Page	1	of	15 forthcoming in Vassilios Karakostas and Dennis Dieks (eds.) (2013), "Recent Progress in Philosophy	of	Science:	Perspectives	and	Foundational	Problems",	The	Third	European	Philosophy of	Science	Association	Proceedings,	Dordrecht:	Springer. Mechanistic	explanation:	asymmetry	lost Samuel	Schindler* Abstract In a recent book and an article, Carl Craver construes the relations between different	levels	of	a	mechanism,	which	he	also	refers	to	as	constitutive	relations, in	terms	of	mutual	manipulability	(MM).	Interpreted	metaphysically,	MM	implies that inter‐level relations are symmetrical. MM thus violates one of the main desiderata of scientific explanation, namely explanatory asymmetry. Parts of Craver's writings suggest a metaphysical interpretation of MM, and Craver explicitly commits to constitutive relationships being symmetrical. The paper furthermore explores the option of interpreting MM epistemologically, as a means for individuating	mechanisms. It is argued that	MM then is redundant. MM	should	therefore	better	be	abandoned. 1 Introduction Ever since	Machamer,	Darden, and	Craver (2000)'s landmark article "Thinking about mechanisms", mechanistic explanations-thought to be the most pervasive	kinds	of	explanation	in	the	biological	sciences-have	become	a	major research topic in the philosophy of science. In a nutshell, Machamer et al. characterize mechanisms as being "composed of both entities (with their properties)	and	activities.	Activities	are	the	producers	of	change.	Entities	are	the things that engage in activities" (3). To	provide a	mechanistic explanation	of a phenomenon, then is "to	explain	how it	was	produced" by a	mechanism (ibid.). The production of the phenomenon in question by a mechanism, call it MPP (mechanistic production of the explanandum phenomenon), is thus absolutely central to the mechanistic conception of explanation. Although not explicitly *	Centre	for	Science	Studies,	Department	of	Physics	and	Astronomy,	Aarhus	University, Munkegade	120,	Building	1520,	8000	Aarhus,	Denmark,	samuel.schindler@ivs.au.dk. Page	2	of	15 highlighted	by	Machamer	et	al.,	MPP	thus	ensures	that	the	mechanistic	account of explanation captures one of the most important desiderata on accounts of explanation: explanatory asymmetry. Mechanisms explain phenomena, but phenomena do not explain mechanism, because mechanisms produce phenomena and not vice versa. The direction of explanation thus follows the direction of a mechanism's production of the relevant phenomenon. This assumption is in fact analogous to an assumption made by large parts of the philosophical literature on causation, perhaps most explicitly put by Salmon (1998,	129):	"The	asymmetry	of	explanation	is	inherited	from	the	asymmetry	of causation"	(see	also	Strevens	2008b,	24f.	and	76‐7).	And	indeed,	although	mostly concerned with the descriptive project of drawing to the attention of philosophers the importance of mechanistic explanations, Machamer et al. do express	broad	and	general	sympathy	with	a	causal	process	theory	for	MPP	in	the tradition of Salmon's (1984) early work on causation. Process theories of causation, however, have	widely been acknowledged to fail on various counts (Hitchcock	1995). In his recent book (Craver 2007) and an article (Craver and Bechtel 2006),	Craver	offers important	refinements	of the	original	mechanistic	account by	Machamer et al. Amongst other things, Craver proposes to understand the relation	between the	mechanism	and the	explanandum	phenomenon (i.e.,	MPP relation) in terms of "mutual manipulability", which, by a reviewer, has been judged	"one	of	the	main	achievements	of	the	book"	(Levy	2009,	141). It	will	be the	purpose	of	this	paper,	to	assess	this	aspect	of	Craver's	account. This is how I proceed. In Section 2 I introduce Craver's notion of mutual manipulability	(MM)	as	an	explication	of	MPP.	I	argue	that	Craver's	explication	of MPP strips the mechanistic account of explanation of its ability to capture explanatory asymmetry. In Section 3 I explore	ways in	which this undesirable consequence	might	be	avoided.	One	option	I	highlight	is	the	interpretation	of	MM as	a	purely	epistemological	criterion	for	identifying	MPP's.	As	I	argue	in	Section 4, however, there is clear textual evidence that Craver intends MM as an explication of the	meaning of MPP, which I argue is a genuinely	metaphysical project. Regardless, I show in Section 5 that, if interpreted in pure epistemological terms, MM becomes redundant. In Section 6 I conclude this Page	3	of	15 paper by recommending the abandonment of MM and by pointing to one characteristic of	mechanisms that the proponents of the	mechanistic approach might want to focus on in order to justify the need for special accounts of mechanistic	explanations. 2 Explanatory	asymmetry	lost? Just like Machamer at al. (2000), Craver (2007, 6‐7) defines mechanisms as "entities and activities organized such that they exhibit the explanandum phenomenon". Craver's (and Machamer et al.'s) standard example for a mechanistic explanation is the explanation of the neuronal action potential, which "is explained by reference to component parts of the action potential mechanism",	whereby examples for component entities are ions, ion channels, protein chains, etc. and examples for component activities are diffusion processes,	and	changes	in	confirmation	(121‐2). Craver	distinguishes	between	a	lower	and	an	upper	level	in	mechanisms (6‐7).	At	the	lower	level	he	locates	the	entities	X	and	their	properties	or	activities , and the 'upper' level is constituted	by the	phenomenon to	be	explained.	The 'mechanism as a whole', i.e., X, , and the explanandum phenomenon, Craver denotes as	S. Furthermore	he treats the	phenomenon to	be explained	as	being equivalent to	S's	activity	.	MPP	then, in	Craver's terminology, is	S's	‐ing	(i.e. the explanandum phenomenon) being "exhibited" or "produced" (Craver uses both terms) by the activities of the mechanism's components (X's ‐ing). Furthermore,	even	though	of	minor importance in	the following, for	Craver	(as for Machamer et al.) mechanisms often consist of multiple levels. That is, the upper level	of	one	mechanism	may	be	a	component	of	a lower level	of	another mechanism,	and	so	on.	More	importantly,	Craver	sharply	distinguishes	between intra‐level and inter‐level relations (Craver and Bechtel 2006; Craver 2007). Whereas intra‐level relations are causal relations, inter‐level relations are not; they are so‐called constitutive relations. Constitutive relations-in contrast to causal	relations-are	symmetric,	synchronous,	and	part‐whole	relations	(153‐4). Although	not	made	very	explicit	by	Craver,	constitutive	relationships	are	meant to	specify	MPP,	as	we	shall	see	in	the	following. Page	4	of	15 In order to elucidate inter‐level relationships in mechanisms, Craver (2007),	following	Woodward	(2003),	adopts	the	notion	of	an	ideal	intervention: "an	ideal	intervention	I	on		with	respect	to		is	a	change	in	the	value	of		that changes	, if at all, only via the change in	" (154). Interventions need	not be performable	by	humans,	nor	need	they	be	physically	possible.	All	that	is	required is that they	be logically	possible (see	Woodward	2003,	127ff.).	Craver (2007)'s explication	of	inter‐level	relations	in	terms	of	ideal	interventions	consists	of	two parts,	which	together	form	his	mutual	manipulability	criterion	(MM): (CR1):	When		is	set	to	the	value	1	in	an	ideal	intervention,	then		takes	on	the value	of	f(1).	(155) (CR2):	When		is	set	to	the	value	1	in	an	ideal	intervention,	then		takes	on	the value	of	f(1).	(159) Apparently, both CR1 and CR2 have the structure of Woodwardian active counterfactuals, i.e., counterfactuals whose antecedents are "made true by interventions" (Woodward 2000, 199), which Woodward intends to pick out causal	relationships.	And	yet,	Craver	denies	that	neither	CR1	nor	CR2	do	so.	As mentioned above, the combination of CR1 and CR2 (i.e. MM) is supposed to individuate constitutive relations, which, according to Craver, are not causal relations. In	accordance	with the	convention in the	contemporary literature	on causation to refer to a causal relation between X and Y as X "being causally relevant" to Y (cf.	Woodward	2003, 39), Craver also refers to CR1 and	CR2 as criteria for constitutive relevance. More specifically, "one can change the explanandum phenomenon by intervening to change a component [of a mechanism]",	and	vice	versa,	"one	can	manipulate	the	component	by	intervening to	change	the	explanandum	phenomenon"	(153).	Craver	concludes	that many,	if	not	most,	causal	relationships	are	unidirectional.	In	contrast,	all constitutive	dependency	relationships	are	bidirectional	(153). And since inter‐level relationships are symmetrical relationships, they are therefore "only	uncomfortably viewed	as causal" (p. 153).	Another reason that Craver mentions for constitutive relationships being distinct from causal relationships,	which	I	will	not	be	concerned	with	in	much	detail	here, is	that in the	former,	"'s	taking	on	a	particular	value	is	not	temporally	prior	to	's	taking Page	5	of	15 on	its	value"	(151‐2),	in	other	words,	constitutive	relationships	are	'synchronic', whereas	causal	relationships	are	not.	However,	contrary	to	what	Craver	seems	to suggest,	a	relationship	being	symmetrical	does	not	imply	that	it	cannot	be	causal. In fact, the account of causation Craver explicitly allows for 'cyclic' causal relations	where	a	change	in	the	cause	variable	brings	about	a	change	in	the	effect variable	and	vice	versa	(Woodward	2003,	396).	Furthermore,	as	Leuridan	(2012, fn. 27 and	29) points out,	many relations of interest in neurobiology (Craver's subject)	are	causal	feedback	loops,	i.e.,	symmetrical	causal	relationships. Craver's explicit commitment to inter‐level relationships being symmetrical relationships raises the following concern: if inter‐level relationships	really	are	symmetrical,	what	is it in	Craver's	account	that	ensures that the desideratum of explanatory asymmetry is respected? Recall, on the original mechanistic account by Machamer et al., explanatory asymmetry is respected, because mechanisms produce phenomena, but not vice versa. The direction of explanation simply follows the direction of production. So what happened	to	the	production	relationship	in	Craver's	account,	which	I	referred	to as	MPP,	and	which	Craver,	like	Machamer	et	al.,	characterizes	as	a	central	feature of	mechanisms?	At	one	point	in	his	book,	Craver	seems	to	say	that	MPP	is	to	be spelled out in terms of Woodwardian counterfactuals picking out causal relations: to say that one stage of a	mechanism is	productive of another (as I suggest in Machamer	et	al.	2000;	Craver	and	Darden	2001) is to	say, at least in	part, that one has the ability to	manipulate one item by intervening to change another. (93‐4;	added	emphasis) As mentioned above, however, for Craver inter‐level relations are not causal relations. So if the above quotation were to refer to MPP (i.e. an inter‐level relation) Craver would clearly contradict himself. So despite speaking of "production" in this	one	passage, suggesting reference to	MPP,	Craver is	better read	as	spelling	out	intra‐level	causal	relations	here.	This	still	leaves	us	with	the question	of	how	MPP	is	to	be	understood. Page	6	of	15 In personal communication,1 Craver is ready to give up on a 'literal' reading	of	MPP;	he	suggests	MPP	be	interpreted	metaphorically	instead.	That	is, whenever we say that the explanandum phenomenon is "produced" by the mechanism, what we should say more carefully is that the phenomenon is constituted or "made up" by the	mechanism, very	much in accordance with a constitutive understanding of inter‐level relations. This is in line	with the one taken	in	Craver	and	Bechtel	(2006): The	causal	claims,	when	made	explicit,	are	all	intra‐level.	But	we	continue	to	talk about	bottom	up	causal	relation	[from	one	to	another	level]	when	we	are	being quick	or	informal	as	long	as	we	understand	that	the	change	at	the	higher	level	is mediated	by,	or	explicable	in	terms	of,	a	mechanism	[and	its	constitutive relations].	(ibid.,	557) In	this	context	it	is	furthermore	interesting	to	note	that	the	idea	of	MPP,	i.e.,	the idea	of	the	mechanism's	producing	the	explanandum	phenomenon,	is	altogether absent from Craver and Bechtel's joint paper. But again, construing	MPP as a symmetrical relation (namely as CR) subjects the mechanistic account to the problem	of explanatory symmetry. If the relation	between the	mechanism	and the	phenomenon	is	symmetrical,	what	is	it	that	prevents	us	from	saying	that	the phenomenon	also	explains	the	mechanism?	In	response	to	this	question,	Craver (personal communication) is ready to embrace a deflationary "explanatory pluralism", implying that phenomena might as well explain mechanisms. But perhaps	there	are	more	appealing	options	for	Craver. 3 Explanatory	asymmetry	saved? There is a much simpler response to the problem of explanatory symmetry available	to	Craver.	He	could	point	out	that	MM	is	only	part	of	how	constitutive relationships are to be understood. Another important aspect of constitutive relationships is that they are part‐whole relationships (see above and Craver 2008,	153‐4).	Part‐whole	relationships	are	asymmetrical	relationships: if	X is	a part	of	S then	S	cannot	be	part	of	X.	Explanatory	asymmetry	between	X's	‐ing 1	Email	communication	with	the	author	of	this	paper	on	02‐22‐2012. Page	7	of	15 and	S's	‐ing	would	thus	be	secured.2	The	problem	with	this	response,	however, is	that	it	stands	in	outright	contradiction	with	Craver's	assertion	that	constitutive relationships are symmetrical: either constitutive relations are part‐whole relations	or	they	are	symmetrical.	Craver	cannot	have	it	both	ways.	Although	the former option seems much more plausible in the face of the problem of explanatory asymmetry, part‐whole relations are underdeveloped in Craver's account (see Leuridan 2012).	What is	more, Craver puts a lot of stress on the symmetry property of constitutive relations and characterizes it as one of the crucial	differences	between	constitutive	and	causal	relations	(Craver	and	Bechtel 2006;	Craver	2007).	At	any	rate,	there	appear	to	be	only	two	options	for	Craver: he	either	gives	up	on	the idea	that	constitutive	relations	are	symmetrical	or	he tries to save explanatory asymmetry whilst holding onto constitutive relationships	being	symmetrical	(and	giving	up	on	part‐hood).	The	former	option appears	much	more	plausible.	It	would	allow	Craver	to	embrace	the	asymmetry property of part‐whole relations and thereby explanatory asymmetry. But because	Craver	is	so	adamant	about	constitutive	relations	being	symmetrical,	let us	briefly	consider	the	latter	option,	before	exploring	the	former	option. Take	one	of	Woodward's	preferred	examples,	the	ideal	gas	law.	This	law relates	variables	of	pressure	(P),	the	volume	of	a	gas	(V)	and	temperature	(T)	in the formula PV=RT (R is the gas constant). Clearly this is a symmetrical relationship. We can intervene on P to change T, and conversely, we can intervene on T to change P (by holding fixed V, respectively). Now, assuming (with	Woodward)	that	the	ideal	gas	law	is	an	explanatory	generalization,	in	each of the above scenario, explanatory asymmetry is preserved despite the relationship being a symmetric relationship. In the one context T is the cause variable	and	P	the	effect	variable,	and	in	another	context,	P	is	the	cause	variable and	T	the	effect	variable.	In	the	first	scenario	a	change	in	T	explains	the	change	in P (but not vice versa), and in the second scenario P explains T (but not vice versa).3 There is of course no a priori reason why this insight could not be 2	Interestingly,	Craver	did	not	make	this	reply	in	the	abovementioned	email	correspondence.	I thank	an	anonymous	referee	for	making	me	consider	this	option. 3	This	was	suggested	to	me	by	Bert	Leuridan. Page	8	of	15 extrapolated to the context of mechanistic explanations. However this extrapolation is only of a limited sort. It extends only to intra‐level but not to inter‐level relationships in mechanisms. That is, on one particular level of a mechanism	it	might	make	sense	to	say,	as	in	the	above	example,	that	a	change	in X1's	1‐ing	explains	(in	a	minimal	sense)	a	change in	X2's	2‐ing,	and	vice	versa (just in	case,	of	course,	X1	and	X2	are	related	as	P	and	T	above).	Further, it	also makes	sense	(now	between	different	levels	of	mechanisms)	to	say	that	if	we	can intervene	on	's	to	change		(the	explanandum	phenomenon),	then	's	explain	. However,	crucially,	it	makes	little	or	no	sense	to	say	the	reverse,	namely	that	 explains , even if CR2	were satisfied. After all, the explanandum phenomenon cannot	explain	the	mechanism,	at	least	not	in	the	standard	sense	of	the	term.	It therefore	seems	that	there	is	no	straightforward	way	in	which	Craver	could	stick to	his	symmetry	thesis	and	save	explanatory	asymmetry. Again,	Craver	could	simply	give	up	on the	symmetry thesis. Indeed,	MM implies the symmetry	of inter‐level relations	only if it is to	be	understood	as a metaphysical	explication	of	inter‐level	relations,	i.e.,	as	specifying	the	meaning	of inter‐level relations. If read in epistemological terms, that is, as a criterion for identifying	inter‐level	relations	(in	contrast	to,	say,	mere	correlations	between	a mechanism and some phenomenon), MM has no implications for the directionality	of inter‐level relations.	At least	a	priori, there is	no	contradiction between	inter‐level	relations	being	asymmetrical	and	our	means	for	identifying them	being	applicable	in	both	directions	of	this	relation	(i.e.,	bottom‐up	and	top‐ down). An epistemological interpretation of MM would also allow Craver to reconcile MM with the asymmetry of part‐hood. Unfortunately for Craver, however, there is strong textual evidence that Craver aims for a	metaphysical explication	of	constitutive	relevance	in	terms	of	MM.	So	before	we	can	consider	a pure epistemological interpretation of MM, we need to consider that textual evidence	in	more	detail. 4 The	metaphysics	of	mechanisms First	and foremost,	as	pointed	out	above,	part	of	Craver's	motivation to	amend the	original	mechanistic	account	by	Machamer	et	al.	is	clearly	owed	to	the	wish Page	9	of	15 to explicate the meaning of MPP. Such endeavor is generally regarded as a genuinely metaphysical. Second, Craver's concession that MM implies the symmetry of inter‐level relations clearly presupposes that MM is interpreted metaphysically. Again, if MM were a mere epistemological criterion, nothing would follow for the direction of inter‐level relations. Furthermore, Craver makes	clear	that	he	wishes	to	provide	a	'normative'	account	that	can	"demarcate [mechanistic]	explanation	from	other	kinds	of	scientific	achievements",	and	that can "reveal criteria for assessing explanations", in other words it "should prescribe	norms	of	explanation"	(20;	original	emphasis).	All	this	he	could	not	do if	he	were	to	interpret	MM	epistemologically	rather	than	metaphysically.	One	e.g. cannot assess whether an explanation is a good explanation without having provided at least a partial answer to the question of what constitutes an explanation,	i.e.,	a	question	about	the	meaning	of	explanation. But again, there are indeed also a number of passages in Craver's book that	suggest	that	Craver	views	MM	as	an	epistemological	criterion.	Primarily,	this is	suggested	by	the	context	in	which	Craver	explicates	inter‐level	relations.	This context is formed by Craver's pointing to the various inter‐level experimental strategies that can be used, inter‐level bottom‐up and top‐down, to establish certain entities and activities as being part of a certain	mechanism. Verbatim, Craver	says: I	build	my	positive	account	[of	mechanisms]	by	considering	the	experimental strategies	that	neuroscientists	use	to	test	whether	a	given	entity,	activity, property,	or	organizational	feature	is	relevant	to	the	behavior	of	the	mechanism as	a	whole	[...]	(140;	added	emphasis).4 Furthermore,	Craver	appears	to	think	that	a	non‐reductive	analysis	of	causation (or	constitution)	based	on	active	counterfactuals	implies	that	the	analysis	is	non‐ metaphysical.	Explicitly,	he	says	in	his	summary	of	Woodward's	manipulationist account, which he then goes on to use to define constitutive relevance in mechanisms,	that 4	Couch	(2011,	fn.	6)	also	reports	that	Craver	explicitly	embraces	an	epistemological interpretation	of	MM	in	personal	communication. Page	10	of	15 I	do	not	discuss	here	whether	such	metaphysics	[of	causation]	is	required	or what	the	available	metaphysical	options	are.	Even	if	the	manipulationist	view does	not	identify	the	truth‐maker[s]	for	causal	claims,	it	is	nonetheless	an illuminating	analysis	of	the	causal	truths	themselves	[...]	(Craver	2007,	105‐7). This, however, is a misapprehension. The fact that a philosophical analysis is non‐reductive	does	not	imply	that	it	is	non‐metaphysical.	To	see	this	requires	a brief	excursion	into	Woodward's	account. Many philosophical analyses seek to reduce the concept of causation to another concept. Humeans, for instance, reduce causation to mere empirical regularities. David Lewis reduces causation to counterfactual dependence. Woodward's analysis of causation, in contrast, is decidedly non‐reductive. Woodward	defines	causal	relationships	as	generalizations	that	remain	invariant under	interventions.	As	Woodward	acknowledges,	the	notion	of	an	intervention is	itself	a	causal	notion.	In	a	sense,	Woodward's	account	is	thus	circular.	However it is not viciously circular, as	Woodward (2003, 20ff.) points out, because the causal relation that is being appealed to (I causing a change in X) is different from	the	causal	relation	that the	analysis	seeks	to illuminate	(namely,	X	causes Y).	Woodward	goes	on	to	show	that	such	a	non‐reductive	account	can	very	well be	insightful.	It	for	instance	delivers	markedly	different	verdicts	on	a	number	of important issues	(e.g.	action‐at‐a‐distance,	causation	by	prevention	or	absence) than	Salmon	(1984)'s	classical	causal	process	theory	of	causation	does. The crucial question now is of course: is a non‐reductive analysis of causation non‐metaphysical (as Craver would have it)? This is not an uncontroversial matter. In a slightly heated exchange with Strevens (2007, 2008a), Woodward (2008) explicitly denies that his own account is metaphysical.	However	Strevens (2008)	offers	a	number	of convincing reasons why	Woodward	might	in	fact	be	mistaken	about	the	aims	of	his	own	book.	First, Woodward	(2003),	throughout	his	book,	presents	his	analysis	as	a	superior	rival to Lewis's metaphysical analysis of causation. It would be hard to see why Woodward does that if the aim of his project were entirely different from Lewis's. Second, Woodward seeks to provide an account according to which causation is mind‐independent, in other words, an account of the nature of causation	which	is	independent	of	how	we	get	to	know	about	this	relation	(2003, 118ff.).	This	clearly	is	a	metaphysical	endeavor.	Third,	Woodward	states	that	"my Page	11	of	15 aim	is	to	provide	an	account	of	the	meaning	or	content	of	various	locutions,	such as	X	causes	Y"	and	that	"my	project	is	semantic	or	interpretive"	(38).	As	Strevens (2008)	points	out In	modern	times,	such	a	project	is	invariably	interpreted	as	aiming	to	provide truth	conditions	for	the	sentences	or	thoughts	in	question,	and	therefore	as aiming	to	specify	those	representations'	truthmakers.	It	may	look	like	semantics, but	it	is	also	a	kind	of	metaphysics	[...]	it	is	generally	agreed	that	a	word	with	an explicit	definition	has	as	its	extension	whatever	stuff	satisfies	that	definition.	If Woodward's	causal	semantics	is	a	truth‐conditional	semantics,	he	is	inevitably, unavoidably,	ineluctably	committed	to	producing	an	account	of	the	truthmakers for	causal	talk,	a	metaphysics	of	causal	facts,	whatever	his	protestations.	(184) Back	to	Craver.	If	Craver	wishes	to	spell	out	the	meaning	of	the	inter‐level relation	in	terms	of	constitution	then,	by	Strevens's	lights,	he	inextricably commits	himself	to	a	metaphysical	project.	Since	there	are	clear	signs	that	Craver does	wish	to	elucidate	the	meaning	of	inter‐level	relations	(see	above),5	his project,	contrary	to	what	he	says,	does	commit	him	to	a	metaphysics	of constitutive	relations.	And	since	Craver's	spelling	out	of	inter‐level	relations implies	inter‐level	relations	being	symmetrical,	i.e.,	a	consequence	which	he	is happy	to	embrace,	the	mechanistic	account	no	longer	captures	explanatory asymmetry.	So	let	us	now	finally	consider	the	possibility	of	interpreting	MM, contrary	to	much	of	what	(is	implied	by	what)	Craver	says	about	it,	as	a	merely epistemological	tool	for	individuating	constitutive	relationships. 5 Individuating	mechanism	boundaries The	main	epistemological	function	of	MM	I	alluded	to	briefly	above	is	this:	it concerns	the	delineation	of	the	boundaries	of	mechanisms	(Craver	2007,	141ff). In	other	words	MM	is	supposed	to	specify	(i)	which	entities	and	activities	are, and	which	ones	are	not,	part	of	the	lower	level	of	a	mechanism	with	respect	to	a particular	explanandum	phenomenon	(this	function	is	performed	by	CR2),	and, conversely,	(ii)	which	phenomenon	is	picked	out	by	a	particular	mechanism	(this function	is	performed	by	CR1).	Craver	gives	the	following	example	(152). 5	This	is	also	what	Craver	told	me	in	personal	communication. Page	12	of	15 The	cognitive	capacity	of	word‐stem	completion,	in	which	a	subject	is presented	with	a	list	of	words	and	afterwards	asked	to	complete	the	word	stems of	the	words	presented	previously,	is	affected	by	changing	the	heart	rate	of	the subject.	That	is,	if	one	were	to	change	the	heart	rate	of	the	subject	(by	e.g. torturing	the	subject),	the	subject's	capacity	to	complete	word	stems	would invariably	change.	According	to	CR1	alone,	however,	the	heart	rate	would erroneously	be	deemed	as	constitutively	relevant	for	the	phenomenon	of	word‐ stem	completion;	the	heart	rate	normally	would	not	be	considered	a	part	of	the mechanism	of	word	stem	completion,	or	so	Craver	reasons.	Here	is	where	CR2 comes	in.	Engaging	subjects	in	word‐stem	completion	will	not,	under	normal circumstances,	result	in	a	different	heart	rate.	Hence	CR2	is	not	satisfied	by	the example	and	the	heart	rate	is	therefore	not	to	be	deemed	part	of	the	mechanism of	word	stem	completion. It	is	questionable	whether	MM	really	fulfills	the	purpose	Craver	foresees for	it.	To	see	this,	note	that	the	persuasiveness	of	the	above	example	rests	on	the implicit	assumption	that	the	relevant	mechanism	for	the	capacity	of	word	stem completion	is	a	cognitive	mechanism.	Only	then	the	heart	rate	appears	irrelevant. But	not	in	all	contexts	need	this	be	so.	In	contexts	in	which	one	is	interested	in, for	instance,	the	capacity	of	completing	word	stems	as	compared	to	the	absence of	any	cognitive	capacity,	the	heart	rate	appears	to	be	indeed	a	part	of	the relevant	mechanism.	In	other	words,	whether	or	not	a	mechanism	(or	part thereof)	is	relevant	to	the	explanation	of	a	phenomenon	is	subject	to	pragmatic considerations.	In	fact	Craver	is	very	well	aware	of	this.	He	for	instance highlights	the	importance	of	contrast	classes	in	the	specification	of	the explanandum	phenomenon	(2007,	202ff.).	And Once	the	explanandum	phenomenon	has	been	specified	[...]	it	is	then	an	objective matter	whether	or	not	it	is	possible	to	make	a	difference	to	that	effect	by manipulating	certain	antecedent	variables	and	which	manipulations	of	that variable	in	fact	make	a	difference.	My	account	is	thus	no	more	pragmatic	than any	view	of	causation	or	explanation	would	have	to	be.	(204) In	a	more	recent	publication,	Craver	(2009)	states	even	more	explicitly	that [...]	the	spatial	and	causal	boundaries	of	mechanisms	depend	on	the epistemologically	prior	delineation	of	relevance	boundaries.	But	relevance	to what?	The	answer	is:	relevance	to	the	phenomena	that	we	seek	to	predict, Page	13	of	15 explain,	and	control.	Within	the	boundaries	of	a	mechanism	are	all	and	only	the entities,	activities,	and	organizational	features	relevant	to	the	phenomenon selected	as	our	explanatory,	predictive,	or	instrumental	focus.	(591;	added emphasis) The	epistemological	function	CR2	is	supposed	to	perform,	namely	the	picking	out of	a	particular	mechanism,	given	a	particular	explanandum	phenomenon,	is therefore	not	necessary.	It	is	plausibly	carried	out	by	pragmatic	considerations: we	choose	a	particular	phenomenon	we	want	to	explain,	predict,	etc.	and	then we	ask,	by	reverse	engineering,	as	it	were,	what	causes	are	responsible	for	the phenomenon	in	question.	MM	as	an	epistemological	criterion	in	Craver's	account looks	therefore	redundant. 6 Conclusion When	interpreted	metaphysically,	MM,	in	violation	of	a	central	desideratum	of explanation,	implies	explanatory	symmetry	and	is	inconsistent	with	the	part‐ hood	characterization	of	constitutive	relevance	(as	I	argued	in	Section	2	and Section	3).	On	the	other	hand,	when	interpreted	epistemologically,	MM	does	not give	us	any	extra	purchase	on	the	individuation	of	mechanisms	(as	argued	in	the last	section).	I	therefore	believe	that	MM	is	better	to	be	abandoned	altogether.	I thus	disagree	with	Leuridan	(2012)	who	concludes	his	detailed	discussion	of	MM by	suggesting	that	mechanistic	inter‐level	relations	be	interpreted,	contrary	to Craver,	as	relations	of	mutual	causation.	But	such	a	proposal	is	of	course	just	as much	subject	to	my	criticism	of	explanatory	asymmetry	being	lost	(see	also Section	3).	Rather	I	think	that	inter‐level	relations,	i.e.	the	productive	relation MPP,	are	perhaps	best	understood	in	terms	of	unidirectional	Woodwardian counterfactuals	picking	out	causal	relations,	without	invoking	top‐down counterfactuals	(contra	Craver).	But	of	course,	there	would	then	be	no	need	for	a specific	mechanistic	account	of	explanation	(seeWoodward	2002).	So	is	there?	A crucial	feature	of	mechanisms	is	the	organization	of	entities	and	their	spatio‐ temporally	concerted	interaction	that	produces	a	phenomenon.	In	order	to accommodate	this	feature	of	mechanisms,	Woodward	(2011)	suggests	that counterfactuals	describing	causal	relations	in	mechanisms	possess „characteristic	spatio‐temporal	signatures"	(427).	I	take	it	that	this	suggestion translates	into	mechanisms	being	specifiable	in	terms	of	conjunctions	of	active Page	14	of	15 counterfactuals	with	complex	antecedents	of	the	following	form: <If	an appropriate	intervention	I	had	changed	the	value	of	an	"entity	variable"	X	at	time t1	and,	if	an	appropriate	intervention	I	had	changed	the	value	of	an	"entity variable"	Y	at	time	t2,	and	...	etc.,	then	the	explanandum	phenomenon	would have	been	produced	by	the	mechanism	comprising	entities	X,	Y,	etc.>.	Since	it seems	awkward	to	say	that	variables	have	a	"location",	spatial	information	might be	a	little	harder	to	accommodate	than	this.	At	any	rate,	barring	concerns	about the	modularity	assumption	that	Woodward	makes	(Cartwright	2002),	such	an amendment	of	Woodwardian	counterfactuals	to	accommodate	the	genuine mechanistic	feature	of	organization	and	concerted	interaction	of	mechanism‐ components	seems	to	be	unproblematic	in	principle. Acknowledgements This article has profited significantly from discussions with Bert Leuridan, Kareem Khalifa, and Petri Ylikoski. Marcel Weber, Alexander Reutlinger, and an anonymous referee	of this journal provided	helpful comments on earlier	drafts of this paper. Carl Craver	kindly	clarified	his	views	for	me	in	personal	communication.	I'm	grateful	to	all	of these individuals. I furthermore want to thank the audience at the EPSA 2011 conference	in	Athens	(Greece)	for	feedback. 7 References Cartwright	N	(2002)	Against	modularity,	the	causal	Markov	condition,	and	any link	between	the	two:	Comments	on	Hausman	and	Woodward.	The	British Journal	for	the	Philosophy	of	Science	53	(3):411‐453. Couch	MB	(2011)	Mechanisms	and	constitutive	relevance.	Synthese	183	(3):375‐ 388. Craver	CF	(2007)	Explaining	the	brain	:	mechanisms	and	the	mosaic	unity	of neuroscience.	Oxford:	Oxford	University	Press. -	-	-	(2009)	Mechanisms	and	natural	kinds.	Philosophical	Psychology	22 (5):575‐594. Craver	CF,	W	Bechtel	(2006)	Top‐down	Causation	Without	Top‐down	Causes. Biology	&	Philosophy	22	(4):547‐563. Hitchcock	CR	(1995)	Salmon	on	explanatory	relevance.	Philosophy	of	Science	62 (2):304‐320. Leuridan	B	(2012)	Three	problems	for	the	mutual	manipulability	account	of constitutive	relevance	in	mechanisms.	The	British	Journal	for	the Philosophy	of	Science	63	(2):399‐427. Levy	A	(2009)	Carl	F.	Craver,	Explaining	what?	Review	of	explaining	the	brain: mechanisms	and	the	mosaic	unity	of	neuroscience.	Biology	and	Philosophy 24	(1):137‐145. Machamer	P,	L	Darden,	CF	Craver	(2000)	Thinking	about	mechanisms. Philosophy	of	Science	67	(1):1‐25. Page	15	of	15 Salmon	W	(1984)	Scientific	explanation	and	the	causal	structure	of	the	world. Princeton,	N.J.:	Princeton	University	Press. -	-	-	(1998)	Causality	and	explanation.	New	York:	Oxford	University	Press. Strevens	M	(2007)	Review	of	Woodward,	Making	Things	Happen.	Philosophy	and Phenomenological	Research	74	(1):233‐249. -	-	-	(2008a)	Comments	on	Woodward,	Making	Things	Happen.	Philosophy and	Phenomenological	Research	77	(1):171‐192. -	-	-	(2008b)	Depth	:	an	account	of	scientific	explanation.	Cambridge,	Mass.: Harvard	University	Press. Woodward	J	(2003)	Making	things	happen:	A	theory	of	causal	explanation:	Oxford University	Press,	USA. -	-	-	(2008)	Response	to	Strevens.	Philosophy	and	Phenomenological Research	77	(1):193‐212. -	-	-	(2011)	Mechanisms	revisited.	Synthese:1‐19. Woodward	J.	(2002)	What	is	a	mechanism?	A	counterfactual	account.	Philosophy of	Science	69	(3):366‐377.