In	Paul	Lodge	and	Tom	Stoneham,	eds.,	Locke	and	Leibniz	on	Substance	(Oxford: Routledge,	2015).	Penultimate	version. Donald	L.	M.	Baxter University	of	Connecticut January	6,	2014 Hume	on	Substance:	A	Critique	of	Locke The	ancient	theory	of	substance	and	accident	is	supposed	to	make	sense	of	complex unities	in	a	way	that	respects	both	their	unity	and	their	complexity.	On	Hume's	view such	complex	unities	are	only	fictitiously	unities.	This	result	follows	from	his thoroughgoing	critique	of	the	theory	of	substance. I	will	characterize	the	theory	Hume	is	critiquing	as	it	is	presented	in	Locke. My	reading	of	Locke	will	follow	Jonathan	Bennett	in	presupposing	what	he	calls	the "Leibnizian	interpretation"	of	the	relevant	texts.1	Locke	uses	the	word	'substance'	in two	senses.	In	one	sense,	an	individual	or	mass	such	as	"a	Man,	Horse,	Gold,	Water, etc."	is	a	substance.	In	another	sense	a	substance	is	the	principle	of	unity	and identity	for	an	individual	in	which	its	accidents	such	as	"Colour	or	Weight"	inhere.	It is	a	"Substratum,	"	a	"pure	Substance	in	general"	(ECHU	2.23.1-4).	Call	substance	in the	first	sense	"individual	substance"	and	in	the	second	sense	"pure	substance."2	In the	New	Essays	Leibniz	locates	both	senses	in	Locke.3 1	Bennett	1987:	197.	Ayers'	alternative	interpretation	is	motivated	by	his assumption	that	the	Leibnizian	interpretation	takes	Lockean	substratum	to	be	like prime	matter.	I	find	this	a	problematic	assumption.	Substratum	is	meant	to	be	a principle	of	unity;	something	divisible	like	prime	matter	would	need	a	principle	of unity.	Ayers	1977:	78. 2	Locke	tends	rather	to	use	"Substance	in	general"	and	"particular	sorts	of Substances"	for	the	two	senses,	but	I	find	his	terminology	less	clear	and	more cumbersome. 3	NE	145-6,	218. 2 Hume's	view	in	the	Treatise	is	that	pure	substance	is	a	fiction,	as	is	complex individual	substance.	Only	simple	individual	substances	are	real.	Surprisingly, however,	they	turn	out	to	be	the	so-called	accidents	that	were	supposed	to	inhere	in a	pure	substance,	namely	qualities	and	perceptions. I	will	discuss	the	seven	main	parts	of	Hume's	view:	(I)	that	we	have	no	idea	of pure	substance,	(II)	that	there	is	no	complex	individual	substance,	except	in	a	loose sense,	(III)	that	the	fiction	of	complex	individual	substance	arises	in	a	way	parallel	to that	of	the	fiction	of	identity	through	time,	and	(IV)	results	in	the	fiction	of	pure substance,	(V)	that	simple	qualities	and	perceptions	satisfy	the	definition	of individual	substance,	(VI)	that	there	is	no	such	thing	as	inherence,	and	(VII)	that there	is	no	such	thing	as	pure	substance. Hume's	views	on	substance	are	often	mentioned	without	being	discussed	in detail.	Kemp	Smith,	Stroud,	and	Garrett,	for	example,	mostly	summarize	various claims	of	Hume	in	the	course	of	expounding	on	his	theory	of	the	idea	of	personal identity.4	In	contrast,	I	will	attempt	to	present	a	systematic	treatment	of	Hume	on substance. I.	No	idea	of	pure	substance Locke	holds	that	an	individual	substance	consists	of	a	pure	substance	plus	the accidents	that	inhere	in	the	pure	substance.	Thus	the	idea	of	an	individual	substance is	a	complex	idea	consisting	of	an	idea	of	a	pure	substance	plus	ideas	of	its	accidents. "The	Ideas	of	Substances	are	such	combinations	of	simple	Ideas,	as	are	taken	to represent	distinct	particular	things	subsisting	by	themselves;	in	which	the 4	Kemp	Smith	1941:	Ch.	23,	Stroud	1977:	Ch.	6,	Garrett	1997:	Ch.	8. 3 supposed,	or	confused	Idea	of	[pure]	Substance,	such	as	it	is,	is	always	the	first	and chief"	(ECHU,	2.12.6). Hume,	following	Locke,	takes	the	accidents	of	an	individual substance	to	be	its	qualities.	When	Hume	denies	that	we	have	an	idea	of	substance, he	is	not	denying	that	we	have	an	idea	of	a	collection	of	qualities	(THN	1.4.3.5).	Thus it	must	be	the	idea	of	pure	substance	that	he	is	questioning.	He	finds	that	we	can have	no	impression	from	which	such	an	idea	could	be	copied.	His	objection	echoes Berkeley's:5 1.	All	we	have	impressions	of	are	at	root	either	sensory	qualities	or	passions. 2.	Qualities	and	passions	are	accidents,	on	the	substance/accident	theory. 3.	Therefore,	we	cannot	have	an	impression	of	pure	substance. 4.	Therefore,	by	the	Copy	Principle,	we	have	no	idea	of	pure	substance. Hume	later	renews	this	objection,	making	clear	the	Berkeleyan	presupposition	that ideas	represent	by	resembling.	Since	an	impression	is	nothing	like	a	pure	substance, it	cannot	represent	a	pure	substance.	Therefore,	there	is	no	impression	of	a	pure substance,	and	so	no	idea	of	one.6	With	this	claim	Locke	would	agree:	"So	that	of [pure]	Substance,	we	have	no	Idea	of	what	it	is,	but	only	a	confused	obscure	one	of what	it	does"	(ECHU,	2.13.19). II.	No	complex	individual	substance As	for	individual	substance,	the	definition	that	Hume	uses	is	"something	which	may exist	by	itself"	(THN	1.4.5.5).	This	accords	with	Locke's	assumption	that	the	pure substance	is	for	the	accidents	the	"Cause	of	their	Union,	as	makes	the	whole	subsist 5	THN	1.1.6.1.	PHK	I.8. 6	THN	1.4.5.3. 4 of	itself,"	whereas	the	accidents	cannot	"subsist	by	themselves"	(ECHU,	2.23.6, 2.23.1). When	appealing	to	an	argument	by	Malezieu	during	his	discussion	of	infinite divisibility,	Hume	concludes,	"But	the	unity,	which	can	exist	alone,	and	whose existence	is	necessary	to	that	of	all	number,	is	of	another	kind,	and	must	be	perfectly indivisible,	and	incapable	of	being	resolv'd	into	any	lesser	unity"	(THN	1.2.2.3,	my emphasis).	So	something	that	can	exist	by	itself	is	perfectly	indivisible.	So	there	are no	complex	individual	substances,	only	simple	ones.	I	will	explain	this	argument. Radical	as	the	conclusion	is,	it	is	not	quite	as	radical	as	it	sounds.	There	will	be	a loose	sense	in	which	complex	individual	substances	exist. When	arguing	that	some	parts	of	space	are	simple	Hume	adds	"another argument	propos'd	by	a	noted	author,	which	seems	to	me	very	strong	and	beautiful" (THN	1.2.2.3).	It	is	based	on	the	following	passage	in	Malezieu: [9]	Moreover,	when	I	carefully	consider	the	existence	of	things,	I	very clearly	understand	that	existence	pertains	to	units	and	not	to	numbers.	I	shall explain	my	meaning. [10]	Twenty	men	exist	only	because	each	man	exists;	number	is	only an	extrinsic	denomination,	or	better,	a	repetition	of	units	to	which	alone existence	pertains.7	(my	emphasis) 7	Malezieu	(1705),	in	a	section	of	Book	9	entitled	"Réflexions	sur	les incommensurables."	The	translation	is	from	Ryan	2012:	110. 5 Only	units	exist.	A	number	of	things	is	not	a	unit,	but	rather	is	many	repeated	units. Consequently	a	number	of	things	does	not	strictly	speaking	exist,	except	insofar	as the	repeated	units	each	exist.	Hume	puts	it	this	way: 'Tis	evident,	that	existence	in	itself	belongs	only	to	unity,	and	is	never applicable	to	number,	but	on	account	of	the	unites	[units],	of	which	the number	is	compos'd.	Twenty	men	may	be	said	to	exist;	but	'tis	only	because one,	two,	three,	four,	&c.	are	existent;	and	if	you	deny	the	existence	of	the latter,	that	of	the	former	falls	of	course.	(THN	1.2.2.3) From	this	insight,	a	Humean	argument	can	be	constructed. (1)	To	be	complex	is	to	have	numerically	distinct	parts. (2)	Anything	with	numerically	distinct	parts	is	many	things,	not	a	single thing. (3)	Only	single	things	really	exist. (4)	So	nothing	complex	really	exists. Hume	takes	premise	(1)	to	be	true	by	definition.	Premises	(2)	and	(3)	are	part	of	the Malezieu	view	that	Hume	endorses. One	might	try	to	resist	(1)	by	appealing	to	Armstrong's	claim	that	there	can be	complexes	that	lack	parts	but	have	constituents--the	key	difference	being	that	no two	complexes	can	have	all	the	same	parts,	whereas	two	distinct	complexes	can have	all	the	same	constituents.8	However,	Hume's	argument	uses	'parts'	in	the	most general	sense,	so	the	argument	would	be	the	same	even	if	"parts	or	constituents" were	substituted	for	"parts"	in	(1)	and	(2).	The	key	is	the	numerical	distinctness	of 8	Armstrong	1991:	189-90. 6 the	composing	things,	not	whether	composition	is	extensional.	In	any	event,	Locke could	not	agree	that	a	complex	substance	has	only	constituents	in	Armstrong's sense,	since	one	of	the	constituents	would	be	the	pure	substance,	i.e.,	the	principle	of unity	and	identity.	If	two	complexes	had	all	the	same	constituents	including	the same	pure	substance,	then	they	would	be	identical	and	not	two. Understanding	the	full	generality	of	(2)	requires	looking	at	the	examples	that Hume	gives:	(i)	"any	determinate	quantity	of	extension,"	(ii)	"twenty	men,"	(iii)	"the whole	globe	of	the	earth,"	(iv)	"the	whole	universe."	We	may	be	tempted	to	think that	(ii)	and	perhaps	(iv)	are	pluralities	as	opposed	to	(i)	and	(iii)	which	are	wholes with	parts.	Locke	would	seem	to	be	thinking	this	way	when	he	distinguishes	single substances	from	collective	substances	(ECHU,	2.12.6).	However,	Hume	discusses	all of	these	as	examples	of	things	whose	unity	is	"merely	a	fictitious	denomination,"	and makes	no	distinction	between	them.	They	are	all	cases	of	"number"	composed	of "unites"	(THN	1.2.2.3).	Some	confirmation	is	provided	in	the	Dialogues	where Cleanthes	says	"the	uniting	of	these	parts	into	a	whole,	like	the	uniting	of	several distinct	countries	into	one	kingdom,	or	several	distinct	members	into	one	body,	is performed	merely	by	an	arbitrary	act	of	the	mind,	and	has	no	influence	on	the nature	of	things"(DNR	9).	Hume	assimilates	wholes	to	mere	pluralities. Note	that	by	'plurality'	I	mean	several	things	collectively,	not	some	single thing	that	is	somehow	made	up	of	them.	A	way	to	understand	Hume's	assimilation	is to	try	to	see	the	strangeness	of	the	contemporary	view	that	the	whole	is	a	single thing	in	addition	to	its	parts.	On	that	view	to	hold	a	six-pack	of	beer	is	automatically to	hold	seven	items--the	six	cans	plus	the	six-pack--even	neglecting	the	plastic 7 yoke.9	But	it	would	seem	that	the	six-pack	just	is	the	six	cans	and	nothing	in addition.	If	so,	then	the	six-pack	is	really	a	plurality,	and	not	a	single	thing	at	all.	10 An	obstacle	to	understanding	Hume's	view	is	our	penchant	for	confusing united	with	unitary.	United	things	are	many	things	in	some	relation.	A	unitary	thing is	a	single	thing.11	When	Hume	talks	of	unity	or	being	one,	he	is	talking	of unitariness	or	being	unitary. Premise	(3)	is	a	version	of	the	ancient	tenet	that	being	and	unity	are convertible.12	Leibniz	thought	it	could	be	seen	to	be	true	just	by	paying	attention	to a	shift	in	emphasis:	"That	what	is	not	truly	one	entity	is	not	truly	one	entity	either."13 Note	that	the	use	of	a	grammatically	singular	expression	to	refer	to	many	things collectively	(e.g.	'plurality',	'multitude',	'aggregate'),	does	not	entail	that	a	plurality, say,	really	exists.	However	referred	to,	they	are	many	things,	strictly	speaking. One	might	think	that	in	his	version	of	the	Malezieu	argument,	Hume	meant only	that	wholes	depend	on	each	and	all	their	parts,	not	that	wholes	are	their	parts collectively.14	However	such	a	dependence	between	distinct	things,	a	whole	and	a part,	contravenes	Hume's	central	principle	that	there	are	no	necessary	connections 9	Even	David	Lewis,	who	argues	that	mereological	composition	is	analogous	to numerical	identity,	still	takes	a	mereological	sum	to	be	something	in	addition	to	its parts,	numerically	distinct	from	each	(1991:	81-87). 10	I	argue	that	to	have	the	six-pack	be	genuinely	a	single	thing	as	well,	would	require that	existence	as	a	single	thing	be	relativized	to	what	I	call	"counts."	See	Baxter 1988. 11	Cf.	Hume's	observation	that	we	tend	to	confuse	an	identical	object	with	a succession	of	related	objects,	even	though	"this	to	an	accurate	view	affords	as perfect	a	notion	of	diversity,	as	if	there	was	no	manner	of	relation	among	the objects"	(THN	1.4.6.6). 12	Aristotle	1941:	Metaphysics,	XI.3,	1061a16. 13	LA	121.	See	also	NE	146	and	Baxter	1995. 14	Garrett	2009:	437. 8 between	distinct	things;	distinct	things	are	separable.15	I	will	return	to	this	point	in section	VI. Interpreting	Hume	as	denying	the	existence	of	complex	individuals	is	not quite	as	radical	as	it	sounds.	It	is	certainly	not	an	interpretation	to	be	refuted	by appealing	to	Hume's	references	to	complex	individuals.	For	instance,	Hume	says, "For	as	the	compound	idea	of	extension,	which	is	real,	is	compos'd	of	such	ideas	[i.e. simple	ones];	were	these	so	many	non-entities,	there	wou'd	be	a	real	existence compos'd	of	non-entities;	which	is	absurd"	(THN	1.2.3.14).	Here	the	complex individual	in	question	is	an	idea--the	compound	idea	of	extension.	Here	Hume	says that	this	idea	is	real.	So,	the	objection	goes,	he	thinks	at	least	one	complex	individual is	real.	So	it	exists,	contra	my	interpretation.16 However,	Hume	has	already	explained	in	the	Malezieu	passage	what	to	make of	references	to	complex	individuals.	Twenty	men	"may	be	said	to	exist"	if	each	of the	twenty	exists.	Though	saying	that	a	plurality	exists	is	strictly	false,	saying	it	can be	appropriate	if	this	member	of	the	plurality	exists	and	that	one	exists	and	that	one exists,	etc.	and	if,	perhaps,	these	many	are	related	in	some	salient	way.	The	same goes	for	any	purportedly	complex	individual.	In	taking	it	to	exist,	the	several individuals	composing	it	"may	be	consider'd	as	an	unite."	But	Hume	is	clear	that	the unity	is	fictitious.	"That	term	of	unity	is	merely	a	fictitious	denomination,	which	the mind	may	apply	to	any	quantity	of	objects	it	collects	together;	nor	can	such	an	unity any	more	exist	alone	than	number	can,	as	being	in	reality	a	true	number"	(THN 15	THN	1.1.7.3,	1.4.5.5. 16	I'm	grateful	to	Jani	Hakkarainen	for	this	objection. 9 1.2.2.3).	Its	unity	is	fictitious,	but	given	the	fiction	we	may	appropriately	say	of	the complex	individual	that	it	exists. Take,	for	example,	Hume's	query, On	the	contrary,	does	he	not	evidently	perceive,	that	from	the	union	of these	points	there	results	an	object,	which	is	compounded	and divisible,	and	may	be	distinguish'd	into	two	parts,	of	which	each preserves	its	existence	distinct	and	separate,	notwithstanding	its contiguity	to	the	other?	(THN	1.2.4.6) An	object	"may	be	said	to	exist"	if	what	are	regarded	as	its	parts	are	contiguous. In	the	same	way,	even	a	complex	substance	"may	be	said	to	exist"	as	in	the following	passage: I	have	already	prov'd,	that	we	have	no	perfect	idea	of	substance;	but that	taking	it	for	something,	that	can	exist	by	itself,	'tis	evident	every perception	is	a	substance	and	every	distinct	part	of	a	perception	a distinct	substance.	(THN	1.4.5.24) On	the	assumption	that	a	substance	is	unitary,	this	passage	appears	to	commit Hume	to	unitary	perceptions	having	unitary	perceptions	as	parts.	However,	Hume	is explicitly	rejecting	that	assumption	in	this	passage.	His	talk	of	proving	"that	we	have no	perfect	idea	of	substance"	refers	back	to	"Of	the	antient	philosophy"	where	he argues	that	in	our	idea	of	a	substance	we	only	feign	its	unity.17	Nonetheless	a complex	substance,	such	as	a	complex	perception,	"may	be	said	to	exist."	Having done	that,	the	complex	perception	meets	the	criterion	of	a	substance:	ability	to	exist 17	THN	1.4.3.5.	See	my	section	III. 10 by	itself.	That	is,	the	many	perceptions	taken	collectively	that	are	"it"	can	exist without	any	other	thing.	And,	of	course,	so	can	each	of	the	many	perceptions	taken individually. All	Hume's	references	to	complex	individuals	are	thus	consistent	with	his Malezieunian	conclusion	that	nothing	complex	really	exists,	strictly	speaking.	It follows	that	a	Lockean	individual	substance	which	is	a	complex	of	pure	substance plus	accidents	does	not	exist,	strictly	speaking. III.	The	fiction	of	complex	individual	substance For	Hume	the	idea	of	a	purported	individual	substance	is	really	just	a	collection	of ideas	of	various	qualities.	For	instance	the	idea	of	a	peach	is	just	the	ideas	of	its color,	taste,	figure,	solidity,	etc.18	And	for	Hume	there	is	no	real	unity	or	existence	to such	collections	other	than	the	individual	existences	of	their	members.	How	does the	fiction	arise	that	such	collections	of	qualities	are	unitary	individuals? The	question	would	seem	to	apply	as	well	to	Locke's	view.	He	speaks	of	the several	ideas	of	accidents	used	collectively	to	think	of	an	individual	substance, "which	by	inadvertency	we	are	apt	afterward	to	talk	of	and	consider	as	one	simple Idea,	which	is	indeed	a	complication	of	many	Ideas	together"	(ECHU,	2.23.1).	Given that	Locke	often	uses	'idea'	to	refer	to	qualities	qua	objects	of	thought,	there	is	the same	problem	of	what	makes	us	take	many	qualities	to	instead	be	a	unitary individual. Even	though	a	whole	is	just	a	plurality	that	does	not	strictly	speaking	exist, the	many	individuals	can	be	considered	as	a	unit	and	so	the	purported	whole	can 18	THN	1.4.3.5. 11 appropriately	be	said	to	exist.	Presumably	the	degree	to	which	they	are	related	is reflected	in	the	degree	to	which	the	mind	tends	to	consider	them	a	unity.19	Hume explores	these	degrees	in	his	discussion	of	the	idea	of	personal	identity,	moving from	a	mass	of	matter,	to	an	assemblage	for	some	purpose,	to	an	organic	unity.20	In "Of	the	antient	philosophy"	he	explains	how	the	relations	between	several	qualities may	be	so	strong	that	the	mind	is	first	moved	to	attribute	simplicity	to	the	qualities taken	collectively,	and	then	is	moved	to	feign	a	pure	substance	to	justify	this attribution.	He	compares	this	tendency	to	our	tendency	to	attribute	identity	to	a succession	of	related	objects.21 There	is	some	temptation	to	think	that	sometimes	by	'simplicity'	Hume	just means	identity	at	a	time.	However,	as	we	shall	see,	it	is	what	we	might	call simplicity-in-complexity	that	parallels	Hume's	account	of	identity	through	time,	not just	simplicity	itself.	Simplicity	is	just	unitariness,	not	a	medium	betwixt	unity	and number	as	identity	must	be.22 Let	me	first	summarize	Hume's	account	of	the	idea	of	identity	through	time, then	describe	its	application	to	a	succession	of	related	objects.	Then	I	can	bring	out the	parallels	in	his	account	of	attributing	simplicity	to	a	plurality. 19	Cf.	LA	121. 20	THN	1.4.6.8-12. 21	THN	1.4.3.5	and	its	summary	at	THN	1.4.6.22. 22	Only	non-complex	things	are	unitary	(THN	1.2.2.3).	To	suppose	something	to	be simple	is	to	suppose	it	to	be	"ONE	thing"	(1.4.3.2).	A	couple	of	places	where simplicity	is	unitariness,	though	only	in	some	respect,	is	THN	1.1.7.18,	where	Hume talks	of	the	simplicity	of	figure	and	body	figured	(even	though	the	body	itself	is complex)	and	THN	1.4.5.14	where	Hume	says	impressions	of	touch	are	simple (except	when	considered	with	regard	to	their	extension).	I'm	grateful	to	Jani Hakkarainen	for	discussion. 12 For	Hume,	the	idea	of	identity	is	the	idea	of	"the	invariableness	and uninterruptedness	of	any	object	thro'	a	suppos'd	variation	of	time"	(THN	1.4.2.30). To	be	invariable	and	uninterrupted	is	to	be	steadfast,	where	something	steadfast	is not	a	succession	of	things.	To	have	duration	in	time	is	to	be	a	succession.23	So	the idea	of	identity	is	the	fiction	of	being	somehow	both	a	succession	and	not	a succession.	Hume	brings	in	successiveness	and	steadfastness	as	surrogates	for	being many	distinct	things	and	being	one	single	thing.	Identity,	for	Hume,	is	essentially	a combination	of	being	one	thing	and	being	many	things.	It	is	essentially	identity-indifference,	as	the	British	Idealists	put	it.24	We	heirs	of	Frege	are	dismissive	of	this notion,	but	we	shouldn't	be.	Hume's	account	of	identity	is	meant	to	respond	to	a fundamental	problem	with	identity	that	Fregeans	overlook--a	problem	I've	termed "Hume's	Difficulty."25	The	problem	is	how	we	can	represent	there	as	being something	that	is	perhaps	one	thing	and	perhaps	many	distinct	things.	The	problem is	raised	by	the	fact	that	we	can	be	unsure	of	an	identity.	We	can	meet	someone	and later	meet	someone	and	be	unsure	whether	or	not	they	are	identical.	Furthermore we	can	imagine	them	either	way.	This	ability	to	alternate	between	thinking	of	them as	one	person	and	thinking	of	them	as	two	distinct	persons	is	what	Hume	is	trying	to capture	in	his	idea	of	identity.	He	calls	it	an	idea	of	a	"medium	betwixt	unity	and 23	Thus	for	Hume	there	are	two	ways	to	take	up	time:	being	a	steadfast	object	and being	a	succession	with	duration.	See	Baxter	2008:	30-31. 24	See	Bradley	1897:	Chs.	2	and	3,	Appendix Note	C;	Bradley	1928:	Bk.	1,	Ch.	5,	Secs. 1-9. 25	Baxter	2008:	54,	83. 13 number."	More	accurately	it	is	an	idea	that	alternates	between	being	a	thought	of something	as	one	single	thing	and	as	many	distinct	things.26 Once	one	has	the	idea	of	identity,	it	is	often	applied	to	distinct	but	closely related	objects	in	succession.	Perhaps	they	resemble	or	are	causally	connected. One's	successive	ideas	of	them	are	thus	connected	by	the	principles	of	association	of ideas.	As	a	result,	thinking	of	the	succession	gives	the	same	feeling	to	the	mind	as	it has	when	contemplating	a	steadfast	object.	The	easy	transition	of	the	mind	between one	idea	and	the	next	feels	just	about	the	same	as	the	feeling	of	repose	when	the mind	continues	the	same	idea,	as	it	does	when	continuing	to	think	of	a	steadfast object.27	In	such	a	case	the	idea	of	identity	is	substituted	and	so	the	succession	in question	is	alternately	viewed	with	an	accurate	idea	of	it	as	many	distinct	things	and with	a	fictitious	idea	of	it	as	one	single	thing.28 Hume	goes	on	to	give	a	parallel	account	of	the	attribution	of	simplicity	to	a bundle	of	qualities.	The	relation	between	them	is	the	one	Locke	noted:	constantly occurring	together,	which	for	Hume	is	a	combination	of	contiguity	and	causation.29 Hume	says	that	the	action	of	the	mind	in	considering	a	simple	object	feels	very similar	to	that	of	contemplating	many	strongly	related	objects. 26	THN	1.4.2.29. 27	THN	1.4.2.33,	1.4.3.3,	1.4.6.16. 28	I	note	that	it	is	hard	to	reconcile	Hume's	characterization	of	numerical	identity	as a	medium	betwixt	unity	and	number	with	his	contention	that	numerical	identity	is the	opposite	of	difference	in	number	(THN	1.1.5.10).	I'm	grateful	to	Annemarie Butler	for	pointing	out	the	import	of	this	passage. 29	ECHU,	2.23.1.	THN	1.1.6.2,	1.4.5.12.	At	THN	1.4.5.12	Hume	uses	'inseparability'	to refer	to	the	relation	of	cause	and	effect,	but	he	does	not	mean	inseparability	in	the metaphysical	sense	that	by	his	Real	Separability	Principle	is	equivalent	to	numerical identity.	He	has	already	made	clear	that	cause	and	effect	are	not	inseparable	in	this sense	(THN	1.3.3.3). 14 We	entertain	a	like	notion	with	regard	to	the	simplicity	of	substances, and	from	like	causes.	Suppose	an	object	perfectly	simple	and indivisible	to	be	presented,	along	with	another	object,	whose	coexistent	parts	are	connected	together	by	a	strong	relation,	'tis	evident the	actions	of	the	mind,	in	considering	these	two	objects,	are	not	very different.	(THN	1.4.3.5) For	Hume	the	idea	of	simplicity	does	not	have	the	complexity	of	the	idea	of	identity. A	simple	thing	is	just	one	single	thing,	and	regarding	it	as	simple	is	to	regard	it	as	a single	thing.	There	is	no	alternation	between	incompatible	ideas	in	the	idea	of simplicity.	However,	in	the	case	at	hand,	Hume	cannot	mean	that	the	idea	of simplicity	is	simply	substituted	for	the	idea	of	complexity	when	contemplating	the many	strongly	related	things.	For	he	says,	"But	the	mind	rests	not	here."	The	mind goes	on	to	be	struck	by	the	multiplicity	of	the	objects	it	is	trying	to	regard	as	one simple	thing.	"Whenever	it	views	the	object	in	another	light,	it	finds	that	all	these qualities	are	different,	and	distinguishable,	and	separable	from	each	other."	So	it feigns	an	unknown	something	to	justify	the	idea	of	the	bundle's	unity-something that	"may	give	the	compound	object	a	title	to	be	call'd	one	thing,	notwithstanding	its diversity	and	composition"	(THN	1.4.3.5).	This	feigned,	unknown	something	is	pure substance. If	this	account	is	meant	to	be	parallel	to	the	account	of	substituting	the	idea of	identity	for	the	idea	of	many	things	in	succession,	then	Hume	must	mean	that, rather	than	simply	substituting	the	idea	of	simplicity,	we	alternate	between	the	idea of	simplicity	and	the	idea	of	complexity.	He	could	not	mean	anything	else.	Several 15 coexistent	things	are	too	obviously	different	from	something	simple.	Take	a	peach or	a	melon	and	contrast	it	with	a	spatially	minimal	point.	The	latter	is	simple.	The former	obviously	is	not,	and	it	would	be	difficult	blithely	to	let	an	idea	representing the	peach	as	simple	to	be	all	there	is	to	it.	The	causes	of	substituting	the	idea	of simplicity	are	not	strong	enough,	apparently,	to	just	give	up	on	the	evident complexity.	The	only	recourse	is	to	alternate	between	the	idea	of	simplicity	and	the idea	of	complexity	just	as	happens	in	the	idea	of	identity	through	time.	In	the present	case	this	would	also	be	an	alternation	between	a	simple	idea	representing the	peach	as	one	single	thing	and	a	collection	of	ideas	representing	the	peach	as many	distinct	things.	So	the	result	would	be	another	version	of	an	idea	of	a	medium betwixt	unity	and	number.	If	the	idea	of	identity	through	time	were	a	way	of addressing	Hume's	Difficulty,	then	this	idea	of	simplicity	amidst	complexity	would be	as	well. IV.	The	fiction	of	pure	substance When	we	first	acquire	the	idea	of	identity,	it	is	from	the	contemplation	of	a	steadfast object.	We	have	a	habit	of	regarding	everything	to	have	duration	and	can't	help	but try	to	regard	the	steadfast	object	as	having	it	even	though	it	manifestly	does	not.	The alternation	of	viewpoints	is	a	way	the	mind	tries	to	obscure	the	incompatibility	of attributing	being	many	to	something	manifestly	one.	That	the	object	is	manifestly one	is	lost,	however,	in	the	subsequent	application	of	the	idea	of	identity	to successions	of	related	objects.	It	is	in	such	cases	that	we	feign	some	cause	of unitariness	to	compensate	for	the	fact	that	the	objects	are	manifestly	many.	For 16 example,	in	the	case	of	the	fiction	of	personal	identity	we	feign	"a	soul,	and	self,	and substance"	(THN	1.4.6.6). The	same	thing	happens	in	the	case	of	taking	a	bundle	of	qualities	to	be	a single	individual.	The	qualities	are	manifestly	many,	but	to	justify	the	viewpoint	of them	as	one	and	unitary,	we	feign	a	pure	substance.	What	happens	in	the	mind when	we	are	feigning	a	pure	substance?	The	mind	represents	the	peach	as	many	via the	collection	of	ideas	of	its	qualities.	But	how	does	the	mind	represent	the	peach	as one	simple	thing?	Hume	doesn't	say.	My	guess	is	that	the	mind	uses	the	idea	of simplicity	in	general,	i.e.	uses	any	simple	idea	regarded	just	insofar	as	it	is	simple.30 So	the	strong	relation	between	the	qualities	leads	the	mind	to	attribute simplicity	to	them.	This	is	"considering	them	as	an	unite"	in	the	strongest	sense.	The manifest	multiplicity	of	the	qualities	puts	pressure	on	this	attribution,	which	leads to	the	fiction	of	an	unknown	and	unknowable	pure	substance	to	justify	it.	The	pure substance	is	meant	to	provide	the	deep	simplicity	of	a	manifest	multiplicity.	The result	is	the	fiction	of	a	complex	individual	substance	made	a	unity	by	a	pure substance. V.	Perceptions	and	Qualities	as	substances Since	there	are	strictly	speaking	no	complex	things	to	count	as	individual substances,	then	if	there	are	individual	substances	they	have	to	be	simple.	The definition	of	individual	substance	that	Hume	relies	on	is	again	"something	which	may exist	by	itself."	However,	Hume	says,	"this	definition	agrees	to	every	thing,	that	can possibly	be	conceiv'd;	and	never	will	serve	to	distinguish	substance	from	accident, 30	Likewise	in	feigning	the	fiction	of	a	soul	underlying	identity	through	time,	the mind	would	use	the	idea	of	steadfastness	in	general. 17 or	the	soul	from	its	perceptions"	(THN	1.4.5.5).	Among	the	things	we	can	conceive are	qualities	and	perceptions.	So	qualities	and	perceptions	are	individual substances,	by	that	definition.31 Here	Hume	directly	attacks	Locke's	assumption	about	"those	Qualities,	we find	existing,	which	we	imagine	cannot	subsist,	sine	re	substante,	without	something to	support	them"	(ECHU	2.23.2).	To	the	contrary,	Hume	argues	that	their independent	existence	is	easily	conceived. Hume	argues	this	to	be	the	case	with	a	compressed	argument	that	glosses over	a	number	of	distinctions	he	ought	to	be	making.	The	argument	makes	use	of	his Separability	Principle,	which	is	really	a	family	of	principles	that	should	be	teased apart.32 Hume	holds	that	all	things	numerically	distinct	are	separable	in	reality,	and conversely.	Let	me	call	this	principle,	the	Real	Separability	Principle.	Strictly speaking,	it	is	not	a	principle	for	Hume,	since	he	derives	it	from	two	others:	what	I will	call	the	Mental	Separability	Principle	and	the	Conceivability	Principle. Hume	states	the	Mental	Separability	Principle	early	on: First,	We	have	observ'd,	that	whatever	objects	are	different	are distinguishable,	and	that	whatever	objects	are	distinguishable	are separable	by	the	thought	and	imagination.	And	we	may	here	add,	that these	propositions	are	equally	true	in	the	inverse	and	that	whatever 31	See	THN	1.4.5.24	and	the	discussion	of	this	passage	in	my	section	II. 32	Garrett	speaks	simply	of	the	Separability	Principle	and	says	it	and	its	converse	are new	with	Hume	(1997:	58).	See	also	Bricke	1980:	68.	However,	see	Descartes's discussion	of	distinctions	in	Principles,	Part	I,	Principles	LX-LXII,	in	CSM:	160-212,	as well	as	medieval	and	ancient	antecedents	such	as	those	cited	in	Bosley	2006.	See Laird	against	the	mental	separability	principle	(1931:	82-83). 18 objects	are	separable	are	also	distinguishable,	and	that	whatever objects	are	distinguishable	are	also	different.	(THN	1.1.7.3) I	assume	from	context	that	by	"in	the	inverse"	Hume	means	what	we	mean	by 'conversely'. When	Hume	says	'different'	he	means	numerically	distinct.	For	instance,	in discussing	time,	he	argues	that	the	impression	of	time	that	one	has	in	hearing	five musical	notes	is	not	"different"	from	the	impressions	of	the	notes	themselves,	which is	to	say	it	is	"not	a	sixth	impression"	(THN	1.2.3.10).	Were	it	different,	it	would	be	a sixth	impression,	i.e.,	numerically	distinct.	Likewise	at	one	point	he	explicitly	uses the	phrase	'numerically	different'.33 When	Hume	says	that	things	are	"distinguishable,"	he	means	that	we	can think	of	them	as	numerically	distinct.	As	he	argues	in	his	passage	on	distinctions	of reason,	where	we	cannot	think	of	things	as	numerically	distinct,	they	are	"in	effect the	same	and	undistinguishable."	Any	apparent	distinguishing	of	"them"	is	really	a distinguishing	of	things	related	to	"them."34 When	Hume	says	objects	are	"separable	by	the	thought	and	imagination"	he means	that	it	is	possible	to	cease	thinking	of	one	while	continuing	to	think	of	the other.	Presumably	we	would	do	this	by	ceasing	to	have	an	idea	of	the	one	while continuing	to	have	an	idea	of	the	other.	When	things	are	inseparable	by	the	thought and	imagination	it	is	not	possible	to	think	of	one	while	ceasing	to	think	of	the	other. For	instance,	"A	person,	who	desires	us	to	consider	the	figure	of	a	globe	of	white 33	THN	1.3.1.1. 34	THN	1.1.7.18.	For	additional	discussion	of	less	than	numerical	distinction	see section	VI. 19 marble	without	thinking	on	its	colour,	desires	an	impossibility"	(THN	1.1.7.18).35 So	the	Mental	Separability	Principle	is	that	objects	are	numerically	distinct only	if	we	can	think	of	them	as	distinct,	and	we	can	think	of	them	as	distinct	only	if we	can	continue	to	think	of	one	while	ceasing	to	think	of	the	other. Presumably	we	mentally	separate	the	objects	of	thought	by	separating during	the	course	of	thinking	the	vehicles	of	thought--the	ideas.	For	example	we might	mentally	separate	a	cause	and	an	effect	by	separating	during	the	course	of thinking	the	idea	of	the	cause	from	the	idea	of	the	effect.	Assuming	that	this	is correct,	Hume	must	be	presupposing	another	proposition	about	separability:	if ideas	are	distinct	then	they	are	separable	in	the	course	of	thought.	Call	it	the	Idea Separability	Principle.	As	a	shortcut	in	reasoning,	he	will	sometimes	use	the	Idea Separability	Principle	interchangeably	with	the	Mental	Separability	Principle,	but strictly	speaking	they	should	be	distinguished. Note	that	to	retain	an	idea	of	one	thing	while	ceasing	to	have	an	idea	of	the other	is	to	clearly	think	of	the	first	thing	as	existing	without	the	other. The	other	main	principle	is	the	Conceivability	Principle.	Hume	says	that "nothing	of	which	we	can	form	a	clear	and	distinct	idea	is	absurd	and	impossible" (THN	1.1.7.6),	and	"	'Tis	an	establish'd	maxim	in	metaphysics,	that	whatever	the mind	clearly	conceives	includes	the	idea	of	possible	existence,	or	in	other	words,	that nothing	we	imagine	is	absolutely	impossible"	(THN	1.2.2.8),	and	"Whatever	is	clearly conceiv'd	may	exist;	and	whatever	is	clearly	conceiv'd,	after	any	manner,	may	exist 35	Cf.	Descartes	who	uses	the	phrases	"only	distinct	.	.	.	by	thought"	and	"differ	only in	thought."	The	Principles	of	Philosophy,	Part	I,	Principle	LXII	'Of	the	distinction created	by	thought'	in	CSM. 20 after	the	same	manner"	(THN	1.4.5.5),	and	"whatever	we	conceive	is	possible"	(THN 1.4.5.10).	Here	Hume	uses	a	principle	tracing	back	to	Bishop	Tempier's condemnations	in	1277,	that	the	clearly	conceivable	is	possible.36 Using	the	two	main	principles,	Hume	reasons	as	follows: 1. If	things	are	distinct,	then	we	can	think	of	them	as	distinct,	i.e.	can clearly	conceive	of	one	continuing	to	exist	without	the	other. 2.	The	clearly	conceivable	is	possible. 3.	So	the	one	can	continue	to	exist	without	the	other. In	other	words,	he	uses	the	Conceivability	Principle	to	infer	from	the	Mental Separability	Principle	a	result	about	real	separability-viz.,	that	any	numerically distinct	things	are	really	separable.	Call	this	result	the	Real	Separability	Principle.37 There	are	three	places	in	which	Hume	uses	this	reasoning	about	external	objects.38 The	use	that	concerns	us	here,	the	third,	occurs	in	his	discussion	of substance.	Unfortunately	he	adds	a	new	level	of	complexity	by	appealing	to	the mental	separability	of	ideas	themselves	as	objects	of	thought.	He	is	clear	that	he	is adding	this	new	level: As	we	conclude	from	the	distinction	and	separability	of	their	ideas, that	external	objects	have	a	separate	existence	from	each	other;	so when	we	make	these	ideas	themselves	our	objects,	we	must	draw	the 36	See	Bosley	and	Tweedale	1997:	editors'	introductions,	pp.	xx-xxi,	440-441. See	also	Grant	1982. 37	Note	that	the	converse	this	principle	follows	easily,	since	nothing	can	exist without	itself. 38	THN	1.2.5.3,	1.3.3.3,	and	1.4.5.5. 21 same	conclusion	concerning	them,	according	to	the	precedent reasoning.	(THN	1.4.5.27) He	here	starts	with	the	separability	of	ideas	as	vehicles	of	thought	when	thinking	of external	objects,	and	then	moves	via	an	implicit	appeal	to	the	mental	separability	of the	external	objects to	the	real	separability	of	the	external	objects.	Then	he endorses	the	same	reasoning	from	the	separability	of	ideas	as	vehicles	of	thought when	thinking	of	ideas	to	the	real	separability	of	those	ideas	being	thought	of.	He	is summarizing	what	he	argued	a	little	earlier:39 Whatever	is	clearly	conceiv'd	may	exist;	and	whatever	is	clearly conceiv'd,	after	any	manner,	may	exist	after	the	same	manner.	This	is one	principle,	which	has	been	already	acknowledg'd.	Again,	every thing,	which	is	different,	is	distinguishable,	and	every	thing	which	is distinguishable,	is	separable	by	the	imagination.	This	is	another principle.	My	conclusion	from	both	is,	that	since	all	our	perceptions are	different	from	each	other,	and	from	every	thing	else	in	the universe,	they	are	also	distinct	and	separable,	and	may	be	consider'd as	separately	existent,	and	may	exist	separately.	.	.	(THN	1.4.5.5) The	argument	goes	as	follows: 1. A	perception	is	numerically	distinct	from	everything	else	in	the universe. 39	The	same	reasoning	recurs	in	the	Appendix	to	the	Treatise:	"Whatever	is	distinct, is	distinguishable;	and	whatever	is	distinguishable,	is	separable	by	the	thought	or imagination.	All	perceptions	are	distinct.	They	are,	therefore,	distinguishable,	and separable,	and	may	be	conceiv'd	as	separately	existent,	and	may	exist	separately, without	any	contradiction	or	absurdity"	(THN	App.12). 22 2. So,	the	perception	is	mentally	separable	from	everything	else (by	separating	the	ideas	of	them	in	the	course	of	thought). 3. So	the	perception	is	really	separable	from	everything	else. 1.	is	given.	2.	follows	by	the	Mental	Separability	Principle.	3.	then	follows	by	the Conceivability	Principle. It	is	important	in	understanding	this	argument	to	distinguish	the	real separability	of	perceptions	from	the	idea	separability	of	perceptions	as	vehicles	of thought.	In	this	argument,	Hume	is	proving	that	perceptions	are	substances.	They can	exist	independently	of	any	of	the	other	perceptions	making	up	the	mind	that they	are	in	(or	any	other	perceptions	for	that	matter),	and	so	can	exist independently	of	the	mind--the	bundle	of	perceptions--that	they	are	in	(and	of	any other	bundle). Now	as	every	perception	is	distinguishable	from	another,	and	may	be consider'd	as	separately	existent;	it	evidently	follows,	that	there	is	no absurdity	in	separating	any	particular	perception	from	the	mind;	that is,	in	breaking	off	all	its	relations,	with	that	connected	mass	of perceptions,	which	constitute	a	thinking	being.	(THN	1.4.2.39) So	a	detailed	understanding	of	Hume	requires	making	these	distinctions between	the	mental	separability	of	objects,	the	real	separability	of	objects,	the	idea separability	in	the	course	of	thought	of	the	ideas	used	in	thinking,	the	mental separability	of	ideas,	and	the	real	separability	of	ideas.	These	are	the	five	different ways	in	which	Hume	appeals	to	separability. The	argument	that	perceptions	are	substances	continues	as	follows: 23 4.	So	a	perception	can	exist	by	itself. 5.	So	by	definition	a	perception	is	a	substance. Hume	has	argued	that	perceptions	are	substances.	He	could	argue	in	just	the same	way	that	qualities	are	substances.	Any	quality	is	really	separable	from anything	that	it	is	numerically	distinct	from.	Therefore	it	can	exist	by	itself. Therefore	it	is	a	substance,	by	definition.	So	Hume	has	shown	that	what	Locke considers	mere	modifications	of	substances--perceptions	and	qualities--are substances	themselves. VI.	No	such	thing	as	inhesion Hume	continues	the	line	of	thought	about	perceptions	and	qualities	to	conclude	that they	do	not	inhere	in	anything.	"Inhesion	in	something	is	suppos'd	to	be	requisite	to support	the	existence	of	our	perceptions.	Nothing	appears	requisite	to	support	the existence	of	a	perception.	We	have,	therefore,	no	idea	of	inhesion"	(THN	1.4.5.6).	It follows	as	well	that	there	is	no	such	thing	as	inhesion. One	might	think	that	Hume	has	gone	too	fast.	He	has	argued	that	perceptions and	qualities	can	exist	independently	of	anything	they	are	numerically	distinct	from. That,	however,	does	not	show	that	they	don't	inhere	in	anything.	They	may	only	be modally	distinct	from	the	substances	they	inhere	in.	The	definition	of	substance	as "something	which	may	exist	by	itself"	should	be	understood	as	something	which	may exist	without	anything	it	is	numerically	distinct	from	and	without	anything	it	is modally	distinct	from. 24 Suarez	takes	the	modal	distinction	to	be	a	less	than	numerical	distinction involving	asymmetric	inseparability.	The	mode	is	inseparable	from	the	substance, but	not	vice-versa.40 Hume	does	seem	to	attribute	to	"theologians"	some	sort	of	less	than numerical	distinction	between	substance	and	modification.	Their	three	criticisms	of Spinoza	that	he	presents	depend	on	taking	differences	between	modes	to	apply	to the	undifferentiated	substance	that	they	depend	on.	That	is	fair	only	if	the	modes are	not	numerically	distinct	from	the	substance.	As	Hume	says,	according	to	"the scholastic	way	of	talking	.	.	.	a	mode,	not	being	any	distinct	or	separable	existence, must	be	the	very	same	with	its	substance	.	.	.	"41	So	the	supposed	distinction	between mode	and	substance	is	not	supposed	to	be	numerical	distinction. However,	Hume	would	take	the	notion	of	a	less	than	numerical	distinction	as untenable.	He	would	criticize	it	the	way	Ockham	criticized	Scotus's	formal distinction.	As	Ockham	puts	it,	"But	among	creatures	the	same	thing	cannot	be	truly affirmed	and	truly	denied	of	the	same	thing."	According	to	Ockham,	if	there	is	any difference	between	two	(created)	things	then	they	are	numerically	distinct.42	Hume does	not	state	this	principle	explicitly	but	appears	to	presuppose	it	when	he	says	the following	of	a	compound,	altering	body	that	we	regard	as	a	simple,	identical	thing: "The	acknowledg'd	composition	is	evidently	contrary	to	this	suppos'd	simplicity,	and the	variation	to	the	identity"	(THN	1.4.3.2).	Identical	things	cannot	differ.	As	we would	put	it,	he	holds	Leibniz's	Law. 40	Suarez	1947:	sect.	2,	no.	6,	p.	44.	See	also	Descartes,	Principles	I.LXI	in	CSM. 41	THN	1.4.5.22-25. 42	Ockham	exempts	things	pertaining	to	God,	such	as	the	Persons	of	the	Trinity. William	of	Ockham,	Ordinatio	I,	distinction	ii,	qu.	6,	in	Spade	1994:	156. 25 If	a	substance	is	independent	of	any	other	existent	whatsoever,	whereas	a mode	is	not,	then	they	differ.	If	they	differ	they	are	numerically	distinct.	So	they	are not	merely	modally	distinct. Confirmation	that	Hume	considered	and	rejected	any	sort	of	modal distinction	is	given	when	he	says,	"Our	perceptions	are	all	really	different,	and separable,	and	distinguishable	from	each	other,	and	from	every	thing	else,	which	we can	imagine;	and	therefore	'tis	impossible	to	conceive,	how	they	can	be	the	action	or abstract	mode	of	any	substance"	(THN	1.4.5.27). One	might	object	that	Hume	himself	gives	an	example	of	asymmetric inseparability.	A	whole	cannot	strictly	speaking	exist	without	having	all	its	parts. "But	supposing	some	very	small	or	inconsiderable	part	to	be	added	to	the	mass,	or substracted	from	it;	tho'	this	absolutely	destroys	the	identity	of	the	whole,	strictly speaking;	yet	as	we	seldom	think	so	accurately,"	etc.	(THN	1.4.6.8).	The	parts,	being distinct,	are	all	separable	from	each	other.	So	a	part	can	exist	without	the	whole.	So here	is	an	asymmetric	inseparability.	Hakkarainen	argues	that	this	asymmetric inseparability	forces	us	to	recognize	a	third	type	of	distinction	in	Hume.	In	addition to	real	distinctions	(i.e.	numerical	distinctions)	and	distinctions	of	reason	(i.e. conceptual	distinctions)	we	must	see	Hume	as	committed	to	what	Hakkarainen, following	David	Lewis,	calls	a	"partial	distinction."	This	partial	distinction	is	a special	version	of	the	modal	distinction	for	wholes	and	their	parts. Hakkarainen	is	candid	that	Hume	explicitly	countenances	no	such	third distinction.	However,	he	contends,	Hume	needs	it	to	resolve	a	glaring	contradiction 26 between	Hume's	Separability	Principle	and	the	fact	that	Hume	explicitly	says	that wholes	depend	on	their	parts.	43 I	think	there	is	a	better	way	to	go	that	is	truer	to	the	text.	Hume	has	said	that the	definition	of	a	substance--"something	which	may	exist	by	itself"--applies	to	"every thing,	that	can	possibly	be	conceiv'd"	(THN	1.4.5.5).	If	a	whole	can	be	conceived, then	it	can't	depend	on	its	parts.	It	must	be	able	to	exist	without	them.	So	a	whole	is not	a	thing	that	can	possibly	be	conceived.	As	Hume's	Malezieu	argument	concluded, a	whole	of	parts,	strictly	speaking,	does	not	exist.	"It"	is	nothing	but	the	many	parts, perhaps	in	some	relation.	Clearly	they	cannot	exist	collectively	unless	each	of	them exist.	But	each	can	exist	without	them	all	existing	collectively. These	facts	give	the appearance	of	asymmetric	inseparability	if	wholes	are	said	to	exist.	However,	there is	no	asymmetric	inseparability	between	things	that	strictly	speaking	exist. Thus	Hume	has	an	answer	to	the	charge	that	his	rejection	of	inhesion overlooked	the	possibility	of	a	modal	distinction	between	mode	and	substance.	So he	can	safely	conclude	that	inhesion	is	impossible. VII.	No	such	thing	as	pure	substance If	there	is	such	a	thing	as	a	pure	substance,	it	is	the	principle	of	unity	and identity	for	an	individual	in	which	accidents	inhere.	As	Locke	says,	it	is	"the supposed,	but	unknown	support	of	those	Qualities,	we	find	existing,	which	we imagine	cannot	subsist	.	.	.	without	something	to	support	them,"	and	which	is	the "Cause	of	their	Union"	in	an	individual	substance	(ECHU	2.23.2,	6). However,	first, nothing	can	be	a	principle	of	unity	and	identity	for	a	collection	of	distinct	qualities, 43	Hakkarainen	2012:	56-7. 27 nor	a	succession	of	them,	as	argued	in	II	and	III.	Adding	an	additional	thing	to several	distinct	things	does	not	result	in	unitariness.44	Second,	nothing	can	be something	in	which	something	that	differs	from	it	inheres,	since	there	is	no	such thing	as	inherence,	as	argued	in	VI.	There	is	no	dependence	between	numerically distinct	things	and	no	modal	distinction.	For	these	two	reasons	individually	and jointly,	there	is	no	such	thing	as	pure	substance. Appeal	to	a	principle	of	unity	and	identity	in	which	distinct	accidents	inhere is	the	way	that	substance/accident	theorists	like	Locke	try	to	have	complex	unities without	them	breaking	up	into	numerically	distinct	independent	existences. However	for	Hume,	the	breakup	is	unavoidable,	given	(i)	the	Convertibility	of	Unity and	Being,	(ii)	the	Real	Separability	principle,	and	(iii)	Leibniz's	Law.	From	there	it	is up	to	natural	psychological	principles	to	explain	why	we	hold	the	fiction	of	complex unities--a	fiction	so	firmly	believed	that	philosophers	dreamt	up	the	theory	of substance	and	accident	to	try	to	capture	it.45,	46 44	Cf.	Aristotle,	"A	substance	cannot	consist	of	substances	present	in	it	in	complete reality	(Aristotle	1941,	Metaphysics	VII.13,	1039a2-14).	See	also	Leibniz's	claim "that	a	substance	is	not	divisible	into	two"	(Discourse	on	Metaphysics,	section	9,	in AG). 45	See	THN	1.4.3.1. 46	I'm	grateful	to	Toby	Napoletano	for	research	assistance.