What	is	the	Philosophy	of	Consciousness? Uriah	Kriegel Introduction	to	U.	Kriegel	(ed.),	Oxford	Handbook	of	the	Philosophy	of	Consciousness Consciousness	is	one	of	the	most	elusive	phenomena	of	the	natural	world.	But	it	is, after	all,	part	of	the	natural	world.	It	has	presumably	evolved	at	some	point,	as	a result	of	certain	natural	processes	taking	place	within	the	causally	integrated spatiotemporal	system	we	call	Nature.	What	need	is	there,	then,	for	a	philosophy	of consciousness?	As	a	natural	phenomenon,	should	it	not	submit	to	theoretical explanation	by	the	natural	sciences?	There	is	no	philosophy	of	owls;	owls	are natural	phenomena,	so	the	theory	of	owls	is	part	of	a	natural	science	–	zoology.	We now	also	have	a	lively	science	of	consciousness,	conducted	by	cognitive	psychologists and	neuroscientists	among	others.	What	do	philosophers	have	to	contribute	here? One	might	answer:	our	empirical	knowledge	about	consciousness	is	so fragmentary	and	incomplete,	at	this	early	stage	of	scientific	inquiry,	that philosophers	may	be	called	upon	to	offer	more	or	less	disciplined	speculations about	the	part	of	the	story	about	which	we	simply	do	not	as	yet	have	sufficient scientific	knowledge.	It	would	follow	that,	as	scientific	knowledge	of	consciousness grows,	the	need	for	a	philosophy	of	consciousness	will	recede	–	until	it	will disappear	and	be	entirely	replaced	by	the	science	of	consciousness. The	assumption	behind	the	present	volume	is	that	philosophy	has	a	more significant	role	to	play	in	shaping	our	understanding	of	consciousness;	that	even	a complete	science	of	consciousness	will	involve	certain	lacunae	calling	for philosophical	supplementation.	The	plausibility	of	this	notion	depends	in	part	on what	one	calls	"a	science."	To	bracket	verbal	issues,	we	will	concern	ourselves	here with	three	areas	in	which	the	science	of	consciousness	as	pursued	today	leaves certain	questions	unaddressed.	The	next	three	sections	introduce	and	discuss	these three	areas;	each	serves	as	a	motivating	introduction	to	one	of	the	volume's	three parts. 2 1 Phenomenal	Grounding:	The	Varieties	of	Conscious	Experience Science	tries	to	explain.	But	to	get	busy	explaining,	we	first	need	to	be	clear	on	what are	the	phenomena	which	need	explaining.	That	is,	we	need	to	be	clear	on	our explananda	before	launching	our	explanatory	enterprise.	A	science	of	consciousness, in	particular,	needs	a	clear	list	of	the	kinds	or	types	of	conscious	experience	that may	occur	in	the	natural	world,	the	scientific	explanation	of	which	would	be welcome. Early	psychologists	in	the	1870s,	alive	to	this	dual	task,	distinguished between	descriptive	psychology	and	explanatory	psychology.	Descriptive	psychology had	the	task	of	describing	psychological	phenomena	–	manifest	mental	occurrences –	as	well	as	providing	an	initial	taxonomy	of	them;	in	this	task	introspection	had	a central	role.	Explanatory	psychology,	in	contrast,	had	the	task	of	providing	causal explanations	for	the	occurrence	of	the	relevant	phenomena,	to	record	regularities	in their	occurrence,	and	to	articulate	the	natural	laws	governing	their	(regular) occurrence;	here	introspection	was	comparatively	subsidiary,	with	"physiology"	(in essence:	neuroscience)	playing	the	more	central	role.	Many	psychological phenomena	(e.g.,	visual	experiences)	are	introspectible,	but	they	occur	as	the	result of	subpersonal	neurophysiological	processes	that	are	not	themselves	introspectible. In	this	division	of	labor,	descriptive	psychology	had	the	job	of	delineating	the domain	of	phenomena	to	which	explanatory	psychology	is	answerable.	However, the	introspective	method	on	which	it	relied	soon	fell	into	disrepute.	The	two	main issues	were	(i)	the	persistent	irresolvability	of	introspective	disagreements	and	(ii) the	apparent	theory-ladenness	of	introspective	judgments.	Behaviorists	were incensed	about	"psychology	exceptionalism":	the	fact	that	psychology	just	did	not look	like	a	regular,	bonafide	natural	science,	in	which	objective	standards	of	testing and	replication	could	be	discerned	and	certain	methods	were	available	for	resolving disagreements	(Watson	1913:	163).	This	story	is	familiar.	What	is	little	understood is	the	way	the	behaviorist	critique	effectively	and	decisively	banished	the	research area	of	descriptive	psychology	from	the	science	of	the	mind.	Explanatory	psychology, with	its	neurophysiological	methods	and	goals,	was	little	affected	by	the	behaviorist turn.	What	needed	radical	reframing,	for	the	behaviorist,	was	not	our	conception	of how	to	explain	in	psychology,	but	our	conception	of	what	needs	explaining	in psychology.	The	new	explananda	were	not	subjectively	experienced	internal occurrences,	but	pieces	of	publicly	observable	behavior. Crucially,	when	cognitivists	mounted	their	critique	of	behaviorism,	starting in	the	late	1950s,	their	critique	was	in	truth	very	partial:	they	insisted	that psychological	explanation	could	advert	to	internal,	genuinely	psychological	posits 3 ("internal"	as	opposed	to	behavioral,	"genuinely	psychological"	as	opposed	to merely	neurological).	But	they	left	intact	the	behaviorist's	conception	of	the explananda	of	psychology.	Still	today,	what	psychologists	(or	"cognitive	scientists") try	to	explain	are	just	bits	of	behavior	(including	verbal	reports).	Internal	mental processes	are	invoked	in	post-behaviorist	psychology,	but	they	are	treated	as explanatory	posits	–	theoretical	entities	posited	in	the	context	of	trying	to	make sense	of	behavioral	phenomena,	not	phenomena	with	which	we	have	pre-scientific acquaintance	and	for	which	we	would	like	to	provide	scientific	explanation. In	this	respect,	consciousness	science	differs	from	other	scientific	disciplines. Chemistry	tries	to	explain	chemical	phenomena,	zoology	tries	to	explain	zoological phenomena,	and	so	on.	But	psychology	does	not	try	to	explain	psychological phenomena;	it	tries	to	explain	behavioral	phenomena.	Furthermore,	while	the zoologist	works	with	certain	theoretical	posits,	such	as	DNA,	with	which	we	have	no pre-scientific	familiarity,	she	also	recognizes	manifest	zoological	phenomena,	such as	owls	and	wings,	with	which	we	do	have	pre-scientific	familiarity;	these	she	treats as	observable	explananda	rather	than	as	unobservable,	purely	theoretical explanatory	posits.	In	contrast,	the	psychologist	treats	subjective	experiences	as theoretical	posits,	as	though	we	have	no	pre-scientific	familiarity	with	them.	But	in truth,	it	is	evident	that	we	very	much	do	have	pre-scientific	and	indeed	pre-theoretic familiarity	with	experiential	phenomena,	such	as	feeling	embarrassed	at	a	party, smelling	a	camembert,	suddenly	understanding	what	the	speaker	meant	in	response to	our	question,	and	so	on.	If	DNA	did	not	help	explain	anything,	we	would	never suspect	its	existence.	But	if	the	experience	of	sadness	did	not	help	explain	anything, we	would	all	still	believe	that	there	is	such	a	thing	as	feeling	sad. In	taking	behavioral	rather	than	psychological	phenomena	as	its	explananda, and	in	treating	all	psychological	occurrences	as	mere	theoretical	posits,	mainstream academic	psychology	involves	a	form	of	exceptionalism	after	all.	No	other	scientific discipline	banishes	its	own	proprietary	phenomena	from	its	domain	of	explananda, substituting	to	them	alternative	explananda;	and	none	proceeds	as	though	its proprietary	phenomena	are	purely	theoretical	posits	even	when	we	clearly	have pre-scientific	familiarity	with	them.	Perhaps	this	exceptionalism	is	ultimately justified,	say	on	methodological	grounds.	Still,	none	of	this	makes	the	experiential phenomena	with	which	we	have	pre-scientific	familiarity	go	away.	Descriptive psychology	may	have	been	banished	from	scientific	psychology,	but	the	experiential phenomena	themselves	–	the	subject	matter	of	descriptive	psychology	–	cannot	be banished	by	fiat. This	opens	up	a	first	area	where	the	philosopher	has	a	contribution	to	make to	our	understanding	of	consciousness.	Wearing	the	moth-eaten	cap	of	the 4 descriptive	psychologist,	the	philosopher	may	hope	to	produce	a	respectable inventory	of	experiential	phenomena,	as	well	as	to	impose	initial	order	in	it (through	taxonomy	and	topology). Two	central	questions	define	the	core	of	this	endeavor.	First:	of	all	the possible	forms	of	conscious	experience,	which	ones	have	psychological	reality	in	our conscious	life?	(Depending	on	one's	ambition,	the	"our"	can	be	interpreted	in	a variety	of	ways,	from	ranging	only	over	normal	human	adults	to	ranging	over	every sentient	being.)	Second:	of	all	the	psychologically	real	phenomena	of	conscious	life, which	can	be	analyzed	into	combinations	of	other,	more	fundamental	experiential phenomena,	and	which	must	be	treated	as	fundamental,	unanalyzable,	primitive types	of	conscious	experience?	The	types	of	experience	which	are	not	only psychologically	real	but	also	fundamental	and	unanalyzable	–	the	"experiential primitives"	in	our	stream	of	consciousness,	if	you	will	–	are	the	holy	grail	of descriptive	psychology.	Collectively,	they	constitute	the	complete	fund	of experiential	ingredients	the	combinations	and	recombinations	of	which	yield	every subject's	total	conscious	experience	at	a	time. It	is	the	fashion	of	the	day	to	frame	philosophical	issues	in	terms	of grounding,	the	metaphysical	relation	canonically	picked	out	by	the	locution	"in virtue	of"	in	its	philosophical	use.	Not	to	be	deficient	in	this	particular,	we	may frame	the	second	issue	just	presented	in	terms	of	a	distinction	between	(a)	types	of experience	which	are	grounded	in	(combinations	of)	other	types	of	experience,	and (b)	types	of	experience	which	are	grounded	in	no	others.	The	latter	are	the ungrounded	grounds	of	the	experiential	realm,	the	"building	blocks"	of	our	stream	of consciousness.	Early	introspectionist	psychologists	referred	to	these	as	the	elements of	consciousness,	and	had	indulged	in	bold	speculations	about	their	number. (Edward	Titchener,	probably	the	most	prominent	of	all	introspectionist psychologists,	proffered	that	there	were	around	42,415	"elements	of consciousness"!)	But	although	such	questions	are	incredibly	hard	to	settle,	or	even address	in	an	epistemically	responsible	way,	they	answer	to	certain	facts	of	the matter	that	empirical	cognitive	science	does	not,	perhaps	judiciously,	venture	to contend	with. Part	I	of	this	volume	is	dedicated	to	philosophical	discussions	of	some	of	the most	central	candidate	ungrounded	grounds	of	the	sphere	of	experiential phenomena.	It	opens	with	two	chapters	on	perceptual	experience:	a	chapter	by	Pär Sundström	on	visual	experience	and	a	chapter	by	Casey	O'callaghan	on	the	varieties of	nonvisual	perceptual	experience.	There	follow	three	chapters	on	three	other types	of	phenomenologically	lively	kinds	of	experience:	a	chapter	by	Frédérique	de Vignemont	on	bodily	experience,	a	chapter	by	Julien	Deonna	and	Fabrice	Teroni	on 5 emotional	experience,	and	a	chapter	by	Amy	Kind	on	imaginative	experience.	Few philosophers	would	deny	the	very	existence	of	these	types	of	experience,	but	plenty of	philosophers	have	maintained	that	these	types	of	experience	can	be	accounted	for entirely	in	terms	of	the	coming-together	of	other,	more	basic	types	of	experience. We	close	Part	I	with	four	chapters	on	more	controversial	types	of	experience,	ones the	very	existence	of	which	has	often	been	called	into	question:	a	chapter	by	Tim Bayne	on	the	experience	of	thinking,	a	chapter	by	Myrto	Mylopoulos	and	Josh Shepherd	on	the	experience	of	agency,	a	chapter	by	Philippe	Chuard	on	the experience	of	time,	and	a	chapter	by	Farid	Masrour	on	the	experiential	unity	of consciousness.	For	each	of	these	putative	experiential	phenomena,	we	can	ask whether	it	is	(a)	an	ungrounded	primitive	of	conscious	life	(an	"experiential primitive"),	(b)	a	psychologically	real	phenomenon	but	one	grounded	in combinations	of	other,	more	fundamental	phenomena	(an	"experiential	derivative"), or	(c)	not	a	psychologically	real	phenomenon	at	all	(a	mere	"experiential	putative,"	if you	will). Consider	the	experience	of	conscious	thinking.	One	possible	view	is	that there	is	simply	no	such	thing	–	thought	processes	are	never	experienced,	though	they may	be	systematically	accompanied	by	certain	experiences,	say	of	auditory	imagery. Another	view	is	that	we	do	experience	some	of	our	thoughts,	but	the	relevant experience	consists	in	some	cocktail	of	perceptual,	imagistic,	and	emotional	feelings coming	together	just	so.	A	third	view	is	that	the	experience	of	conscious	thought sometimes	brings	into	the	picture	a	new,	sui	generis	kind	of	irreducibly	cognitive feeling	–	a	type	of	proprietary	cognitive	phenomenology.	According	to	this	third view,	there	is	at	least	one	aspect	to	the	experience	of	conscious	thought	that constitutes	an	experiential	primitive,	an	experiential	ingredient	that	goes	beyond any	combination	of	ingredients	found	in	other	kinds	of	experience. A	similar	dispute	between	three	such	positions	–	one	broadly	eliminativist, one	broadly	reductivist,	and	one	broadly	primitivist	–	can	be	framed	for	any	of	the putative	types	of	conscious	experience	discussed	in	Part	I	of	this	volume.	The collection	of	all	phenomena	for	which	the	primitivist	view	is	the	correct	one	would give	us	the	fundamental	furniture	of	the	experiential	realm	–	the	"elements	of consciousness."	Clearly,	settling	on	the	right	collection	or	inventory	of	such elements	is	a	foundational	task	of	first	importance	in	our	understanding	of consciousness.	And	it	is	the	philosophy	of	consciousness	that	attends	to	this	task. (To	repeat,	one	could	use	the	term	"science"	in	such	a	way	that	descriptive psychology	is	part	of	the	science	of	consciousness.	My	point	is	just	that	very	few academics	in	science	departments	engage	in	descriptive	psychology;	it	is	typically academics	in	philosophy	departments	who	do.) 6 2 Physical	Grounding:	Theories	of	Consciousness When	we	speak	of	scientific	research	into	consciousness,	we	speak	in	truth	of research	which	targets	in	the	first	instance	salient	correlates	of	consciousness. Neuroscientific	research	is	transparently	framed	in	those	terms,	since	its	official	aim is	to	identify	the	"neural	correlates	of	consciousness."	But	as	we	will	see	in	the	next section,	research	into	consciousness	in	cognitive	psychology	can	similarly	be	framed as	targeting,	in	the	first	instance,	the	cognitive	correlates	of	consciousness.	It	is possible,	of	course,	to	hold	that	consciousness	is	in	fact	nothing	over	and	above	its neural	correlates	and/or	nothing	over	and	above	its	cognitive	correlates.	A	certain kind	of	physicalist	would	hold	the	former	and	a	certain	kind	of	functionalist	would hold	the	latter.	But	these	additional	claims	–	these	nothing-over-and-above	claims	– are	not	themselves	scientific	claims,	and	the	arguments	bearing	on	their	plausibility are	not	on	their	face	empirical	arguments	based	on	laboratory	results.	On	the contrary,	nothing-over-and-above	claims	are	paradigmatically	philosophical	claims and	the	considerations	typically	brought	in	their	support	are	adduced	rather	from the	armchair. In	other	areas,	what	scientists	provide	us	with	makes	it	straightforward whether	a	nothing-over-and-above	claim	is	appropriate.	Once	told	that	H2O molecules	are	present	wherever	water	is	present,	and	that	sufficiently	large coalitions	of	them	exhibit	all	the	features	and	behaviors	water	does,	it	is straightforward	that	H2O	constitutes	not	only	the	"molecular	correlate	of	water"	but water	itself.	Nonetheless,	it	is	still	a	non-empirical,	philosophical	claim	that	water	is nothing	but	H2O	–	albeit	one	that	courts	no	controversy.	In	the	case	of consciousness,	in	contrast,	the	corresponding	claim	that	consciousness	is	nothing but	its	neural	(and/or	cognitive)	correlate	does	engender	a	kind	of	intellectual discomfort. How	best	to	frame	this	discomfort	is	itself	a	topic	of	lively	debate	among philosophers;	the	opening	chapter	of	this	volume,	by	David	Papineau,	offers	one potential	diagnosis.	According	to	Papineau,	there	is	simply	a	specially	resistant	predoxastic	"intuition	of	distinctness"	that	makes	it	hard	for	us	to	take	seriously	the proposition	that	consciousness	might	be	nothing	but	its	neural	correlate.	This	is intended	as	an	alternative	to	the	most	popular	philosophical	diagnosis	of	the challenge	presented	by	consciousness,	namely,	the	notion	that	even	absolutely complete	knowledge	of	a	person's	brain,	or	for	that	matter	of	the	totality	of	physical reality,	would	be	insufficient	to	derive	any	knowledge	of	what	specific	kind	of conscious	experience	the	relevant	person	is	undergoing,	and	even	of	whether	she	is 7 undergoing	any	experience	at	all	(more	on	this	in	§3).	My	own	view	may	be	put	in terms	of	an	"intuition	of	categorical	distinctness":	the	reason	we	cannot	see	how	the subjective	quality	of	conscious	experience	could	be	generated,	let	alone	constituted, by	the	silent	transactions	of	so	many	brain	cells	lying	motionless	inside	the	darkness of	the	skull,	is	that	the	two	appear	to	belong	to	two	different	ontological	categories. All	these	brain	cells	ever	do,	after	all,	is	increase	and	decrease	the	rates	at	which they	transmit	an	electrical	impulse	to	each	other;	the	notion	that	the	subjective quality	of	feeling	sad,	or	smelling	coffee,	or	getting	angrier,	just	is	such	a	change	in cells'	firing	rates	seems	like	a	category	mistake,	something	akin	to	saying	that	the number	5	is	identical	to	that	mango	over	there,	or	that	justice	is	nothing	but	cement mixed	with	wood	fiber. However	we	ultimately	choose	to	frame	the	relevant	intellectual	discomfort, its	result	is	that	there	is	a	controversy	over	the	non-empirical,	philosophical nothing-over-and-above	claim(s)	associated	with	consciousness.	Addressing	and resolving	this	controversy	is	a	second	area	where	philosophical	research	is indispensable	for	a	complete	understanding	of	the	nature	of	consciousness. Accordingly,	Part	II	of	this	volume	is	dedicated	to	discussions	of	some	of	the most	prominent	philosophical	positions	on	the	ultimate	relationship	between consciousness	and	its	physical	(notably	neural)	correlates.	It	opens	with	two chapters	about	the	neural	correlates	of	consciousness:	one	chapter	by	Jorge	Morales and	Hakwan	Lau	on	the	state	of	the	art	in	this	area	of	scientific	inquiry,	and	another chapter	by	me	about	the	different	philosophical	"explanations"	of	the	very	existence of	a	correlation	between	neural	activity	of	consciousness.	There	follow	six	chapters about	the	most	prominent	such	explanations,	three	physicalist	and	three	antiphysicalist:	a	chapter	by	Brie	Gertler	on	dualism,	the	view	(roughly)	that	the experiential	and	the	physical	are	mutually	(metaphysically)	independent,	such	that any	links	between	consciousness	and	its	neural	correlate	are	at	most	causal	and contingent,	not	constitutive	and	necessary;	a	chapter	by	Sam	Coleman	and	Philip Goff	on	so-called	neutral	monism,	the	view	(roughly)	that	consciousness	and	matter are	ultimately	two	facets	of	a	single	underlying	reality,	but	a	reality	somehow neither	physical	nor	mental,	or	perhaps	both	physical	and	mental;	a	chapter	by Michael	Pelczar	on	idealism,	according	to	which	(roughly)	consciousness	is ontologically	prior	to	matter	(or	else	matter	does	not	exist	at	all);	a	chapter	by	Liz Irvine	and	Mark	Sprevak	about	eliminative	physicalism,	which	denies	the	very existence	of	consciousness;	a	chapter	by	Frank	Jackson	on	a	priori	physicalism,	the idea	that	consciousness	is	not	only	grounded	in	its	physical	correlate,	but	is	so	in	an epistemically	transparent	way,	such	that	complete	knowledge	of	a	the	physical	facts regarding	some	person	would	make	it	possible	to	derive	knowledge	of	the	conscious 8 experience	she	is	undergoing;	and	finally	a	chapter	by	Joe	Levine	on	a	posteriori physicalism,	which	holds	that	although	experiential	facts	are	grounded	in	physical facts,	this	link	is	epistemically	opaque	to	us,	in	that	complete	knowledge	of	physical facts	does	not	suffice	to	derive	facts	about	conscious	experience.	(Note	well:	not	all of	these	authors	endorse	the	position	on	which	they	write!)	The	next	three	chapters of	Part	II	explore	three	specific	strategies	often	adopted	by	physicalists	in	trying	to account	for	consciousness	(but	sometimes	embraced	without	a	physicalist	agenda): a	chapter	by	Adam	Pautz	about	representationalist	theories,	according	to	which conscious	mental	activity	is	characterized	essentially	by	its	distinctive	way	of representing	the	world;	a	chapter	by	Josh	Weisberg	on	higher-order representationalist	theories,	according	to	which	conscious	mental	activity	is characterized	rather	by	the	way	it	is	targeted	by	representations	of	it;	and	a	chapter by	Tom	McClelland	on	self-representationalist	theories,	according	to	which	the crucial	characteristic	of	conscious	states	is	that,	whatever	else	they	represent,	they always	also	represent	themselves.	The	final	chapter	of	Part	II,	by	Daniel	Stoljar,	is dedicated	to	the	so-called	epistemic	approach	to	the	problem	of	consciousness,	the view	(roughly)	that	we	simply	lack	knowledge	of	certain	key	empirical	facts	that would	shed	light	on	whether	and	how	consciousness	might	"arise"	from	physical processes;	in	some	versions,	our	ignorance	of	the	relevant	facts	is	principled	and insurmountable,	in	others	it	is	potentially	merely	temporary. This	selection	of	philosophical	theories	on	the	connection	between consciousness	and	its	physical	correlate	is	perforce	partial.	There	are	many	more options	and	sub-options	for	understanding	the	relationship	between	consciousness and	matter.	A	more	systematic	and	more	nearly	complete	map	of	the	available options	is	presented	in	Figure	1.	It	divides	philosophical	takes	on	the	consciousnessmatter	relationship	into	monist	and	dualist.	The	dualist	approaches	it	divides	into substance	dualism	and	property	dualism,	and	property	dualism	it	divides	into naturalistic	and	nonnaturalistic	versions:	the	former	insists	on	a	causal	or	nomic dependence	of	consciousness	on	physical	facts,	while	the	latter	insists	rather	on	the absence	of	any	dependence	of	the	sort.	Naturalistic	dualism	is	then	divided	into interactionist	and	epiphenomenal:	the	former	grants,	while	the	latter	denies,	causal efficacy	to	conscious	experiences.	In	this	organization	of	logical	space,	the	four	main dualist	theories	are	substance	dualism,	nonnaturalistic	property	dualism, interactionist	naturalistic	property	dualism,	and	epiphenomenalist	naturalistic property	dualism. Monist	theories	are	divided	in	Figure	1	into	three	groups:	physicalist monism,	neutral	monism,	and	idealist	monism.	All	three	hold	that	reality	is	"at bottom"	unified	(i.e.,	unified	at	the	fundamental	level,	the	level	of	"ontological 9 bedrock").	But,	roughly	speaking,	one	view	is	that	reality	is	at	bottom	physical, another	is	that	it	is	at	bottom	mental,	and	a	third	that	it	is	at	bottom	something	else, something	which	is	neither	physical	nor	mental	but	is	both	proto-physical	and proto-mental.	Ignoring	important	subdivisions	of	idealist	and	neutral-monist	views, Figure	1	goes	on	to	divide	physicalist	views	into	three	further	groups:	eliminative physicalism,	reductive	physicalism,	and	nonredudctive	physicalism.	The	first	claims that	there	are	no	experiential	properties	or	types;	the	second	that	experiential properties	or	types	are	identical	to	physical	ones;	the	third	that	experiential properties/types	are	not	identical	to	physical	properties/types,	but	the	former	are nonetheless	constitutively	dependent	(and	not	merely	causally	or	nomically dependent)	upon	the	latter.	This	constitutive	dependence	was	traditionally	framed in	terms	of	supervenience,	a	relationship	whereby	one	family	of	properties	(the supervenient	ones)	cannot	vary	without	corresponding	variation	in	another	family of	properties	(the	subvenient	ones,	the	"supervenience	base").	Traditionally,	a further	important	distinction	between	a	priori	and	a	posteriori	physicalism	is	then framed	as	a	distinction	between	two	different	potential	modal	forces	with	which	the experiential	may	supervene	on	the	physical:	a	priori	physicalism	holds	that	the experiential	supervenes	on	the	physical	with	conceptual	necessity,	a	posteriori physicalism	that	it	supervenes	with	merely	metaphysical	necessity.	(Incidentally, naturalistic	dualism	has	sometimes	been	characterized	as	holding	that	the experiential	supervenes	on	the	physical	with	merely	nomic	necessity	on	the physical,	and	experiential	nonnaturalistic	dualism	can	be	seen	as	rejecting	any supervenience	of	the	experiential	on	physical.) It	is	worth	noting	that	in	recent	philosophy	supervenience	relations	have come	to	be	seen	as	mere	symptoms	of	the	aforementioned	more	robust	underlying metaphysical	relation	of	grounding.	Accordingly	there	emerged	in	recent	philosophy of	mind	an	interest	in	"grounding	physicalism"	as	a	potential	position,	either	on	the mind-body	problem	or	on	the	associated	"consciousness-matter	problem"	on	which we	have	focused	here.	It	is	not	always	clear	in	the	relevant	literature	how	grounding physicalism	is	related	to	the	more	traditional	versions	of	physicalism,	but	my	own sense	is	that	it	is	best	viewed	as	a	potentially	better	and	more	accurate reformulation	of	nonreductive	physicalism.	For	grounding	relations	are	not	typically eliminative	about	the	grounded,	so	grounding	physicalism	is	not	a	version	of eliminative	physicalism.	Meanwhile,	reductive	physicalism	is	naturally	seen	as requiring	the	identification	of	consciousness	with	something	physical,	whereas	an important	feature	of	grounding	is	that	it	differs	from	identity	in	being	asymmetric. 10 11 (This	difference	is	moreover	crucial	to	grounding	physicalism's	ability	to	capture the	ontological	priority	of	the	physical	over	the	mental	without	subsuming	or absorbing	the	mental	in	the	physical.)	So	I	think	grounding	is	best	seen	as	that which	metaphysical	or	conceptual	supervenience	is	the	symptom	of,	and accordingly	grounding	physicalism	is	best	seen	as	what	nonreductive	physicalism was	always	supposed	to	be.	Now,	if	we	do	frame	nonreductive	physicalism	in	terms of	grounding	rather	than	supervenience,	then	the	distinction	between	a	priori	and	a posteriori	physicalism	becomes	a	distinction	between	two	kinds	of	grounding,	or two	different	ways	in	which	the	experiential	can	be	grounded	in	the	physical.	The idea	is	that	there	are	distinctions	between	grounding	relations	that	mirror	the distinction	between	conceptual	and	metaphysical	supervenience.	Consider:	the	fact that	Jimmy	is	a	bachelor	is	grounded	in	one	way	in	the	fact	that	Jimmy	is	an unmarried	man,	but	grounded	in	another	way	in	the	fact	that	the	person	who originated	in	the	relevant	sperm	and	the	relevant	egg	is	a	bachelor.	(I	am	indulging here,	merely	for	the	sake	of	illustration,	the	essentiality-of-origin	thesis.)	The	first kind	of	grounding	has	conceptual	(hence	a	priori)	supervenience	for	a	symptom,	the second	has	metaphysical	(hence	a	posteriori)	supervenience	for	a	symptom. Figure	1	thus	distinguishes,	alongside	the	aforementioned	four	types	of dualism,	four	central	forms	of	physicalism:	eliminative	physicalism,	reductive physicalism,	a	priori	nonreductive	physicalism,	and	a	posteriori	nonreductive physicalism.	In	addition	it	recognizes	a	fifth	and	sixth	types	of	monism:	idealism	and neutral	monism.	Then	there	is	the	epistemic	approach,	which	allows	that	we	may simply	not	know,	and	may	even	be	constitutionally	incapable	of	knowing,	which	of these	positions	is	correct.	With	this	plethora	of	options,	certain	decisions	had	to	be made	to	keep	Part	II	of	this	volume	manageable	–	decisions	which	were	guided	not only	by	intellectual	but	also	by	practical	and	sociological	considerations. 3 Psychological	Grounding:	Consciousness	and	Neighboring	Phenomena At	least	historically,	the	problem	of	whether	there	is	more	to	consciousness	than	its physical	correlate	has	been	the	most	central	in	the	philosophy	of	consciousness. Indeed,	this	problem	is	often	referred	to	in	the	philosophical	literature	simply	as "the	problem	of	consciousness."	As	noted,	there	are	different	ways	of	understanding the	source	of	the	problem.	But	a	recurring	theme	in	many	presentations	is	that although	the	problem	concerns	in	the	first	instance	the	relationship	between consciousness	and	its	physical	realizer,	it	ultimately	passes	through	worries concerning	the	relationship	between	consciousness	and	its	functional	or psychological	role	(see	Levine	1983:	357,	Chalmers	1995:	64). 12 It	is	sometimes	said	that	there	is	nothing	more	to	physical	matter	than structure	and	dynamics,	whereas	there	is	something	more	to	consciousness	than structure	and	dynamics,	namely,	intrinsic	feel.	The	point	can	be	put	more	informally and	impressionistically	as	follows:	there	is	no	gap	between	what	a	physical phenomenon	does	and	what	it	is,	whereas	there	is	more	to	what	consciousness	is than	what	it	does	(there	is	also	how	it	feels).	A	physical	phenomenon,	system, mechanism,	process,	or	property	is	what	it	does	–	its	essence	or	nature	is	to	do	just that	which	it	does.	But	although	a	conscious	experience	does	various	things	–	it performs	many	psychological	functions,	i.e.	plays	a	certain	psychological	role	–	its nature	is	not	exhaustively	captured	by	a	complete	enumeration	of	all	it	does.	There appears	to	be	a	"para-functional	residue"	in	consciousness;	the	term	"phenomenal consciousness"	can	be	seen	as	designating	precisely	the	aspect	of	consciousness	that appears	to	go	beyond	the	totality	of	its	psychological	functions	(Block	1995). To	bring	the	point	out,	let	us	compare	consciousness	and	memory.	Both	of them	do	something:	they	play	a	causal	or	functional	role	in	the	subject's	overall psychological	economy.	Crushing	many	subtleties,	let	us	say	that	what	memory	does is	store	information.	What	is	important	to	note	is	that	storing	information	is	so essential	to	memory	that	it	is	natural	to	say	that	memory	just	is	an	informationstorer.	Now,	the	complete	psychological	profile	of	memory	is	surely	more	nuanced than	this;	but	whatever	it	is,	once	we	know	exactly	what	memory	does,	we	can identify	memory	with	that-which-does-that.	Any	system	or	mechanism	that	did	the same	thing	–	that	played	the	relevant	psychological	role	–	would	ipso	facto	count	as a	memory	system.	This	is	why	we	have	no	problem	attributing	memory	to	such inanimate	objects	as	desktop	computers	from	the	seventies.	(To	say	this	is	not	take a	stand	on	whether	computers	could	ever	be	conscious;	it	is	only	to	take	a	stand	on whether	desktop	computers	from	the	seventies	were	conscious	–	the	stand	that	they were	not!) Consciousness	has	its	own	functional	or	psychological	profile,	which	seems considerably	more	multifaceted	than	memory's.	Some	attempt	to	unify	that	profile around	the	notion	of	cognitive	access:	consciousness	makes	information	more available	to	a	wide	variety	of	downstream	mechanisms,	modules,	and	subsystems. Again,	a	full	specification	of	the	functional	or	psychological	role	of	consciousness would	surely	be	more	nuanced	than	this,	but	whatever	it	turns	out	to	be,	it	is	what	I referred	to	in	§2	as	the	"cognitive	correlate	of	consciousness":	it	is	the	specific, singular	contribution	consciousness	makes	to	cognition.	What	is	important	for	our purposes	is	that	it	seems	perfectly	possible,	indeed	rather	easy,	to	imagine	a	system or	mechanism	that	does	exactly	that	(whatever	consciousness	does	within	our cognitive	system)	and	yet	does	not	amount	to	consciousness	at	all.	If	one	day	we 13 isolate	the	complete	psychological	role	of	consciousness,	construct	a	system	made	of silicon	chips	that	perform	the	exact	same	cognitive	functions,	and	embed	that system	in	a	computer,	we	will	not	immediately	take	this	to	settle	the	question	of whether	that	computer	is	conscious.	There	seems	to	be	an	open	question	left	over: Does	the	computer's	performance	of	these	functions	involve	a	subjective	quality,	a phenomenal	character,	an	experiential	feel?	The	very	fact	that	this	question	makes sense,	even	where	the	psychological	role	of	consciousness	has	been	fully	replicated, shows	that	there	is	more	to	our	concept	of	consciousness	than	the	idea	of	thatwhich-does-such-and-such.	In	other	words,	our	concept	of	consciousness	allows	for	a potential	daylight	between	what	consciousness	is	and	what	consciousness	does	–	a daylight	that	our	concept	of	memory,	for	instance,	does	not	tolerate. A	philosopher	could	of	course	maintain	that	although	this	is	true	of	our	naïve, prescientific	concept	of	consciousness,	a	more	sophisticated	scientific understanding	of	consciousness	will	reduce	consciousness	to	its	cognitive	correlate, that	is,	will	construe	consciousness	as	exhausted	by	the	sum	total	of	its psychological	functions.	To	this,	another	philosopher	will	respond	that	such	a reconceptualization	of	consciousness	is	so	radical,	leaving	out	the	very	essence	of consciousness	(namely,	its	subjective	feel),	that	it	amounts	to	effectively	denying	the existence	of	consciousness	as	we	ordinarily	think	of	it.	The	first	philosopher	may then	come	back	with	the	complaint	that	unless	we	accept	her	radical reconceptualization	of	consciousness,	we	will	end	up	with	a	mysterian, nonnaturalist	picture	of	the	world	overly	welcoming	of	scientifically	intractable, vaguely	supernatural	phenomena.	Here	we	approach	the	great	looming	fear	of	the philosophy	of	consciousness:	that	all	said	and	done,	the	only	stable	positions	on	the nature	of	consciousness	are	eliminative	physicalism	and	nonnaturalistic	dualism	– and	that	the	tremendous	industry	of	fashioning	clever	intermediate	positions	is	but an	expression	of	a	futile	hope	to	have	the	cake	and	eat	it	too. The	point	I	want	to	make	is	that	this	is	a	paradigmatically	philosophical debate,	not	a	scientific	one.	At	the	end	of	inquiry,	if	you	please,	the	cognitive psychology	of	consciousness	will	identify	the	complete	cognitive	correlate	of consciousness.	But	the	question	of	the	ultimate	relationship	between	consciousness and	its	cognitive	correlate	cannot	be	settled	by	cognitive	psychology	itself	–	just	as the	question	of	the	ultimate	relationship	between	consciousness	and	its	neural correlate	cannot	be	settled	by	neuroscience	(and	indeed,	just	as	the	question	of	the ultimate	relationship	between	voting	patterns	and	the	behavior	sub-atomic	particles cannot	be	settled	by	particle	physics).	The	claim	that	consciousness	is	nothing	but	its cognitive	correlate,	and	the	claim	that	it	is	not	nothing	but	its	cognitive	correlate,	are 14 philosophical	claims.	Here	too,	then,	we	find	a	distinctive	role	for	philosophy	to	play in	our	overall	understanding	of	the	nature	of	consciousness. Part	III	of	this	volume	is	dedicated	in	large	part	to	studies	of	the	relationship between	consciousness	and	some	of	the	psychological	phenomena	or	mechanisms with	which	it	is	functionally	bound.	It	includes	chapters	on	consciousness' relationship	with	attention	(Chris	Mole),	memory	(Chris	Hill),	action	guidance (Benjamin	Kozuch),	intentionality	(Angela	Mendelovici	and	David	Bourget), perceptual	knowledge	(Berit	Brogaard	and	Eli	Chudnoff),	the	self	(Dan	Zahavi), introspection	and	self-knowledge	(Maja	Spener),	and	even	moral	knowledge	(Josh Shepherd	and	Neil	Levy);	the	volume	closes	with	a	chapter	(by	Mark	Rowlands)	on consciousness'	embodiment. There	are	surely	other	psychological	functions	of	relevance	to	consciousness' functional	role.	But	this	partial	list	captures	some	of	the	most	salient	ones:	conscious awareness	tends	to	display	a	foreground/background	attentional	structure; consciously	perceived	stimuli	tend	to	stay	in	short-term	memory	longer	and	more vividly	than	stimuli	perceived	subliminally	or	in	blindsight;	conscious	experiences seem	to	guide	certain	of	our	on-the-fly	actions;	they	tend	to	be	intentionally	directed at	objects	and	features	in	the	external	world;	they	tend	to	induce	and	justify perceptual	beliefs	about	the	external	world,	as	well	as	introspective	beliefs	about oneself	and	one's	"inner	world."	So	much	is	beyond	serious	doubt.	The	question	is whether	these	intimate	relations	consciousness	bears	to	attention,	memory,	action, intentionality,	perceptual	judgment,	introspection,	and	the	self	are	causal	and contingent	or	on	the	contrary	constitutive	and	necessary.	Causal	and	contingent relations	are	to	be	studied	by	cognitive	science;	the	philosopher's	distinctive contribution	is	to	examine	the	prospects	for	constitutive,	essential	connections between	consciousness	and	these	neighboring	psychological	phenomena. Again,	we	may	also	put	the	question	in	terms	of	grounding:	Is	consciousness grounded	in	some	of	its	neighboring	psychological	phenomena,	such	that	once	a cognitive	system	performs	the	relevant	psychological	functions,	it	is	guaranteed	to also	exhibit	(phenomenal)	consciousness?	Or	does	consciousness	ultimately outstrip	any	collection	of	psychological	functions	and	is	thus	a	sui	generis	mental phenomenon?	Grounding	being	a	metaphysical	relation,	such	questions	of grounding	pertain	to	the	metaphysics	of	consciousness,	not	to	the	empirical	science of	consciousness.	There	is	no	laboratory	instrument	that	can	detect	grounding relations	in	nature.	The	question	is	not	empirically	tractable. Conclusion 15 Given	a	certain	conscious	experience	–	say,	feeling	grief-stricken	about	a	relative's death,	or	nervous	about	an	upcoming	appointment	–	there	are	at	least	three	kinds	of question	about	relations	of	grounding	or	constitutive	dependence	that	the experience	bears	to	(increasingly	removed)	other	phenomena:	(1)	Is	the	experience grounded	in/constitutively	dependent	upon	other	types	of	conscious	experience? (2)	Is	it	grounded	in/constitutively	dependent	upon	psychological	phenomena	other than	conscious	experience?	(3)	Is	it	grounded	in/constitutively	dependent	upon certain	physical	(or	otherwise	non-mental)	processes	or	states?	These	questions define	a	sphere	of	inquiry	where	philosophical	reflection	on	consciousness	seems indispensable.	This	volume	attempts	to	cover	some	of	the	most	prominent	issues that	arise	concerning	each	of	these	distinctively	philosophical	questions. References § Block,	N.J.	(1995),	'On	a	Confusion	About	the	Function	of	Consciousness',	in	Behavioral	and	Brain Sciences	18:	227-247. § Chalmers,	D.J.	(1995),	'The	Puzzle	of	Conscious	Experience,	in	Scientific	American	(December): 62-68. § Levine,	J.	(1983),	'Materialism	and	Qualia:	The	Explanatory	Gap',	in	Pacific	Philosophical Quarterly	64:	354-361. § Watson,	J.B.	(1913),	'Psychology	as	the	Behaviorist	Views	It',	in	Psychological	Review	20:	158177.