Mechanisms,	Then	and	Now: From	Metaphysics	to	Practice Stathis	Psillos	&	Stavros	Ioannidis Dept	of	History	and	Philosophy	of	Science University	of	Athens University	Campus 15771	Athens, Greece psillos@phs.uoa.gr sioannidis@phs.uoa.gr Abstract For	many old and new	mechanists,	Mechanism is both a	metaphysical position and	a thesis	about scientific	methodology. In this	paper	we	discuss the relation between	the	metaphysics	of	mechanisms	and	the	role	of	mechanical	explanation in	the	practice	of	science,	by	presenting	and	comparing	the	key	tenets	of	Old	and New Mechanism. First, by focusing on the case of gravity, we show how the metaphysics of Old Mechanism constrained scientific explanation, and discuss Newton's critique of Old Mechanism. Second, we examine the current mechanistic metaphysics, arguing that it is not warranted by the use of the concept of mechanism in scientific practice, and	motivate a thin conception of mechanism	(the	truly	minimal	view),	according	to	which	mechanisms	are	causal pathways	for	a	certain	effect	or	phenomenon.	Finally,	we	draw	analogies	between Newton's critique of Old Mechanism and our thesis that the metaphysical commitments of New Mechanism are not necessary in order to illuminate scientific	practice. 1.	Introduction The	mechanical	worldview	of	the	seventeenth	century	was	both	a	metaphysical thesis	and	a	scientific	theory.	It	was	a	metaphysical	thesis	insofar	as	it	was committed	to	a	reductionist	account	of	all	worldly	phenomena	to	configurations of	matter	in	motion	subject	to	laws.	In	particular,	it	was	committed	to	the	view that	all	macroscopic	phenomena	are	caused	by,	and	hence	are	accounted	for,	the interactions	of	invisible	microscopic	material	corpuscles.	Margaret	Wilson captured	this	view	succinctly: The	mechanism	characteristic	of	the	new	science	of	the	seventeenth	century	may	be briefly	characterised	as	follows:	Mechanists	held	that	all	macroscopic	bodily	phenomena result	from	the	motions	and	impacts	of	submicroscopic	particles,	or	corpuscles,	each	of which	can	be	fully	characterised	in	terms	of	a	strictly	limited	range	of	(primary) properties:	size,	shape,	motion	and,	perhaps,	solidity	and	impenetrability	(1999,	xiii-xiv). But	this	metaphysical	thesis	did,	at	the	same	time,	license	a	scientific theory	of	the	world,	viz.,	a	certain	conception	of	scientific	explanation	and	of theory-construction.	To	offer	a	scientific	explanation	of	a	worldly	phenomenon	X was	to	provide	a	configuration	Y	of	matter	in	motion,	subject	to	laws,	such	that	Y could	cause	X.	A	mechanical	explanation	then	was	(a	species	of)	causal The final version of this article has been published in: Falkenburg B., Schiemann G. (eds) Mechanistic Explanations in Physics and Beyond. European Studies in Philosophy of Science, vol 11. Springer, Cham, pp 11-31. Published version: https://link.springer.com/chapter/10.1007/978-3-030-10707-9_2. 2 explanation:	to	explain	that	Y	causes	X	was	tantamount	to	constructing	a mechanical	model	of	how	Y	brings	about	X.	The	model	was	mechanical	insofar	as it	was	based	on	resources	licensed	by	the	metaphysical	worldview,	viz.,	action	of particles	by	contact	in	virtue	of	their	primary	qualities	and	subject	to	laws	of motion.1 Nearly	four	centuries	later,	the	mechanical	worldview	has	become prominent	again	within	philosophy	of	science.	It's	become	known	as	'the	New Mechanical	Philosophy'	and	has	similar	aspirations	as	the	old	one.	New Mechanism,	as	Stuart	Glennan	puts	it, says	of	nature	that	most	or	all	the	phenomena	found	in	nature	depend	on	mechanisms- collections	of	entities	whose	activities	and	interactions,	suitably	organized,	are responsible	for	these	phenomena.	It	says	of	science	that	its	chief	business	is	the construction	of	models	that	describe,	predict,	and	explain	these	mechanism-dependent phenomena	(2017,	1). So,	New	Mechanism	too	is	both	a	view	about	science	and	about	the	metaphysics of	nature.	And	yet,	in	New	Mechanism	the	primary	focus	has	been	on	scientific practice,	and	in	particular	on	the	use	of	mechanisms	in	discovery,	reasoning	and representation	(cf.	Glennan	2017,	12).	The	focus	on	the	metaphysics	of mechanisms	has	emerged	as	an	attempt	to	draw	conclusions	about	the	ontic signature	of	the	world	starting	from	the	concept	of	mechanism	as	it	is	used	in	the sciences.	According	to	Glennan,	as	the	research	into	the	use	of	mechanism	in science	developed,	"it	has	been	clear	to	many	participants	in	the	discussion	that metaphysical	questions	are	unavoidable"	(2017,	12).	It	is	fair	to	say	that	New Mechanism	aims	to	ground	the	metaphysics	of	mechanisms	on	the	practice	of mechanical	explanation	in	the	sciences. The	chief	aim	of	this	paper	is	to	discuss	the	relation	between	the metaphysics	of	mechanisms	and	the	role	of	mechanical	explanation	in	the practice	of	science.	It	will	do	that	by	presenting	and	comparing	the	key	tenets	of Old	and	New	Mechanism.	Section	2	will	be	devoted	to	the	seventeenth	century Mechanism.	It	will	present	the	basic	contours	of	the	mechanistic	metaphysics and	show	how	it	constrained	scientific	explanation,	focusing	on	the	case	of gravity.	In	this	section,	we	will	also	discuss	Isaac	Newton's	critique	of mechanism	and	highlight	the	significance	of	his	key	thought,	viz.,	that	causal explanation	should	identify	the	causes	and	the	laws	that	govern	their	action, irrespective	of	whether	or	not	these	causes	can	be	taken	to	satisfy	certain (mostly	metaphysically	driven)	constraints,	such	as	being	modelled	in	terms	of configurations	of	matter	in	motion.	Section	3	will	focus	on	the	current mechanistic	metaphysics	and	show	that	it	is	not	warranted	by	the	use	of	the concept	of	mechanism	in	scientific	practice.	It	will	show	that	the	currently popular	minimal	general	characterisation	of	a	mechanism	is	still	metaphysically inflated	in	various	ways	and	will	motivate	a	thin	conception	of	mechanism, which	is	not	committed	to	any	views	about	the	ontological	signature	of mechanism.	This	thin	conception-what	we	call	'truly	minimal	mechanism'- 1 For	the	purposes	of	this	paper,	we	ignore	issues	of	mind-body	causation	and	we	focus	on	bodybody	causation.	We	also	ignore	divisions	among	mechanists	concerning	the	nature	of	corpuscles, the	existence	of	vacuum	etc. 3 takes	it	that	mechanisms	are	causal	pathways	for	a	certain	effect	or phenomenon.	Finally,	in	section	4	we	will	draw	analogies	between	Newton's critique	of	Old	Mechanism	and	our	critique	of	New	Mechanism.	Briefly	put,	the point	will	be	that	causal	explanation	in	the	sciences	is	legitimate	even	if	we bracket	the	issue	of	"what	mechanisms	or	causes	are	as	things	in	the	world" (Glennan	2017,	12);	or	the	issue	of	what	activities	are	and	how	they	are	related to	powers	and	the	like.	The	metaphysical	commitments	of	New	Mechanism	are not	necessary	in	order	to	illuminate	scientific	practice. 2.	Old	Mechanism:	from	Metaphysics	to	Practice A	rather	typical	example	of	the	interplay	between	the	metaphysical	worldview and	the	scientific	conception	of	the	world	in	the	seventeenth	century	was	the attempted	mechanical	explanation	of	gravity. 2.1	Mechanical	Models	of	Gravity Let	us	start	with	René	Descartes.	The	central	aim	of	the	3rd	and	4th	part	of Descartes's	Principia	Philosophiae,	published	in	1644,	was	the	construction	of	an account	of	natural	phenomena.	In	Cartesian	physics,	the	possible	empirical models	of	the	world	are	restricted	from	above	by	a	priori	principles	which capture	the	fundamental	laws	of	motion	and	from	below	by	experience.	Between these	two	levels	there	are	various	theoretical	hypotheses,	which	constitute	the proper	empirical	subject-matter	of	science.	These	are	mechanical	hypotheses; they	refer	to	configurations	of	matter	in	motion.	As	Descartes	explains	in	(III,	46) of	the	Principia,	since	it	is	a	priori	possible	that	there	are	countless configurations	of	matter	in	motion	that	can	underlie	the	various	natural phenomena,	"unaided	reason"	is	not	able	to	figure	out	the	right	configuration	of matter	in	motion.	Mechanical	hypotheses	are	necessary	but	experience	should be	appealed	to,	in	order	to	pick	out	the	correct	one: [W]e	are	now	at	liberty	to	assume	anything	we	please	[about	the	mechanical configuration],	provided	that	everything	we	shall	deduce	from	it	is	{entirely}	in conformity	with	experience	(III,	46;	1982,	106). These	mechanical	hypotheses	aim	to	capture	the	putative	causes	of	the phenomena	under	investigation	(III,	47).	Hence,	they	are	explanatory	of	the phenomena.	Causal	explanation-that	is,	mechanical	explanation-proceeds	via decomposition.	It	is	a	commitment	of	the	mechanical	philosophy	that	the behaviour	of	observable	bodies	should	be	accounted	for	on	the	basis	of	the interactions	among	their	constituent	parts	and	particles;	hence,	on	the	basis	of unobservable	entities.	In	(IV,	201;	1982,	283),	Descartes	states	that	sensible bodies	are	composed	of	insensible	particles.	But	to	get	to	know	these	particles and	their	properties	a	bridge	principle	is	necessary;	that	is,	a	principle	that connects	the	micro-constituents	with	the	macro-bodies.	According	to	this principle,	the	properties	of	the	minute	particles	should	be	modelled	on	the properties	of	macro-bodies.	Here	is	how	Descartes	put	it: Nor	do	I	think	that	anyone	who	is	using	his	reason	will	be	prepared	to	deny	that	it	is	far better	to	judge	of	things	which	occur	in	tiny	bodies	(which	escape	our	senses	solely because	of	their	smallness)	on	the	model	of	those	which	our	senses	perceive	occurring	in large	bodies,	than	it	is	to	devise	I	know	not	what	new	things,	having	no	similarity	with those	things	which	are	observed,	in	order	to	give	an	account	of	those	things	[in	tiny 4 bodies].	{E.g.,	prime	matter,	substantial	forms,	and	all	that	great	array	of	qualities	which many	are	accustomed	to	assuming;	each	of	which	is	more	difficult	to	know	than	the	things men	claim	to	explain	by	their	means}	(IV,	201;	1982,	284). In	this	passage	Descartes	does	two	things.	On	the	one	hand,	he	advances	a continuity	thesis:	it	is	simpler	and	consonant	with	what	our	senses	reveal	to	us	to assume	that	the	properties	of	micro-objects	are	the	same	as	the	properties	of macro-objects.	This	continuity	thesis	is	primarily	methodological.	It	licenses certain	kinds	of	explanations:	those	that	endow	matter	in	general,	and	hence	the unobservable	parts	of	matter,	with	the	properties	of	the	perceived	bits	of	matter. It	therefore	licenses	as	explanatory	certain	kinds	of	unobservable	configurations of	matter;	viz.,	those	that	resemble	perceived	configurations	of	matter.	On	the other	hand,	however,	Descartes	circumscribes	mechanical	explanation	by	noting what	it	excludes;	that	is	by	specifying	what	does	not	count	as	a	proper	scientific explanation.	He's	explicit	that	the	Aristotelian-scholastic	metaphysics	of substantial	forms	and	powerful	qualities	is	precisely	what	is	abandoned	as explanatory	by	the	mechanical	philosophy2. All	this	was	followed	in	the	investigation	of	the	mechanism	of	gravity	and the	(in)famous	vortex	hypothesis	according	to	which	the	planets	are	carried	by vortices	around	the	sun.	A	vortex	is	a	specific	configuration	of	matter	in motion-matter	revolving	around	a	centre.	The	underlying	mechanism	of	the planetary	system	then	is	a	system	of	vortices: [T]he	matter	of	the	heaven,	in	which	the	Planets	are	situated,	unceasingly	revolves,	like	a vortex	having	the	Sun	as	its	center,	and	[...]	those	of	its	parts	which	are	close	to	the	Sun move	more	quickly	than	those	further	away;	and	[...]	all	the	Planets	(among	which	we {shall	from	now	on}	include	the	Earth)	always	remain	suspended	among	the	same	parts	of this	heavenly	matter	(III,	30;	1982,	196). The	very	idea	of	this	kind	of	configuration	is	suggested	by	experience,	and by	means	of	the	bridge	principle	it	is	transferred	to	the	subtle	matter	of	the heavens.	Hence,	invisibility	doesn't	matter.	The	bridge	principle	transfers	the explanatory	mechanism	from	visible	bodies	to	invisible	bodies.	More	specifically, the	specific	continuity	thesis	used	is	the	motion	of	"some	straws	{or	other	light bodies}...	floating	in	the	eddy	of	a	river	where	the	water	doubles	back	on	itself and	forms	a	vortex	as	it	swirls"	(op.cit.).	In	this	kind	of	motion	we	can	see	that the	vortex	carries	the	straws	"along	and	makes	them	move	in	circles	with	it" (op.cit.).	We	also	see	that some	of	these	straws	rotate	about	their	own	centers,	and	that	those	which	are	closer	to the	center	of	the	vortex	which	contains	them	complete	their	circle	more	rapidly	than	those which	are	further	away	from	it	(op.cit.). More	importantly	for	the	explanation	of	gravity,	we	see	that although	these	whirlpools	always	attempt	a	circular	motion,	they	practically	never describe	perfect	circles,	but	sometimes	become	too	great	in	width	or	in	length	(op.cit.). 2	In	(IV,	204;	1982,	286)	Descartes	accepts	that	scientific	explanation	does	not	require	the	truth of	the	claims	about	the	microconstituents	of	things.	In	the	next	paragraph,	however,	he	argues that	his	explanations	have	'moral	certainty'	(IV,	205;	1982,	286-7). 5 Given	the	continuity	thesis,	we	can	transfer	this	mechanical	model	to	the	motion of	the	planets	and	"imagine	that	all	the	same	things	happen	to	the	Planets;	and this	is	all	we	need	to	explain	all	their	remaining	phenomena"	(op.cit.).	Notably, the	continuity	thesis	offers	a	heuristic	for	discovering	plausible	mechanical explanations. Christiaan	Huygens	(1690)	came	to	doubt	the	vortex	theory	"which formerly	appeared	very	likely"	to	him	(1997,	32).	He	didn't	thereby	abandon	the key	tenet	of	mechanical	philosophy.	For	Huygens	too	the	causal	explanation	of	a natural	phenomenon	had	to	be	mechanical.	He	said	referring	to	Descartes: Mr	Descartes	has	recognized,	better	than	those	that	preceded	him,	that	nothing	will	be ever	understood	in	physics	except	what	can	be	made	to	depend	on	principles	that	do	not exceed	the	reach	of	our	spirit,	such	as	those	that	depend	on	bodies,	deprived	of	qualities, and	their	motions	(1997,	1-2). Huygens	posited	a	fluid	matter	that	consists	of	very	small	parts	in	rapid	motion in	all	directions	and	which	fills	the	spherical	space	that	includes	all	heavenly bodies.	Since	there	is	no	empty	space,	this	fluid	matter	is	more	easily	moved	in circular	motion	around	the	centre,	but	not	all	parts	of	it	move	in	the	same direction.	As	Huygens	put	it	"it	is	not	difficult	now	to	explain	how	gravity	is produced	by	this	motion"	(1997,	16).	When	the	parts	of	the	fluid	matter encounter	some	bigger	bodies,	like	the	planets:	"these	bodies	[the	planets]	will necessarily	be	pushed	towards	the	center	of	motion,	since	they	do	not	follow	the rapid	motion	of	the	aforementioned	matter"	(op.cit.).	And	he	added: This	then	is	in	all	likelihood	what	the	gravity	of	bodies	truly	consists	of:	we	can	say	that this	is	the	endeavor	that	causes	the	fluid	matter,	which	turns	circularly	around	the	center of	the	Earth	in	all	directions,	to	move	away	from	the	center	and	to	push	in	its	place	bodies that	do	not	follow	this	motion	(op.cit). In	fact,	Huygens	devised	an	experiment	with	bits	of	beeswax	to	show	how	this movement	towards	the	centre	can	take	place. Newton	of	course	challenged	all	this,	and	along	the	lines,	the	very	idea that	causal	explanation	should	be	mechanical.	But	before	we	take	a	look	at	his reasons	and	their	importance	for	the	very	idea	of	scientific	explanation,	we should	not	fail	to	see	the	broader	metaphysical	grounding	of	the	mechanical project.	For,	as	we	noted,	in	the	seventeenth	century	Mechanism	offered	the metaphysical	foundation	of	science. 2.2	Mechanical	vs	Non-Mechanical	Explanation The	contours	of	this	endeavour	are	well-known.	Matter	and	motion	are	the 'ultimate	constituents'	of	nature;	or,	as	Robert	Boyle	(1991,	20)	put	it,	the	"two grand	and	most	catholic	principles	of	bodies".	Hence,	all	there	is	in	nature	(but clearly	not	the	Cartesian	minds)	is	determined	(caused)	by	the	mechanical affections	of	bodies	and	the	mechanical	laws.	Here	is	Boyle	again: [T]he	universe	being	once	framed	by	God,	and	the	laws	of	motion	being	settled	and	all upheld	by	his	incessant	concourse	and	general	providence,	the	phenomena	of	the	world thus	constituted	are	physically	produced	by	the	mechanical	affections	of	the	parts	of 6 matter,	and	what	they	operate	upon	one	another	according	to	mechanical	laws	(1991, 139). The	Boylean	conception,	pretty	much	like	the	Cartesian,	took	it	that	the new	mechanical	approach	acquired	content	by	excluding	the	then	dominant account	of	explanation	in	terms	of	"real	qualities":	the	scholastics	"attribute	to them	a	nature	distinct	from	the	modification	of	the	matter	they	belong	to,	and	in some	cases	separable	from	all	matter	whatsoever"	(1991,	15-16).	Explanation based	on	real	qualities,	which	are	distinct	(and	separable)	from	matter,	is	not	a genuine	explanation.	They	are	posited	without	"searching	into	the	nature	of particular	qualities	and	their	effects"	(1991,	16).	They	offer	sui	generis explanations:	why	does	snow	dazzle	the	eyes?	Because	of	"a	quality	of	whiteness that	is	in	it,	which	makes	all	very	white	bodies	produce	the	same	effect"	(1991, 16).	But	what	is	whiteness?	No	further	story	about	its	nature	is	offered,	but	just that	it's	a	"real	entity"	inhering	in	the	substance:	why	do	white	objects	produce this	effect	rather	than	that?	Because	it	is	in	their	nature	to	act	thus. Descartes	made	this	point	too	when,	in	his	Le	Monde,	he	challenged	the scholastic	rivals	to	explain	how	fire	burns	wood,	if	not	by	the	incessant	and	rapid motion	of	its	minute	parts.	In	his	characteristic	upfrontness,	Descartes contrasted	two	ways	to	explain	how	fire	burns	wood.	The	first	is	the	Aristotelian way,	according	to	which	"the	'form'	of	fire,	the	'quality'	of	heat,	and	the	'action'	of burning"	are	"very	different	things	in	the	wood"	(Descartes	2004,	6).	The	other	is his	own	mechanistic	way:	when	the	fire	burns	wood, it	moves	the	small	parts	of	the	wood,	separating	them	from	one	another,	thereby transforming	the	finer	parts	into	fire,	air,	and	smoke,	and	leaving	the	larger	parts as	ashes	(2004,	6). This	causal	explanation,	based	as	it	is	on	matter	in	motion,	is	preferable precisely	because	it	is	explanatory	of	the	burning;	in	contrast,	the	Aristotelian	is not,	precisely	because	it	does	not	make	clear	the	mechanism	by	which	the	fire consumes	the	wood: [Y]ou	can	posit	'fire'	and	'heat'	in	the	wood	and	make	it	burn	as	much	as	you	please:	but	if you	do	not	suppose	in	addition	that	some	of	its	parts	move	or	are	detached	from	their neighbours	then	I	cannot	imagine	that	it	would	undergo	any	alteration	or	change	(2004, 6). To	the	then	dominant	account	of	real	qualities	the	new	mechanical metaphysics	juxtaposed	a	different	view	of	qualities.	For	something	to	be	a quality	it	should	be	determined	by	the	mechanical	affections	of	matter,	that	is,	by "virtue	of	the	motion,	size,	figure,	and	contrivance,	of	their	own	parts"	(Boyle 1991,	17).	Hence,	there	can	be	no	change	in	qualities	unless	there	is	a	change	in mechanical	affections.	Though	"catholic	or	universal	matter"	is	common	to	all bodies	(being,	as	Boyle	(1991,	18)	put	it,	"a	substance	extended,	divisible,	and impenetrable"),	it	is	diversified	by	motion,	which	is	regulated	by	laws. The	key	point	then	is	that	the	mechanical	account	of	nature	is	both	a metaphysical	grounding	of	science	and	a	way	to	do	science:	offering	mechanical explanations	of	the	phenomena.	It	covers	everything,	from	the	very	small	to	the very	large.	Here	is	Boyle	again: 7 For	both	the	mechanical	affections	of	matter	are	to	be	found,	and	the	laws	of	motion	take place,	not	only	in	the	great	masses	and	the	middle-sized	lumps,	but	in	the	smallest fragments	of	matter;	and	a	lesser	portion	of	it,	being	as	well	a	body	as	a	greater,	must,	as necessarily	as	it,	have	its	determinate	bulk	and	figure	(1991,	143). The	metaphysical	grounding	of	mechanical	explanation	renders	it	a distinct	kind	of	explanation,	which	separates	it	sharply	from	rival	accounts. Concomitantly,	it	becomes	very	clear	what	counts	as	a	non-mechanical alternative.	An	explanation	couched	in	terms	of	"nature,	substantial	forms,	real qualities,	and	the	like"	is	"unmechanical"	(1991,	142).	But	a	sui	generis	chemical account	of	nature	is	unmechanical	too.	As	Boyle	put	it: [T]hough	chemical	explications	be	sometimes	the	most	obvious	and	ready,	yet	they	are not	the	most	fundamental	and	satisfactory:	for	the	chemical	ingredient	itself,	whether sulphur	or	any	other,	must	owe	its	nature	and	other	qualities	to	the	union	of	insensible particles	in	a	convenient	size,	shape,	motion	or	rest,	and	contexture,	all	which	are	but mechanical	affections	of	convening	corpuscles	(1991,	147). The	opposition	to	both	of	these	non-mechanical	accounts	is	weaved	around	a certain	metaphysical	account	of	the	world	as	fundamentally	mechanical	and	a reductive-decompositional	account	of	scientific	explanation	itself. 2.3	Newton	against	Mechanism When	Newton	offered	a	non-mechanical	account	of	gravity,	he	primarily challenged	the	idea	that	legitimate	scientific	explanation	ought	to	be mechanistic.	There	is	a	sense	in	which	Newton	prioritised	explanation	by unification	under	laws	and	not	by	mechanisms.	This	is	seen	in	the	Preface	to	the second	(1713)	edition	of	the	Principia,	authored	by	Roger	Cotes	under	the supervision	of	Newton.	In	this	preface,	Cotes	presents	Newton's	method	as	a midway	(a	via	media)	between	Aristotelianism	and	Mechanism.	To	be	sure, mechanical	explanations	were	an	improvement	over	the	Scholastic	explanations because	they	relied	on	demonstrations	on	the	basis	of	laws.	Still,	taking	"the foundation	of	their	speculations	from	hypotheses",	the	mechanists	are	"merely putting	together	a	romance	[i.e.	fiction],	elegant	perhaps	and	charming,	but nevertheless	a	romance"	(Newton	2004,	43). Thus	put,	the	point	sounds	epistemic;	it	concerns	the	increased	risk involved	in	hypothesising	a	mechanism	which	is	supposed	to	underpin,	and hence	to	causally	explain,	a	certain	phenomenon.	Cotes	adds: But	when	they	[the	mechanists]	take	the	liberty	of	imagining	that	the	unknown	shapes and	sizes	of	the	particles	are	whatever	they	please,	and	of	assuming	their	uncertain positions	and	motions,	and	even	further	of	feigning	certain	occult	fluids	that	permeate	the pores	of	bodies	very	freely,	since	they	are	endowed	with	an	omnipotent	subtlety	and	are acted	on	by	occult	motions:	when	they	do	this,	they	are	drifting	off	into	dreams,	ignoring the	true	constitution	of	things,	which	is	obviously	to	be	sought	in	vain	from	false conjectures,	when	it	can	scarcely	be	found	out	even	by	the	most	certain	observations (Newton	2004,	43). Still,	it's	fair	to	say	that	Newton's	via	media	was	based	on	a	different understanding	of	scientific	explanation:	it	should	look	for	causes-hence, scientific	explanation	should	be	causal-but	the	sought	after	causes	need	not	be mechanical.	Newton's	way,	Cotes	says,	is	to	"hold	that	the	causes	of	all	things	are 8 to	be	derived	from	the	simplest	possible	principles",	but	unlike	the	mechanists's way,	it	"assume(s)	nothing	as	a	principle	that	has	not	yet	been	thoroughly proved	from	phenomena".	The	"explication	of	the	system	of	the	world	most successfully	deduced	from	the	theory	of	gravity"	is	the	"most	illustrious" example	of	Newton's	way	(2004,	32). Newton	emphatically	denied	feigning	any	hypotheses	about	the	cause	of gravity.	For	him, it	is	enough	that	gravity	really	exists	and	acts	according	to	the	laws	that	we	have set	forth	and	is	sufficient	to	explain	all	the	motions	of	the	heavenly	bodies	and	of our	sea	(2004,	92). Gravity	according	to	Newton	is	a	non-mechanical	force	since	it operates	not	according	to	the	quantity	of	the	surfaces	of	the	particles	upon	which it	acts	(as	mechanical	causes	used	to	do),	but	according	to	the	quantity	of	the	solid matter	which	they	contain,	and	propagates	its	virtue	on	all	sides	to	immense distances,	decreasing	always	as	the	inverse	square	of	the	distances	(op.cit.). He	added	that	the	very	motion	of	the	comets	makes	it	plausible	to	think	that	the regular	elliptical	motion	of	the	planets	(as	well	as	of	their	satellites)	cannot "have	their	origin	in	mechanical	causes"	(2004,	90). In	his	already	mentioned	Discourse	on	the	Cause	of	Gravity	(1690),	Huygens expressed	his	dissatisfaction	with	Newton's	failure	to	offer	a	mechanical explanation	of	the	cause	of	gravitational	attraction.	Favouring	his	own explanation	of	gravity	in	terms	of	the	centrifugal	force	of	the	subtle	and	rapidly moving	matter	that	fills	the	space	around	the	Earth	and	the	other	planets, Huygens	noted	that	Newton's	theory	supposes	that	gravity	is	"an	inherent quality	of	corporeal	matter".	"But",	he	immediately	added,	such	a	hypothesis "would	distance	us	a	great	deal	from	mathematical	or	mechanical	principles" (1997,	35). Yet	Huygens	had	no	difficulty	in	granting	that	Newton's	law	of	gravity	was essentially	correct	when	it	comes	to	accounting	for	the	planetary	system.	As	he put	it: I	have	nothing	against	Vis	Centripeta,	as	Mr.	Newton	calls	it,	which	causes	the	planets	to weigh	(or	gravitate)	toward	the	Sun,	and	the	Moon	toward	the	Earth,	but	here	I	remain	in agreement	without	difficulty	because	not	only	do	we	know	through	experience	that	there is	such	a	manner	of	attraction	or	impulse	in	nature,	but	also	that	it	is	explained	by	the	laws of	motion,	as	we	have	seen	in	what	I	wrote	above	on	gravity	(1997,	31). Explaining	the	fact	that	gravity	depends	on	the	masses	and	diminishes	with distance	"in	inverse	proportion	to	the	squares	of	the	distances	from	the	centre" (1997,	37),	were,	for	Huygens,	clear	achievements	of	Newton's	theory	despite the	fact	that	the	mechanical	cause	of	gravity	remained	unidentified. Commitment	to	mechanical	explanation	was	honoured	by	Gottfried Wilhelm	Leibniz	too.	In	a	piece	titled	"Against	Barbaric	Physics:	Toward	a Philosophy	of	What	There	Actually	Is	and	Against	the	Revival	of	the	Qualities	of the	Scholastics	and	Chimerical	Intelligences"	(written	between	1710	and	1716), he	defended	the	mechanical	view	by	arguing	that	corporeal	forces	should	be grounded	mechanically	when	it	comes	to	their	application	to	the	natural	world. 9 Leibniz	was	very	clear	that	though	he	allowed	"magnetic,	elastic,	and	other	sorts of	forces",	they	are	permissible	"only	insofar	as	we	understand	that	they	are	not primitive	or	incapable	of	being	explained,	but	arise	from	motions	and	shapes" (Leibniz	1989,	313).	So,	forces	are	necessary,	but	a	condition	for	their applicability	to	the	natural	world	is	that	they	are	seen	as	"arising	from	motions and	shapes".	What	he	took	it	to	be	"barbarism	in	physics"	was	to	posit	sui generis,	that	is	non-mechanically	grounded,	"attractive	and	repulsive"	forces	that act	at	a	distance	(1989,	314-315).	Newton's	gravity	was	supposed	to	be	such	a barbaric	force! In	a	letter	he	sent	to	Nicolaas	Hartsoeker	(Hanover,	10	February	1711), Leibniz	makes	it	clear	that	the	proper	scientific	explanation	should	be mechanical.	It	is	not	enough	for	scientific	explanation	to	identify	the	law	by means	of	which	a	certain	force	acts;	what	is	also	required	is	the	specification	of the	mechanism	by	means	of	which	it	acts.	The	mechanism	is,	clearly,	on	top	of the	law	and	given	independently	of	it.	Without	the	mechanism	the	power	is	"an unreasonable	occult	quality".	He	says: Thus	the	ancients	and	the	moderns,	who	own	that	gravity	is	an	occult	quality,	are	in	the right,	if	they	mean	by	it	that	there	is	a	certain	mechanism	unknown	to	them,	whereby	all bodies	tend	towards	the	center	of	the	earth.	But	if	they	mean	that	the	thing	is	performed without	any	mechanism	by	a	simple	primitive	quality,	or	by	a	law	of	God,	who	produces that	effect	without	using	any	intelligible	means,	it	is	an	unreasonable	occult	quality,	and	so very	occult,	that	it	is	impossible	it	should	ever	be	clear,	though	an	angel,	or	God	himself, should	undertake	to	explain	it	(Newton	2004,	112). Newton	couldn't	disagree	more.	In	an	unsent	letter	written	circa	May	1712	to the	editor	of	the	Memoirs	of	Literature,	Newton	referred	explicitly	to	Leibniz's letter	to	Hartsoeker,	and	stressed	that	it	is	not	necessary	for	the	introduction	of a	power-such	as	gravity-to	specify	anything	other	than	the	law	it	obeys;	no extra	requirements	should	be	imposed,	and	in	particular	no	requirement	for	a mechanical	grounding: And	therefore	if	any	man	should	say	that	bodies	attract	one	another	by	a	power	whose cause	is	unknown	to	us,	or	by	a	power	seated	in	the	frame	of	nature	by	the	will	of	God,	or by	a	power	seated	in	a	substance	in	which	bodies	move	and	float	without	resistance	and which	has	therefore	no	vis	inertiae	but	acts	by	other	laws	than	those	that	are	mechanical:	I know	not	why	he	should	be	said	to	introduce	miracles	and	occult	qualities	and	fictions into	the	world.	For	Mr.	Leibniz	himself	will	scarce	say	that	thinking	is	mechanical	as	it must	be	if	to	explain	it	otherwise	be	to	make	a	miracle,	an	occult	quality,	and	a	fiction (Newton	2004,	116). Note	well	Newton's	point.	The	fact	that	an	explanation	does	not	conform to	a	certain	mechanical	framework	does	not	make	it	fictitious,	occult	or miraculous.	Non-mechanical	explanations	are	legitimate	insofar	as	they	identify the	law	that	covers	or	governs	a	certain	phenomenon.	Hence,	Newton	promotes a	methodological	shift:	causal	explanation	without	mechanisms	but	subject	to laws. Causal	explanation	then	need	not	be	mechanical	to	be	legitimate	and adequate.	This	is	Newton's	key	thought.	In	breaking	with	a	tradition	which brought	under	the	same	roof	a	certain	metaphysical	conception	of	the	world	and a	certain	view	of	scientific	explanatory	practice,	Newton	distinguished	the	two and	laid	emphasis	on	the	explanatory	practice	itself,	thereby	freeing	it	from	a 10 certain	metaphysical	grounding. Though	this	is	not	the	end	of	the	story	of	Old	Mechanism,3	Newton's	key thought,	we	shall	argue,	is	of	relevance	in	the	current	debates	over	New Mechanism,	to	which	we	shall	now	turn	our	attention. 3.	New	Mechanism:	from	Practice	to	Metaphysics It	is	useful	to	differentiate	between	two	ways	to	conceptualise	mechanisms	in the	post-1970	literature.	First,	mechanism	has	been	used	as	a	primarily metaphysical	concept,	mostly	aiming	to	illuminate	the	metaphysics	of	causation. Second,	mechanism	has	been	taken	to	be	a	concept	used	in	science,	and philosophical	accounts	of	mechanism	have	aimed	to	elucidate	this	concept.	To	be sure,	some	philosophical	approaches	to	mechanism,	most	notably	Glennan's (1996),	blend	these	two	conceptions	(the	metaphysical	one	and	the	concept-inuse).	However,	it's	fair	to	say	that	there	are	two	quite	distinct	points	of	origin	of the	recent	philosophical	accounts	of	mechanism:	the	first	starts	from metaphysics	(as	was	the	case	for	Descartes	and	other	old	mechanists),	the second	from	scientific	practice.	Using	this	distinction	between	mechanism	as	a primarily	metaphysical	concept	and	as	a	concept-in-use	in	science,	we	can differentiate	between	two	kinds	of	approaches	to	the	metaphysics	of mechanisms. On	the	first	approach,	the	aim	is	to	show	what	the	connection	is	between mechanism	qua	a	metaphysical	category	and	other	central	metaphysical concepts,	notably	causation.	In	the	context	of	the	metaphysics	of	causation, 'mechanistic'	accounts	are	theories	about	the	link	between	cause	and	effect.	Such theories	are	meant	to	be	anti-Humean	in	that	they	view	causation	as	a productive	relation,	i.e.	the	cause	somehow	brings	about	or	produces	the	effect. The	aim	of	the	mechanistic	view	of	causation	is	to	illuminate	the	productive relation	between	the	cause	and	the	effect	by	positing	a	mechanism	that	connects them,	and	by	explicating	'mechanism'	in	a	suitable	way	such	that	causal sequences	are	differentiated	from	non-causal	ones.	The	central	thought,	then,	is that	A	causes	B	if	and	only	if	there	is	a	mechanism	connecting	A	and	B. Two	kinds	of	views	have	become	prominent:	those	that	characterise	the mechanism	that	links	cause	and	effect	in	terms	of	the	persistence,	transference or	possession	of	a	conserved	quantity	(Mackie	1974;	Salmon	1997;	Dowe	2000); and	those	that	connect	a	mechanistic	account	to	causal	production	with	a	powerbased	one	(see	Harré	(1970)	for	an	early	such	view).	Despite	their	differences, these	views	share	in	common	the	claim	that	mechanisms	are	the	ontological	tie that	constitutes	Hume's	'secret	connexion'.	In	our	(2018),	we	have	called	such mechanisms,	mechanisms-of.	Mechanisms-of	are	ontological	items	that	underlie or	constitute	certain	kind	of	processes,	i.e.	those	that	can	be	deemed	causal. Since	we	have	dealt	with	these	accounts	in	some	detail	elsewhere	(Psillos	2002; Ioannidis	&	Psillos	2018),	we	shall	not	discuss	them	further	here. On	the	second	approach,	working	out	a	metaphysics	of	mechanisms	is	not the	starting	point	but	rather	the	end	point	of	inquiry.	Starting	with	mechanism as	a	concept-in-use	in	science,	one	tries	first	to	give	a	general	characterisation	of 3	For	more	on	the	development	of	Old	Mechanism,	see	Psillos	(2011). 11 this	concept	and	then	to	derive	metaphysical	conclusions,	i.e.	conclusions	about the	(mechanistic)	structure	of	the	world. This	kind	of	bottom-up	inquiry	has	yielded	several	well-known	general accounts	of	mechanisms	as	well	as	theses	about	the	ontic	signature	of	a mechanistic	world. 3.1	The	Metaphysics	of	New	Mechanism Here	are	three	well-known	general	characterisations	of	a	mechanism	in	recent mechanistic	literature: Mechanisms	are	entities	and	activities	organized	such	that	they	are	productive	of	regular changes	from	start	or	set-up	to	finish	or	termination	conditions	(Machamer,	Darden,	& Craver	2000,	3). A	mechanism	for	a	behavior	is	a	complex	system	that	produces	that	behavior	by	the interaction	of	a	number	of	parts,	where	the	interactions	between	parts	can	be characterized	by	direct,	invariant,	change-relating	generalizations	(Glennan	2002,	S344). A	mechanism	is	a	structure	performing	a	function	in	virtue	of	its	component	parts, component	operations,	and	their	organization.	The	orchestrated	functioning	of	the mechanism	is	responsible	for	one	or	more	phenomena	(Bechtel	&	Abrahamsen	2005, 423). The	focus	on	mechanism	as	a	concept-in-use	is	common	to	all	three	accounts; none	of	the	three	accounts	can	be	viewed	as	falling	under	the	rubric	of mechanistic	theories	of	causation.	And	yet,	all	these	and	similar	accounts	yield specific	metaphysical	commitments	about	what	kind	of	things	in	the	world mechanisms	are.	All	these	accounts	are	committed	to	the	thesis	that	a	general characterisation	of	mechanism	must	itself	be	cashed	out	in	metaphysical	terms. Hence,	talk	of	mechanisms	in	science	is	taken	to	have	quite	direct	consequences about	the	kind	of	ontology	presupposed	by	such	talk.	In	order	to	substantiate this	point,	let	us	look	at	the	three	accounts	mentioned	earlier	in	some	more detail. Peter	Machamer,	Lindley	Darden	and	Carl	Craver's	(MDC)	account	is perhaps	the	most	ontologically	inflated,	as	it	is	explicitly	committed	to	both entities	and	activities	as	distinct	and	separate	ontological	categories.	It	is	thus committed	to	a	particular	view	about	the	metaphysics	of	causation:	causation within	mechanisms	is	to	be	characterised	in	terms	of	production,	where	the productive	relation	is	captured	by	the	various	different	kinds	of	activities identified	by	science. Glennan's	case	is	interesting,	since	in	his	(2002)	he	refrains	from	taking mechanisms	to	entail	a	productive	account	of	causation.	Instead,	withinmechanism	interactions	are	characterised	in	terms	of	invariant,	change-relating generalisations.	As	we	will	see	below,	however,	Glennan	has	presently	connected his	account	of	mechanisms	with	a	power-based	understanding	of	causation. Hence,	he	is	committed	to	causal	powers	as	parts	of	the	building	blocks	of mechanisms. Lastly,	William	Bechtel	&	Adele	Abrahamsen's	account	does	not	include	a specific	characterisation	of	what	mechanistic	causation	amounts	to	at	all.	Here, however,	as	in	the	other	two	accounts,	we	have	a	series	of	general	terms	the meaning	of	which	needs	to	be	unpacked.	So,	MDC	include	in	their	accounts 12 'entities'	and	'organisation';	Glennan	includes	'complex	system'	and	'parts';	and Bechtel	&	Abrahamsen	talk	about	'structure',	'function',	'parts'	and	'organisation'. All	these	accounts	suggest	the	further	need	to	explain	what	this	'new mechanical	ontology'	of	entities,	activities,	organisation	of	parts	into	wholes	etc. amounts	to:	what,	in	general	terms,	the	constituents	of	mechanisms	are	and what	are	their	relations	with	more	traditional	metaphysical	categories,	such	as things,	properties,	powers	and	processes. Notably,	there	has	been	a	tendency	recently	to	offer	a	more	minimal general	characterisation	of	a	mechanism.	For	example,	according	to	Phyllis	Illari &	Jon	Williamson: A	mechanism	for	a	phenomenon	consists	of	entities	and	activities	organized	in	such	a	way that	they	are	responsible	for	the	phenomenon	(2012,	120). Glennan's	recent	version	is	almost	identical: A	mechanism	for	a	phenomenon	consists	of	entities	(or	parts)	whose	activities	and interactions	are	organised	so	as	to	be	responsible	for	the	phenomenon	(2017,	17). Glennan	calls	this	account	Minimal	Mechanism.	The	key	motivation	here	is	for	a general	characterisation	of	mechanism	broad	enough	to	capture	examples	of mechanisms	in	different	fields,	from	physics	to	the	social	sciences.	But	even	in this	minimal	mode,	mechanisms,	according	to	Glennan,	"constitute	the	causal structure	of	the	world"	(Glennan	2017,	18). This	minimal	account	of	mechanism	might	appear	to	fit	the	bill	of capturing	a	concept-in-use	in	science.	On	closer	inspection,	however,	it	is committed	to	a	rather	rich	metaphysical	account	of	mechanism:	the	minimal account	is	not	more	minimal	than	the	metaphysically	inflated	accounts	noted above.	The	reason	is	that	both	of	the	foregoing	minimal	accounts	still	invite questions	about	the	ontic	status	of	mechanisms.	For	example:	how	exactly	do entities	and	activities	differ?	What	is	the	relation	between	activities	and interactions?	How	should	organisation	be	understood?	Glennan	(2017,	13) explicitly	talks	about	a	"new	mechanical	ontology"	as	the	upshot	of	the	minimal account.	The	"minimal	mechanism",	he	adds,	"is	an	ontological	characterization of	what	mechanisms	are	as	things	in	the	world"	(2017,	19). New	Mechanism,	then,	aims	to	provide	a	new	ontology	of	mechanisms. We	can	identify	three	commonly	accepted	key	theses	concerning	mechanistic ontology: (1) The	world	consists	of	mechanisms. Thesis	1	is	a	typical	view	among	mechanists:	mechanisms	are	taken	to	be	things in	the	world,	with	objective	boundaries.	Ours	is	a	mechanistic	world.	As	Glennan puts	it	at	the	end	of	his	(2017,	240),	"[t]hat	is	just	how	we	have	found	the	world to	be". (2) A	mechanism	consists	of	objects	of	diverse	kinds	and	sizes	structured	in such	a	way	that,	in	virtue	of	their	properties	and	capacities,	engage	in	a	variety	of different	kinds	of	activities	and	interactions	such	that	a	certain	phenomenon	P	is brought	about. 13 Thesis	2	(or	something	very	similar)	can	be	taken	as	the	common	core	of	the general	characterisation	of	mechanism	as	a	concept-in-use	given	by	new mechanists.	It	identifies	the	components	of	a	mechanism	and	the	relations among	them.	As	mechanisms	are	things	in	the	world	(thesis	1),	their components	are	also	particular	things	in	the	world.	Besides,	these	parts	engage in	activities	by	being	"active,	at	least	potentially"	(Glennan	2017,	21).	Activity	is understood	as	a	manifestation	of	the	powers	things	have.	Glennan	is	quite explicit	that	"Activities	manifest	the	powers	(capacities)	of	the	entities	involved in	the	Activity"	(2017,	31).	Positing	powers	is	supposed	to	explain	why "activities	are	powerful";	being	powerful,	activities	are	what	"an	entity	does,	not merely	something	that	happens	to	an	entity"	(2017,	32).	But	activities	are	not enough.	Interactions	are	needed	too	because	"there	is	no	production	without interaction"	(2017,	22).	"The	fundamental	point	of	ontological	agreement	among the	New	Mechanists",	as	Glennan	(2017,	21	n.6)	puts	it,	is	that	that	entities cannot	exist	without	activities	or	activities	without	entities.	It's	not	hard	to	see that	the	minimal	account	of	mechanism	is	taken	to	imply	or	suggest	a	rather substantive	metaphysical	conception	of	mechanism,	which,	until	further	notice at	least,	is	broadly	neo-Aristotelian. (3) To	explain	a	certain	phenomenon	P	is	to	offer	the	mechanism	that produces	it. Thesis	3	connects	the	previous	theses	with	a	claim	about	explanation	(and	more specifically,	causal	explanation):	since	in	a	mechanistic	world	phenomena	are produced	by	mechanisms,	scientific	explanation	has	to	identify	the	mechanism that	produces	a	certain	phenomenon;	that	is	(by	thesis	2)	to	identify	the organised	entities	and	activities	that	produce	the	phenomenon. Despite	their	differences,	there	are	important	similarities	between	Old and	New	Mechanism	(which	justify	viewing	both	positions	as	mechanistic).	On the	one	hand,	as	we	saw,	new	mechanists	differ	from	their	seventeenth	century predecessors	in	that	they	do	not	start	their	analysis	with	a	metaphysical	concept of	mechanism;	rather,	they	aim	at	giving	a	general	characterisation	of mechanism	as	a	central	concept	of	scientific	practice.	This	characterisation	is non-reductive	in	that	it	is	not	committed	to	the	view	that	mechanisms	are configurations	of	matter	in	motion	subject	to	laws	(and	contact	action).	But,	on the	other	hand,	they	are	committed	to	mechanisms	being	configurations	of powerful	entities	engaged	in	activities	and	interactions.	As	Glennan	puts	it: "Mechanisms	are	particular	and	compound,	made	up	of	parts	(entities)	whose activities	and	interactions	are	located	in	particular	regions	of	space	and	time" (2017,	57). Hence,	New	Mechanism	is	similar	to	seventeenth	century Mechanism,	in	that	it	is	committed	to	a	mechanistic	ontology.	This	ontology (theses	1	&	2	above),	while	not	a	global	metaphysics	in	the	sense	of	the seventeenth	century,	is	still	a	thesis	about	the	ontic	signature	of	the	world.	Here is	Glennan	again: New	Mechanist	ontology	is	an	ontology	of	compound	systems.	It	suggests	that	the properties	and	activities	of	things	must	be	explained	by	reference	to	the	activities and	organization	of	their	parts	(2017,	57). 14 Instead	of	resulting	in	a	'flat'	ontology	where	everything	there	is	consists	in matter	in	motion,	this	new	mechanical	metaphysics	ends	up	with	a	hierarchy	of particular	things-mechanisms-which	may	contain	a	diverse	set	of	entities	and activities,	rather	than	the	limited	set	endorsed	by	the	corpuscularians,	and whose	productivity	is	grounded	in	causal	powers,	rather	than	in	a	few fundamental	laws	of	motion. But	we	can	ask:	are	these	ontological	commitments	really	necessary	in order	to	understand	scientific	practice?	Are	they	licenced	by	the	practice	of science?	Remember	here	that	the	primary	aim	of	new	mechanists	is	to	give	a general	characterisation	of	mechanism	as	a	concept-in-use.	So,	ideally,	the general	account	of	mechanism	should	capture	as	far	as	possible	the	extension	of a	concept-in-use	in	the	various	sciences.	The	minimal	account	of	mechanism discussed	so	far,	though	broad	enough	to	play	this	role,	inflates	the	concept-inuse	by	making	it	amenable	to	a	certain	metaphysical	description	of	its	basic components. Note	that	our	claim	is	not	that	the	metaphysical	questions	are	not philosophically	interesting	questions	to	ask;	they	are,	especially	if	we	are interested	in	giving	an	account	of	the	ontological	structure	of	reality.	Moreover, such	a	kind	of	project	has	to	be	informed	by	what	science	has	to	say	about	the world.	If,	however,	our	aim	is	to	understand	how	a	specific	concept- mechanism-is	used	in	scientific	practice,	these	questions	seem,	at	least	prima facie,	irrelevant,	especially	if	a	general	characterisation	of	mechanism	is	possible that	does	not	include	such	things. 3.2	Mechanism	in	Scientific	Practice A	metaphysically	deflationary	view	of	mechanism	as	a	concept-in-use	that	is broad	enough	to	capture	all	examples	of	mechanisms	that	we	find	in	science seems	indeed	possible.	In	our	(2017)	we	have	argued	that	there	is	an	even	more minimal	account	of	mechanism	than	those	offered	by	Illari	&	Williamson	and Glennan,	which	captures	this	concept-in-use.	This	is	achieved	by	dropping	the reference	to	activities	and	interactions	and	by	understanding	mechanism	as	the causal	pathway	of	a	certain	phenomenon,	described	in	the	language	of	theory. According	to	this	account	that	we	call	Truly	Minimal	Mechanism	[TMM],	a mechanism	in	science	just	is	a	causal	pathway	described	in	theoretical	language: TMM:	mechanism	=	causal	pathway,	described	in	theoretical	language The	central	idea	behind	TMM	is	that	when	scientists	talk	about	a	'mechanism', what	they	try	to	capture	is	the	way	(i.e.	the	causal	pathway)	a	certain	result	is produced.	Say,	for	example,	that	we	want	to	find	out	how	a	certain	disease	state is	brought	about.	What	we	must	look	for	is	a	specific	mechanism,	i.e.	a	causal pathway	that	involves	various	causal	links	between,	for	example,	a	virus	and changes	in	properties	of	the	organism	that	ultimately	lead	to	the	disease	state.	In pathology,	such	causal	pathways	constitute	the	pathogenesis	of	a	disease,	and when	pathologists	talk	about	the	mechanisms	of	a	disease,	it	is	such	pathways that	they	have	in	mind	(cf.	Lakhani	et	al.	2009;	see	also	Gillies	2017,	for	a	similar account	of	mechanisms	in	medicine	as	causal	pathways). Mechanisms	then	and	causation	are	closely	related:	when	two	events	are causally	connected,	there	is	a	mechanism	(that	is,	a	causal	pathway)	that 15 connects	them	and	accounts	for	the	specific	way	that	the	cause	brings	about	the effect.	Also,	scientists	succeed	in	identifying	a	mechanism,	if	they	succeed	in describing	the	relevant	causal	pathway	in	terms	of	the	theoretical	language	of the	particular	scientific	field. In	life	sciences,	this	is	a	typical	use	of	'mechanism'.	In	our	(2017),	we	have discussed	extensively	a	particular	example,	the	case	of	apoptosis,	i.e.	a	particular causal	pathway	(or	mechanism)	of	cell	death.	The	study	of	apoptosis	(and	of mechanisms	of	programmed	and	physiological	cell	death	in	general)	in	fact transcends	particular	biological	fields,	and	has	involved	cytologists, developmental	biologists,	pathologists,	and	molecular	biologists	among	others. Because	of	its	broad	role,	this	case	offers	a	nice	test	case	of	the	concept	of mechanism	as	it	is	used	in	science.	Apoptosis	is	described	in	various	biological disciplines	as	a	'mechanism'	of	cell	death.	The	common	concept	of	mechanism	at play	here,	we	have	argued	in	(2017),	is	that	a	mechanism	just	is	a	causal pathway.	TMM	then	is	the	common	denominator	of	all	uses	of	the	concept	of mechanism	in	biology	and	elsewhere. To	further	see	the	plausibility	of	this	view,	consider	the	relation	between the	concepts	of	mechanism	and	function.	In	the	general	accounts	mentioned earlier,	only	Bechtel	&	Abrahamsen	explicitly	refer	to	the	behaviour	of	a mechanism	in	terms	of	its	function.	But	in	all	accounts,	a	mechanism	is	always	a mechanism	for	a	phenomenon	or	behaviour,	and	this	can	be	taken	as	an	implicit reference	to	the	function	of	the	mechanism,	which	plays	a	central	role	in individuating	the	mechanism.	In	other	words,	there	is	no	mechanism	without	a function,	and	the	function	determines	what,	among	everything	that	happens within	a	complex	system	such	as	an	organism,	counts	as	a	mechanism.	It	is	an open	issue	among	new	mechanists	how	this	commitment	to	function	should	be construed	and	what	its	consequences	for	the	metaphysics	of	mechanisms	are	(cf. Garson	2018;	Craver	2013). However,	if	we	insist	on	an	account	of	mechanism	broad	enough	to capture	all	uses	of	the	concept	in	science,	and	given	that	there	are	scientific	fields where	the	concept	of	function	is	not	present	(e.g.	particle	physics,	solid	state physics,	astrophysics,	cosmology),	an	account	such	as	TMM	seems	preferable.	Of course,	there	are	contexts	(for	example,	in	molecular	biology),	where	a mechanism	is	automatically	a	mechanism	for	a	certain	function;	e.g.	apoptosis	is a	mechanism	for	cell	death,	and	it	also	has	a	homeostatic	function	within	the organism	(see	our	(2017)	for	more	on	this	point	and	its	relevance	for	TMM).	But it	is	not	clear	that	a	mechanism	of	star	formation,	for	example,	has	star formation	as	its	function,	unless	one	takes	function	to	be	what	the	mechanism produces,	i.e.	its	effect.	The	point	here	is	that	if	we	want	to	claim	that	a mechanism	of	cell	death	and	a	mechanism	of	star	formation	are	in	some	sense the	same	kind	of	thing	(i.e.	they	are	both	mechanisms),	that	is	if	we	want	to	give a	general	account	of	a	mechanism	as	a	concept-in-use	across	various	scientific fields,	TMM	seems	the	most	promising	candidate.	At	the	same	time,	TMM	can	be easily	adapted	to	capture	particular	uses	of	'mechanism'	in	various	contexts where	a	specific	notion	of	function	is	presupposed. We	want	to	resist	the	temptation	to	offer	a	metaphysically	inflated account	of	the	causal	pathway,	in	terms	of	an	explicit	specification	of	its ontological	constituents.	A	key	reason	for	this	is	that	the	causal	pathway	should be	described	in	the	theoretical	language	of	a	specific	scientific	field,	and	not	in 16 some	privileged	ontological	language	or	even	in	ontologically	loaded	terms.	This suggests	that	the	form	of	the	description	of	the	mechanism	cannot	be	decided beforehand	and	in	advance	of	how	the	concept	of	mechanism	is	used.	What counts,	each	time,	as	a	legitimate	description	of	a	causal	pathway,	is	something that	has	to	be	decided	by	scientific	practice.	Instead	of	imposing	various metaphysical	categories	as	those	that	constitute	a	general	legitimate	description of	a	mechanism,	it	should	be	left	to	the	scientists	themselves	to	decide	how	best to	describe	mechanisms	using	the	theoretical	language	they	employ	to understand	and	describe	the	world.	TMM	has	the	consequence	that	a	series	of questions	that	new	mechanists	have	been	concerned	with	need	not	concern	us	if our	aim	is	to	understand	scientific	practice.4 If	we	identify	a	mechanism	with	a	causal	pathway,	would	it	be	required	to make	a	commitment	about	what	the	ontological	nature	of	causation	is?	This	does not	seem	necessary	for	understanding	the	concept-in-use.	Scientific	practice establishes	robust	causal	connections,	which	can	be	used	for	understanding	and manipulation,	without	necessarily	being	committed	to	a	single	and	overarching ontic	account	of	causation.	Ultimately,	whatever	fundamental	ontological	theory of	what	causation	is	one	might	have	(e.g.	in	terms	of	causal	powers	or regularities	etc.),	the	identification	of	causal	relationships	is	based	on	theorydescribed	difference-making	relations;	this	is	what	scientists	look	for	when establishing	causal	relations	and	causal	pathways.	In	this	sense,	the	causal pathway	by	means	of	which	a	phenomenon	Y	is	brought	about	by	a	cause	X, given	that	X	initiates	a	chain	of	events	that	leads	to	Y,	is	the	very	network	of theory-described	difference-making	relations	among	the	various	intermediaries of	X	and	Y.	It	is	a	further	question,	and	one	that	is	not	needed	to	be	answered	in order	to	discover	and	use	causal	relations	in	scientific	practice,	what	the	truthmakers	of	these	difference-making	relations	are.	Hence,	the	point	here	is	that	in order	to	understand	what	a	causal	pathway	(and	hence	a	mechanism)	as	a concept-in-use	is,	and	to	identify	mechanisms,	we	do	not	need	a	theory	about	the metaphysics	of	causation:	TMM	is	really,	ontologically	speaking,	a	truly	minimal view5. TMM	is	best	seen	in	the	context	of	a	thesis	that	we	call,	following	Joseph Henry	Woodger	(1929)	and	Robert	Brandon	(1984),	Methodological	Mechanism [MM]	(see	our	(2017)	for	more	on	Woodger	on	MM).	In	his	(1929),	Woodger distinguished	between	two	ways	in	which	a	certain	notion	can	be	employed:	a metaphysical	or	ontological	way	and	a	methodological	one.	The	latter	is	when	a notion	is	used	for	the	purposes	of	description	''independently	of	its	metaphysical interpretation''.	In	this	case,	Woodger	says,	the	notion	''is	employed methodologically,	i.e.	simply	for	the	purpose	of	investigation''	(1929,	31).	The advantage	of	this	use	is	that	the	notion	can	be	used	in	a	certain	practice	and	cast light	on	it	independently	of	whatever	difficulties	(and	controversies)	are	raised 4	These	questions	concern,	for	example,	the	components	and	boundaries	of	mechanisms	(cf. Kaiser	2018),	the	metaphysics	of	causation	(cf.	Matthews	&	Tabery	2018),	and	the	mechanistic levels	(cf.	Povich	&	Craver	2018). 5	There	are	various	other	questions	that	can	be	raised	concerning	TMM:	for	example,	do	we	take pathways	to	be	types	or	tokens?	Here	again,	we	defer	to	practice.	Causal	pathways,	qua	things	in the	world	that	produce	an	effect,	are	concrete	particulars.	But	what	is	described	theoretically	in the	language	of	theory	is	a	type	of	causal	pathway. 17 by	the	intricate	metaphysical	debates	concerning	what	its	worldly	reference	is really	like.	According	to	Woodger,	taking	the	methodological	standpoint amounts	to	asking	''the	methodological	mechanist	what	he	has	to	say	in	support of	his	contention	that	the	mechanical	explanation	is	the	only	one	which	is admissible	in	science''	(1929,	231).	Hence,	MM	is	a	view	about	mechanistic explanation	and	its	admissibility,	and	not	about	the	blueprint	of	the	universe. Methodological	Mechanism,	we	want	to	claim,	is	the	view	that commitment	to	mechanism	in	science	is	adopting	a	methodological	postulate which	licenses	looking	for	the	causal	pathways	for	the	phenomena	of	interest. Hence,	MM	licenses	adopting	TMM. MM	illuminates	practice	in	a	way	that	ontologically	inflated	accounts	of mechanism	do	not.	It	accounts	for	the	centrality	of	mechanisms	in	scientific discovery	and	explanation,	since	according	to	it	discovering	mechanisms	(i.e. causal	pathways)	is	the	central	task	of	science.	At	the	same	time,	however,	it refrains	from	imposing	on	scientific	practice	ontic	constraints	that	are	not licensed	by	it.	According	to	MM,	the	mechanistic	view	need	not	be	taken	as something	stronger	than	a	certain	methodological	commitment	to	a	kind	of explanation.	As	Brandon	has	put	it,	the	question:	'what	is	a	mechanism?' has	no	general	metaphysical	answer,	because	the	business	of	science	is	the	discovery	of mechanisms;	so	we	cannot	delimit	in	any	a	priori	manner	the	mechanisms	of	nature.	[...] The	best	we	can	do	is	to	give	an	open-ended	answer:	a	mechanism	is	any	describable causal	process	(1990,	185,	emphasis	added). This	can	be	generalised	as	follows:	concepts	such	as	mechanism,	that	are central	in	scientific	practice,	should	be	viewed	as	methodological	postulates rather	than	as	presupposing	robust	metaphysical	commitments.	But methodological	postulates	should	be	'open-ended';	otherwise	they	would unnecessarily	limit	research.	Far	from	being	a	trivial	commitment,	MM	is	flexible enough	to	foster	searching	for	mechanisms,	whatever	the	ontic	signature	of	the world	might	be. 4.	Newton	Revisited How	does	all	this	get	connected	to	Newton's	critique	of	the	Old	Mechanism?	In	a letter	to	Leibniz	dated	16	October	1693,	Newton	challenged	him	to	offer	a mechanical	explanation	of	"gravity	along	with	all	its	laws	by	the	action	of	some subtle	matter"	and	to	show	"that	the	motion	of	planets	and	comets	will	not	be disturbed	by	this	matter".	If	this	were	available,	Newton	said,	he	would	be	"far from	objecting".	But	no	such	explanation	was	forthcoming	and	Newton	was happy	to	re-iterate	his	view	that since	all	phenomena	of	the	heavens	and	of	the	sea	follow	precisely,	so	far	as	I	am	aware, from	nothing	but	gravity	acting	in	accordance	with	the	laws	described	by	me;	and	since nature	is	very	simple	[...]	all	other	causes	are	to	be	rejected	(Newton	2004,	108-109). Newton	does	not	simply	say	that	causal	explanation	might	not	be mechanical.	His	point	is	that	causal	explanation	should	be	liberated	from mechanism.	It'd	not	be	enough	to	offer	a	mechanical	account	of	the	cause	of gravity;	the	laws	that	gravity	obeys	should	be	mechanically	explicable	and,	as Newton	repeatedly	stressed,	this	was	not	forthcoming.	Though	causal 18 explanation	matters,	it	doesn't	matter	if	it	is	subject	to	various	(old)	mechanical constraints. We	noted	already	that	the	new	mechanical	conception	of	nature	is	far from	the	seventeenth	century	conception	that	everything	should	be	accounted for	in	terms	of	(configurations	of)	matter	in	motion.	So	it's	far	from	us	to	tar	New Mechanism	with	the	same	brush	as	Old	Mechanism.	For	instance,	the	key ontology	of	the	old	mechanical	picture	was	justified,	by	and	large	a	priori, whereas	the	key	ontology	of	New	Mechanism	is	grounded	in	scientific	practice; in	this	case,	it	is	practice	that	constrains	metaphysics.	Be	that	as	it	may,	we	are now	going	to	argue	that	there	exists	a	kind	of	Newtonian	move	against	New Mechanism	too. What	is	clear	from	the	present	discussion	is	that,	regardless	of	the	main difference	noted	above,	the	new	idea	of	mechanism	is	no	less	metaphysically loaded	than	the	old	one.	Where	the	seventeenth	century	mechanists	looked	for stable	arrangements	of	matter	in	motion	subject	to	laws,	the	twenty-first	century mechanists	look	for	stable	arrangements	of	powerful	entities	engaged	in	various activities	and	interactions.	These	mechanisms	are	supposed	to	be	the	building blocks	of	nature	and	the	scientific	task	is	to	unravel	them.	They	underpin "mechanistic	explanations"	which,	as	Glennan	put	it,	show	"how	the	organized activities	and	interactions	of	some	set	of	entities	cause	and	constitute	the phenomenon	to	be	explained"	(2017,	223).	Mechanistic	explanation	"always involves	characterizing	the	activities	and	interactions	of	a	mechanism's	parts" (2017,	223).	Where	the	seventeenth	century	mechanists	saw	'action	by	contact' as	a	requisite	for	a	proper	mechanical	explanation,	new	mechanists	see	powers and	'activities'. Why	is	Newton's	key	thought	relevant	to	the	modern	debates	about mechanisms?	The	key	thought,	to	repeat,	was	that	causal	explanation	should identify	causes	and	the	laws	that	govern	their	action	irrespective	of	whether	or not	these	causes	can	be	taken	to	satisfy	further	(mostly	metaphysically	driven) constraints.	In	other	words,	Newton	showed	that	certain	causal	explanations	of phenomena	(in	terms	of	non-mechanical	forces)	are	both	legitimate	and complete	insofar	as	they	identify	the	right	causes	and	are	empirically	grounded. We	take	it	that	the	point	MM	stresses,	is,	mutatis	mutandis,	analogous	to Newton's.	The	point	of	MM	is	that	causal	explanation	need	not	be	mechanistic	in the	new	mechanists'	ontic	sense,	and	that	being	couched	in	the	way	new mechanists	propose,	causal	explanation	is	subjected	to	constraints	unwarranted by	scientific	practice.	Insofar	as	mechanism	is	a	concept-in-use	in	science,	it	may well	be	seen	referring	to	the	causal	pathway	of	the	phenomenon	to	be	explained, couched	in	the	language	of	theories.	Preserving	the	spirit	of	Newton's	key thought,	we	might	say	that	causal	explanation	is	legitimate	even	if	we	bracket the	issue	of	"what	mechanisms	or	causes	are	as	things	in	the	world"	(Glennan 2017,	12)	or	the	issue	of	what	activities	are	and	how	they	are	related	to	powers and	the	like.	The	issue	then	is	not	"an	ontological	characterization	of	what mechanisms	are	as	things	in	the	world"	(2017,	19),	but	a	methodological characterisation	of	them	as	causal	pathways	described	in	the	language	of theories. To	press	the	analogy	a	bit	more,	questions	such	as: If	entities,	activities,	and	the	mechanisms	they	constitute	are	compounds,	of	what	are	they compounded?	Where	does	one	entity	or	activity	or	mechanism	end,	and	when	does 19 another	begin?	And	on	what	account	do	we	decide	that	a	collection	of	interacting	entities is	to	count	as	a	whole	mechanism?	(Glennan	2017,	29) are	pretty	much	like	the	questions	concerning	the	cause	of	the	properties	of gravity	that	Newton	thought	need	not	be	asked	and	answered	for	a	scientifically legitimate	conception	of	causal	explanation. We	don't	want	to	claim	that	questions	such	as	the	above	are	not connected	to	scientific	practice.	After	all,	even	the	question	of	the	cause	of gravity	that	Newton	refrained	from	answering,	was	connected	to	scientific practice.	The	point,	rather,	we	take	from	Newton	is	that	answering	these questions	is	not	required	for	offering	adequate	causal	explanations	of	the phenomena	under	study.	Similarly,,	for	MM,	answering	questions	such	as	the above	is	not	required	in	order	to	have	legitimate	mechanistic	explanations.	In other	words,	the	properties	of	mechanism	over	and	above	those	that	are required	by	its	methodological	use	need	not	be	specified;	nor	is	there	an explanatory	lacuna	if	they	are	not. According	to	MM,	the	concept	of	mechanism	as	used	in	practice	need	not, and	should	not,	be	understood	in	a	metaphysically	inflated	sense.	Hence,	new mechanists,	in	offering	such	metaphysically	inflated	accounts,	need	to	show	that such	accounts	are	indeed	indispensable	for	doing	good	mechanistic	science. To	conclude,	as	Newton	remained	agnostic	about	the	underlying mechanism	of	gravity,	so	MM	remains	agnostic	about	the	metaphysical	ground	of any	particular	causal	pathway.	As	in	the	case	of	gravity,	it	is	enough	that mechanisms	qua	causal	pathways	really	exist	and	act	as	they	do. 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