarietyof explanation hat isbroader han (thoughinclusiveof) teleologicalexplanation.What is re-quireds a senseof "function," nd of causal/mechanicalxplanation,hatis couchedin termsof the contributionof an item to some higher-levelmechanism,rrespective f thecontribution f that mechanismo fitness,adaptation,or the good life.Walsh and Ariewdo not recognize he pos-sibilityof contextualexplanation,and I think this is becausethey havecholesterol n heartattacksor a heartattack)andin historicalresearch e.g., explainingthe role of Vesalius' De Fabrica n Harvey'sdiscoveryof the circulationof the blood).More importantlyfor the biological sciences,contextual explanationsappearin de-scriptionsof constitutivemechanismssuch as the mechanismof the kidney'sfiltrationof the blood or the mechanismof theheart'scontraction.These contextualexplanationscite factorsat later stages of the mechanism,but they do not cite them as productiveof X's b-ing.

71


21/23

CARL F. CRAVERnot paidsufficientattention o the importancehemultilevel tructure fmechanistic heoriesand causal/mechanicalxplanations.Once this uni-level bias is corrected, he place for contextualexplanation s apparentenough.Hardcastle 1999)has takensome initialstepstowardsworkingout apragmatics f functional xplanationhat fitsnicelywiththe view of con-textualexplanationhatI have beendeveloping.Shecorrectly mphasizesthat the choiceof a contextualmechanism r "goalstate," or thehighestlevel mechanismdependson accepted heories, he research ommunity,the background ontextof the research,and the biases of individual ci-entists(1999,39).But Hardcastle riticizesCumminson thegrounds hat(i) hisanalysis s restricted o "howa systemactuallybehaves"and hence"misdescribeshe functionsof malformedor brokenthings"and(ii) hisanalysis"overlooksnterspeciesariation" inceevenminordifferencesncontaining ystemsentaildifferencesn functioneven if thepartshave thesamerole(described t somedegreeof abstraction).Hardcastle 999,36-37) Mechanistic ole functionsmay seem to be especiallyvulnerable osuchcriticisms, ndso I closeby suggesting omewaysaround hem thatare,in theend,consistentwith Hardcastle's ragmaticproject.Thefirstobjection an be metbynotingthattheascription f a functionto a malformedor brokenpartis derivativeupon a descriptionof howthat type of part (X) fits into a type of higher-levelmechanism S). Themalformed nd brokenpartcanbe identified s an X by thetypicalprop-ertiesandactivitiesof Xs (withinwhichtheremaybe considerable aria-tion evenamongproperly ormedparts).Parts can be identifiedby, forexample, heirsize, shape,location,composition,and development, ndby their diverseproperties ndactivities.Malformedpartswill not shareall of theseproperties, ndif theyshare oo fewthey maybecomeunrec-ognizableas Xs, but this is most oftennot the case. (Moreextensivear-gumentsof this sort can be found in Amundsonand Lauder1994).Themechanistic ole of the brokenpart only appearsagainstthe fixed back-dropof sharedassumptions bouta typeof mechanismwithinwhichpartsof this type generally or preferably)make importantcontributions.Abrokenkidney,for example,can still be identifiedby its positionin thetorso, its connectionswith the renalvein andartery, ts being composedof nephra,and its characteristic hape, color, and size. The kidney'smechanistic oleis thenidentified gainst he fixedbackdropof a descrip-tion of thewaythecirculatory ystemgenerallyworks,or thewaythat itpreferablyworks,or thewaythat it works n whatever normalorpatho-logical)mechanismhat we seek to understand.Hardcastle's econdobjection imilarlypointsto the needforcloseat-tention to the varyingdegreesof abstraction ndgenerality f biologicalgeneralizationsseee.g.,Schaffner 993;Darden1996)andto thefuzziness

72


22/23

ROLE FUNCTIONS, MECHANISMS, AND HIERARCHYof biological kinds (see, e.g., Waters 1998). Even slight differences nmechanisticcontext entail differentmechanisticrole functions.But theneed to keeptrack of such differencess not uniqueto contextualexpla-nations;the need also arises whenwe ask whether wo events have thesameetiologicalexplanationor whentwo eventshavethe sameconstitu-tive explanation.Judgements f "sameness"n these casesdependuponan agreed-uponoleranceof diversity mong okenswithin ypes.For thisreason, talk of close or distantanalogy,both among mechanismsandamongmechanistic oles, s moreappropriatehantalkof "sameness"ordiscussing nterspecies omparisons.Both for roles and for mechanisms,closenessof analogydependsuponthesimilarity f theircomponents ndsimilarityn theiractive,spatial,andtemporalorganization.6. Conclusion. hediscoveryof an item'smechanistic ole is consideredfirst rate scientificachievement.This is because role ascriptionshelp tointegratehe levels nmultilevelhierarchies nd because earning n item'srole providesa kind of understanding f that item-a contextualexpla-nation of how that item(or typeof item)fits into a nexus of mechanisms.Describingan item'smechanistic ole is a perspectival ffair.Thisper-spectival ake on functionalascription houldbe a reminder hat whatwetake as functionaldescriptions an be tinged n a verydirectway by ourinterestsand biases (see e.g., Amundson2000;Gould 1981).Multilevelmechanismsare framedrelativeto a sharedtoppingoff point. Perhapsgroundingunctionaldescriptionnthe detailsof mechanistic rganizationwillprovideasetof criteriaorassessingheprecision ndaccuracy f func-tionalascriptions ndwillperhapshelpto guardagainstempiricallynade-quate,vague,oroverlyabstract unctional scriptions.nanageof obesitygenesand humorcenters,any clarityconcerningheprocessof assigningmechanistic oles shouldbe a welcomecontribution.Here,I havetriedtosketcha frameworkwithinwhich hisclarification an takeplace.

REFERENCESAllen, Colin, Mark Bekoff, and George V. Lauder (eds.) (1998), Nature's Purposes. Cam-bridge, MA: MIT Press.Amundson, Ron (2000), "Against Normal Function", Studies in the History and Philosophyof the Biological and Biomedical Sciences 31: 33-53.and George V. Lauder (1994), "Function without Purpose:The Uses of Causal RoleFunction in Evolutionary Biology", Biology and Philosophy 9: 443-469. Reprinted inAllen, Bekoff, and Lauder 1998, 335-370.Bechtel, William (1986), "Teleological Functional Analyses and the Hierarchical Organiza-tion of Nature", in N. Rescher (ed.), Teleology and Natural Science. Landham, MD:University Press of America, 2648.and Robert C. Richardson (1993), Discovering Complexity: Decompositionand Lo-calization as Strategies in Scientific Research. Princeton: Princeton University Press.Churchland, Patricia S. and Terrence Sejnowski (1992), The ComputationalBrain. Cam-bridge, MA: MIT Press.

73


23/23

74 CARL F. CRAVERCraver, Carl F. (1998), Neural Mechanisms:On the Structure,Function and DevelopmentofTheoriesnNeuroscience.h.D.Dissertation, ittsburgh: niversity f Pittsburgh.andLindleyDardenforthcoming), DiscoveringMechanismsnNeurobiology: heCaseof SpatialMemory",n P. Machamer,R. Grush,and P. McLaughlineds.),The-ory and Method in Neuroscience.Pittsburgh, PA: University of Pittsburgh Press.Cummins,Robert(1975),"FunctionalAnalysis,"Journal f Philosophy 2:741-765.Re-printednAllen,Bekoff,and Lauder1998,169-196.Pagereferences reto thereprint.(1983),"Analysis ndSubsumptionn the Behaviorismf Hull",Philosophy f Sci-ence 50: 96-111.Darden, Lindley (1991), TheoryChangein Science: Strategiesfrom Mendelian Genetics.NewYork:OxfordUniversityPress.(1996), "Generalizationsin Biology", Studiesin theHistoryandPhilosophy of Science27: 409-419.Glennan,StuartS. (1996),"Mechanisms nd the Natureof Causation",Erkenntnis4:49-71.(1997), "Capacities,Universality,and Singularity",Philosophyof Science 64:605-626.Gould,Stephen .(1981),TheMismeasurefMan.New York:W.W.NortonandCompany.Hardcastle,ValerieG. (1999),"Understandingunctions",n ValerieG. Hardcastleed.),WhereBiology Meets Psychology. Cambridge, MA: MIT Press, 27-43.Harvey, William ([1628] 1963), Movementof the Heart and Blood in Animals: An AnatomicalEssay.Translated y K.J. Franklin.London:Dent.Haugeland, John (1998), Having Thought:Essays in the Metaphysics of Mind. Cambridge,MA: HarvardUniversityPress.Hempel, Carl G. (1965), Aspects of ScientificExplanation. London: Collier Macmillan Pub-lishers.Hitchcock,Christopher1995),"Discussion: almonon ExplanatoryRelevance", hiloso-phy of Science 62: 304-20.(1996),"TheMechanist nd theSnail",Philosophicaltudies84: 91-105.Kauffman, tuartA. (1971),"Articulationf PartsExplanationnBiologyand the RationalSearch for Them", Boston Studies in the Philosophyof Science 8: 257-272.Lycan,WilliamG. (1990),"TheContinuityof Levelsof Nature", n W. G. Lycan(ed.),Mind and Cognition:A Reader. Oxford: Bradford, 77-96.Machamer, eterK.(1977),"Teleology nd SelectiveProcesses", . G.Colodny ed.),Logic,Laws,andLife,Vol.6. Pittburgh, A:University f Pittsburgh ress.LindleyDarden,and CarlF. Craver 2000),"Thinking boutMechanisms", hi-losophy of Science 67: 1-25.Rescher,Nicholas 1955),"Axioms or the PartRelation",Philosophicaltudies : 8-10.Salmon, Wesley C. (1984), Scientific Explanation and the Causal Structure of the World.Princeton: rincetonUniversityPress.(1997),"Causality ndExplanation: Reply o TwoCritiques", hilosophy f Sci-ence.64: 461-477.Schaffner, Kenneth (1993), Discovery and Explanation in Biology and Medicine. Chicago:University f ChicagoPress.Simon, Herbert A. (1969), The Sciences of the Artificial. Cambridge:MIT Press.vanFraassen,Bas C. (1980),TheScientificmage.Oxford:Clarendon ress.Walsh,DenisM. and AndreAriew 1996),"ATaxonomy f Functions",Canadian ournalof Philosophy26: 493-514.Waters,Ken C. (1998),"CausalRegularitiesn the BiologicalWorldof ContingentGener-alizations", Biology and Philosophy 13: 5-36.Wimsatt,William(1974), "Complexityand Organization",n KennethSchaffnerand

RobertS. Cohen eds.),PSA 1972,67-86.(1986),"Formsof Aggregativity",n AlanDonagan,AnthonyN. Perovich, r.,andMichael V. Wedin (eds.), Human Nature and Natural Knowledge.Dordrecht: Reidel,259-291.(1997),"Aggregativity:eductiveHeuristicsorFindingEmergence",hilosophyfScience64 (Proceedings):372-S384.

role functions

Documents