To appear in: Bueno,	O./ Chen, R.-L./ Fagan,	M. B. (eds) (2018): Individuation, Process and Scientific	Practices.	Oxford:	Oxford	University	Press. Individuating	Part-Whole	Relations	in	the	Biological	World Marie	I.	Kaiser Bielefeld	University kaiser.m@uni-bielefeld.de Abstract What are the conditions under which one biological object is a part of another biological object? This paper answers this question by developing a general, systematic account of biological	parthood.	I	specify	two	criteria	for	biological	parthood.	Substantial	Spatial	Inclusion requires biological parts to be spatially located inside or in the region that the natural boundary	of	the	biological	whole	occupies.	Compositional	Relevance	captures	the	fact	that	a biological	part	engages	in	a	biological	process	that	must	make	a	necessary	contribution	to	a condition that is	minimally sufficient to one or	more of the characteristic behaviors of the biological	whole.	Instead	of	emphasizing	the	diversity	of	part-whole	relations	in	the	biological world, this paper asks what biological part-whole relations have in common and what constrains	their	existence,	in	general.	After	presenting	the	two	criteria	for	biological	parthood I	discuss	in	how	far	my	account	can	cope	with	hard	cases	(e.g.,	redundant	parts)	and	I	reveal the	merits	and	limits	of	monism. 2 1 Introduction The view that nature is divided into part-whole hierarchies is deeply embedded in the sciences, for instance, in the biological sciences. Biologists represent objects (e.g., cells) as being	constituted	of	a	certain	collection	of	organized	parts	(e.g.,	DNA,	mitochondria,	proteins, etc.). Assumptions about part-whole relations are involved in classifications of biological objects	into	kinds	(e.g.,	the	assumption	that	fish	have	gills	as	parts,	whereas	mammals	have lungs	as	parts).	Moreover,	the	methodological	principle	that	one	can	understand	the	behavior of	a	whole	by	decomposing	it	into	its	parts	and	studying	the	behavior	of	the	parts	is	central	to generating	knowledge	in	the	biological	sciences (for	the	limitations	of	reductive	methods	see Kaiser	2011,	2015). The importance	of	part-whole relations to the	biological sciences raises the question under	which conditions something is a part of a biological	whole and	what it means that the parts constitute the whole. For instance, when does a vesicle that is transported	from	the	endoplasmic	reticulum	to	the	Golgi	apparatus	in	a	eukaryotic	cell	cease to	be	a	part	of	the	endoplasmic	reticulum	and	become	a	part	of	the	Golgi	apparatus?	Is	the case	that	is	attached	to	the	body	of	a	Caddisfly	larva	and	that	promotes	the	larva's	survival	a part	of the larva	or	does it	belong to the larva's	environment?	Under	which	conditions is	a particular	species	or	abiotic	factor	(e.g.,	nitrogen)	a	part	of	an	ecosystem?	Does	a	virus	that enters	a	host	cell	and	uses	the	cell's	machinery	and	metabolism	to	replicate itself	become	a part	of	the	host	cell? The goal of this paper is to answer questions like these by developing a general, systematic account of biological parthood. Such an account specifies general criteria that guide the individuation of wholes and parts in the biological sciences, and it reveals the conditions	under	which	biological	objects stand in	a	part-whole relation to	each	other. The central question	of the	paper is under	which conditions is one	biological object x a	part	of another	biological	object	y.1 If two	biological	objects stand in	a	part-whole relation to	each other, I will speak of one being a biological part of the other. Hence, biological parthood applies	to	biological	objects	only.2	The	question	under	which	conditions	x	is	a	biological	part of	y is	a special	version	of	what	van Inwagen	has	called the	"special composition	question" (1990, 21). It is a special version because it concerns only part-whole relations in the biological world, rather than the concept of a part in general (which is the object of mereology).	This	paper	is	about	biological	parthood,	not	about	parthood	simpliciter.3 Questions about biological parts are intertwined with questions about biological individuality.	Typically,	the	concept	of	a	biological	individual is	centered	on	but	not	confined to	organisms.	Besides	organisms,	also	parts	of	organisms	(e.g.,	genes	and	cells)	and	groups	of 1	I	take	biological	objects	to	be	those	objects	that	belong	to	the	domain	of	the	biological	sciences. 2	For	reasons	of	simplicity,	I	will	often	speak	of	one	object	being	a	biological	part	of	another	object.	This	must	be read	synonymously	with	the	claim	that	one	biological	object	is	a	part	of	another	biological	object. 3 My focus on biological parthood is not driven by the conviction that biological part-whole relations are fundamentally different from	other kinds of part-whole relations. In fact,	my analysis yields parthood criteria that	might	be	applied	also	to	non-biological	objects,	for	instance,	in	the	chemical	realm. 3 organisms	(e.g.,	populations)	are	discussed	as	being	biological individuals (e.g.,	Clarke	2011; Wilson and Barker 2017). Accordingly, the relata of many part-whole relations in the biological realm will be biological individuals. Rather than focusing on the question what biological	individuals	are	and	how	they	are	individuated,	this	paper	examines	how	biological individuals relate to each other, in particular, which conditions must be satisfied so that biological	individuals	relate	to	each	other	as	parts	and	wholes.	One	of	my	central	claims	will be	that	the	individuation	and	characterization	of	a	biological	individual	as	a	whole	constrains the individuation of its parts in several ways. This paper uncovers the various kinds of constraints	by	formulating	different	criteria	for	biological	parthood. Methodologically,	my	account	of	biological	parthood	arises	from	and	is	sustained	by	an analysis of a wide variety of paradigmatic examples of part-whole relations from the biological sciences. My analysis takes into account actual cases of biological part-whole relations	as	well	as	the	explanatory	and	investigative	strategies	that	biologists	employ	when studying	part-whole	relations.	The	account	of	biological	parthood	that	I	develop	in	this	paper is	thus	an	exercise	in	what	I	call	a	metaphysics	of	biological	practice.	First,	it	is	a	metaphysical account	because it aims	at	describing	a feature	of reality,	namely	part-whole relations that exist	out	there	in	the	biological	world. It	provides	an	understanding	of	the	characteristics	of biological	part-whole	relations	and	of	the	conditions	under	which	they	hold.	Second,	I	agree with	naturalistic	metaphysics	(e.g.,	Ladymann	and	Ross	2007; Chakravartty	2013) in	that	we should	consult	the	sciences	to	develop	metaphysical	views	about	which	kinds	of	entities	exist and how they are like. But instead of considering physical theories only, a	metaphysics of biological	practice	draws	our	attention	to	the	metaphysical	underpinnings	of	the	non-physical sciences,	and	it	develops	metaphysical	claims	that	take	into	account	also	the	non-theoretical aspects	of	biological	practice	(e.g.,	scientific	activities,	epistemic	values,	reasoning	strategies; Chang	2011,	Waters	2014).4	Because	of	the	relatively	broad	scope	of	my	account	(i.e.,	parts and	wholes in the	biological	world, in	general) I focus	on	analyzing	examples	of	part-whole relations that are paradigmatic and figure in successful biological explanations. In addition, my	analysis	is	backed	up	by	philosophical	case	studies	that	are	concerned	with	more	specific kinds	of part-whole relations (e.g.,	DNA sequences	being	parts of the	human	genome) and that investigate the	concrete investigative	practices	and reasoning strategies that	biologists employ	(see,	e.g.,	Kaiser	forthc.). A pluralistically inclined	philosopher	might object that	my goal of paying attention to actual biological practice conflicts with my other goal of developing a general, systematic 4	I	am	aware	of	the	fact	that	a	metaphysics	of	biological	practice	understood	in	that	way,	presupposes	scientific realism and requires an account of which elements of biological practice allow for drawing metaphysical conclusions	(and	which	do	not).	A	promising	approach	is	to	focus	on	successful	and	stable	practices.	However,	it is	a	controversial	question	whether	scientific	success	provides	us	with	access	to	the	world,	as	it	really	is.	Those skeptical	about	this	might	prefer	to	adopt	a	weaker	view	of	metaphysics,	according	to	which	metaphysics	makes general	claims	about	our	conceptual	apparatus,	rather	than	about	the	world	(e.g.,	Strawson	1959).	I	agree	that these are urgent and very interesting questions but they lie beyond the scope	of this paper. Fortunately,	my analysis	of	biological	parthood	is	compatible	with	both	views	of	metaphysics. 4 account	of	biological	parthood.	According	to	the	pluralist,	a	monistic	account	that	specifies	a single	set	of	criteria	supposed	to	apply	to	all	part-whole	relations	in	the	biological	world	will fail	to	capture	the	diversity	of	biological	part-whole	relations	and	of	individuation	practices	in the	biological	sciences	(cf.	Kellert,	Longino,	and	Waters	2006).	I	agree	with	the	pluralist	that	it might	turn	out	that	only	a	pluralistic	notion	of	biological	parthood	that	recognizes	different, perhaps	conflicting	parthood	criteria	(such	as	Wimsatt	1972,	2007)	accounts	for	the	diversity of biological practice. However, diversity and difference is only one aspect of biological practice that philosophers can and should account for (Kaiser 2015, Chapter 2). From a philosophical	perspective, it is	also	interesting	to	zoom	out	and	look	for	generalities	and	for similarities between	different	practices	of individuating	parts and	wholes.	When traditional metaphysicians argue about the general structure of reality, they adopt such a general perspective	and	develop	philosophical	views	that	are	supposed	to	apply	universally.	I	think	we should	not	reject	the	monistic	aspirations	of	traditional	metaphysics	too	hastily	because	we can learn	a lot from	striving for	monism	and from	analyzing the	obstacles	we	meet.	Hence, the	aim	of	my	paper	is	twofold.	First,	I	develop	a	monistic	account	of	biological	parthood	that proposes two general, necessary conditions for the existence of biological part-whole relations, and I show why these criteria are preferable to alternative criteria. Second, my analysis enables me to explain why the search for a monistic account that specifies individually necessary and jointly sufficient criteria for biological parthood is so difficult. Understanding	the	reasons for these	difficulties	delivers	valuable insights into	the	nature	of biological	part-whole	relations. This	paper	is	organized	as	follows.	In	Section	2,	I	distinguish	different	kinds	of	questions that	one can	ask concerning	biological	parthood, and I specify the relata	of	biological partwhole relations. Section 3 develops spatial inclusion as the first criterion for biological parthood and introduces the notion of a natural boundary, which is central to the spatial inclusion criterion. In Section 4, I develop the second criterion for biological parthood, compositional relevance, which refers to the characteristic behaviors of the whole and specifies	the	relevance	condition	in	terms	of	an	insufficient	but	necessary	part	of	a	condition that is unnecessary but sufficient (INUS-condition). In Section 5, I discuss in how far my account	of	biological	parthood	can	deal	with	hard	cases,	such	as	redundant	parts	or	collective parts. I conclude in Section 6 by revealing the merits and limits of a monistic account of biological	parthood. 2 Preliminaries 2.1 Relating	Different	Part-Whole	Questions I	shall	call	the	question	of	whether	a	particular	biological	object	x	(e.g.,	a	vesicle)	is	a	part	of another biological object y (e.g., the Golgi apparatus) the parthood question. It is closely related	to	but	less	demanding	than	the	decomposition	question,	which	asks	for	not	only	one but	for	all	parts	into	which	a	whole	is	partitioned.	My	account	of	biological	parthood	focuses 5 on	the	relation	between	one	biological	part	and	its	whole	(i.e.,	on	the	parthood	question)	and specifies	the	conditions	under	which	this	one-to-one	relation	holds.	From	this focus, it	does not follow	that the	other	parts	of the	whole	are irrelevant to	the	existence	of	a	part-whole relation.	On	the	contrary,	several	authors	have	drawn	attention	to	the	fact	that	a	biological part	always is a	member	of	a team	of	parts that interact	with	each	other in very "intense" (Simon	1962,	Haugeland	1998,	McShea	2000)	or	"productive"	ways	(Machamer,	Darden,	and Craver	2000,	Gillett	2013)	and	that	exhibit	a	special	"jointness" (Fagan	2012).	How,	exactly, the	existence	of	a	particular	part-whole	relation	depends	on	other	parts	of	the	same	whole will	be	examined	in	Section	4.	This	dependency	does	not	imply	that	answering	the	parthood question presupposes answering the decomposition question. We can individuate one particular	biological	object	as	a	part	of	a	whole	without	having	decomposed	the	whole	into	all of	its	parts	(but	not	without	knowing	some	of	the	other	parts). Whether	a	particular	object is	a	biological	part	of	another	object,	the	whole,	does	not only	depend	on	the	other	parts	but	also,	and	in	particular,	on	how	the	whole	is	individuated and	by	which	properties	and	behaviors	it	is	characterized.	I	shall	call	the	question	'What	is	the whole, how can it be individuated and characterized?' the individuation-of-the-whole question.	A central idea that I	will elaborate in the following sections is that	answering the individuation-of-the-whole question constrains answering the parthood question. In other words, which object I pick out as the whole and how I characterize its properties, typical behaviors,	and	spatial	boundaries	constrains	what	counts	as	a	biological	part	of this	whole. For	instance,	biologists	do	not	treat	a	green	alga	that	is	spatially	included	in	a	fungus	as	a	part of	the	fungus.	However,	if	the	whole	is	referred	to	as	a	lichen,	the	green	alga	will	be	a	part	of the	lichen.	Likewise,	if	biologists	conceive	of	a	genome	as	having	a	specific	chemical	structure (i.e.,	as	consisting	of	DNA	only)	transcription	factors	and	histones	fail	to	be	biological	parts	of the genome because they are proteins. By contrast, if biologists characterize a genome in purely	functional	terms	(e.g.,	as	guiding	development)	transcription	factors	and	histones	turn out to be biological parts of the genome	because they are central to gene regulation. The criteria	for	biological	parthood	that	I	develop	in	this	paper	specify	the	different	ways	in	which the	individuation	of	the	whole	constrains	the	individuation	of	its	parts. The individuation-of-the-whole question	must be distinguished from the	demarcation question,	which	concerns the	outer	boundary	of the	whole	and	asks	how	the	whole	can	be demarcated	from	its	context.	One	might	claim	that	answering	the	demarcation	question	boils down to answering the decomposition question because to demarcate an object from its context is nothing but identifying all of its parts and classifying all non-parts as context. In Section 3.1, I shall argue that this view invites a circularity objection, which can only be avoided	by	a	substantial	notion	of	a	spatial	boundary	that	allows	us	to	demarcate	an	object from its environment independently of individuating all of its parts. In other words, I will argue that answering the demarcation question is independent of answering the decomposition question. Identifying what I will refer to as the 'natural boundary' of a 6 biological	object	may	be	one	aspect	of	individuating	the	object	as	a	whole.5	Hence,	answering the	demarcation	question	may	be	part	of	answering	the	individuation-of-the-whole	question. In	sum,	we	must	distinguish	four	questions	concerning	part-whole	relations: Part-Whole	Questions (1) Individuation-of-the-whole	question:	What	is	the	whole,	how	can	it	be	identified? (2) Demarcation	question:	Where	does	the	outer	boundary	of	the	whole	run?	How	can the	whole	be	demarcated	from	its	context? (3) Decomposition	question:	Into	which	collection	of	parts	can	the	whole	be decomposed? (4) Parthood	question:	Is	a	given	biological	object	a	part	of	another	biological	object,	the whole? 2.2 The	Relata	of	Biological	Part-Whole	Relations Claims	about	part-whole	relations	in	the	biological	sciences	typically	concern	types	or	kinds	of objects, not tokens. For instance, all individuals of the kind lichen are composed of green algae and fungi, not only a particular lichen. The fact that science is often concerned	with kinds of part-whole relations, however, does not imply that the part-whole relation itself holds between types or kinds.6 Depending on one's ontology, kinds might be viewed as abstract	entities (e.g.,	universals) that	do	not	exist in space	and time,	or they	might	not	be said	to	exist	at	all	–	at	least	not	independently	of	our	classification	practices.	It	seems	to	me that	the	more	parsimonious	and	less	controversial	assumption	to	start	with	is	that	part-whole relations	in	the	biological	realm	relate	token	objects	that	exist	in	space	and	time.	The	account of	biological	parthood	that	I	develop	in	this	paper	is	thus	a	singularist	account.7	According	to	a singularist account of biological parthood, part-whole relations exist between individual biological	objects	and	claims	about	kinds	of	biological	part-whole	relations	are	generalizations that	arise	from	investigating	particular	part-whole	relations. Even	if	kinds	are	not	the	relata	of	biological	part-whole	relations,	they	are	still	relevant to the conditions	under	which	biological part-whole relations	exist.	Whether two	particular biological objects are related as part and whole depends also on the kinds to which they belong.	To	see	this,	consider	the	question	of	whether	a	virus	that	has	infected	a	host	cell	is	a part	of	it.	Answering	this	question	depends	not	only	on	how	biologists	individuate	the	whole but also on how they classify it. Viruses are surely not among the parts that cells have, in general. But they may be parts of a subtype of cells, namely infected host cells. A virus normally	does	not	contribute	to	but	often	hinders	the	behaviors	that	are	typical	for	objects	of 5 For instance, biologists spatially characterize a cell as being surrounded by a cell	membrane and insects as being	surrounded	by	an	exoskeleton	with	a	particular	shape	and	structure. 6	This is	not	a	trivial	point	because	some	authors	claim	that	part-whole	or	constitutive	relations	exist	between properties	or	types	of	events	(e.g.,	Harbecke	2010). 7	My	singularist	account	is	in	accordance	with	the	fact	that	the	part-whole	relation	in	the	classical,	mereological sense	is	conceived	of	as	a	first-order	relation	between	individuals. 7 the	kind	"cell",	such	as	growth,	DNA	replication,	protein	synthesis,	and	cell	division.	A	token object that	belongs to the	kind	"host cell",	by	contrast, is characterized	by	different typical behaviors,	such	as	reduced	cell	defenses,	viral	replication,	and	release	of	viruses.	The	virus	is	a biological	part	of	a	cell	of	the	kind	host	cell	because	the	virus	is	essential	to	the	characteristic behaviors	of	host	cells.8	I	will	pick	up	this	point	in	Section	4.1	when	I	explain	the	notion	of	a characteristic behavior in more detail and incorporate it into my second criterion for biological parthood. To be clear,	my claim that kinds are relevant to the conditions under which biological part-whole relations exist does not imply that the existence of biological part-whole relations depends on classification preferences of individual biologists. Not any biological	kind	that	one	might think	of is	scientifically legitimate	and thus	can	be	said	to	be real	(in	a	promiscuous	way;	Dupré	1993,	36).	Hence,	from	the	fact	that	biological	part-whole relations	exist	only	relative	to	specific	kinds	it	does	neither	follow	that	biological	parthood	is subjective,	nor	does	it	follow	that	my	account	of	biological	parthood	is	epistemological	rather than	metaphysical	in	character. Some philosophers might agree that part-whole relations exist between tokens but reject	the	view	that	material	objects	–	understood	in	accordance	with	endurantism9	–	are	the appropriate relata of part-whole relations. Process ontologists argue that an	object-bias or "substance	paradigm" (Seibt 2016) fails to acknowledge the	processual nature	of the living world	(e.g.,	Whitehead	1929;	Rescher	1996,	2000;	Seibt	2003;	Dupré	2012;	O'Malley	2014).	I agree that the ontological category of processes (or occurrents, which include processes, events,	and	states)	is	important	to	capture	the	constantly	changing,	complex	biological	world (see	Kaiser	and	Krickel	2016).	However, this	does	not commit	one to the radical claim that "everything is process" (Bickhard 2011, 95)10 and that part-whole relations must exist between	processes	only.	It	is	possible	to	identify	material	objects	as	the	relata	of	part-whole relations	and	yet	to	acknowledge	the	importance	of	processes.	As	my	analysis	will	show,	the processes in	which biological objects are involved co-determine	whether these objects are related as parts and wholes. A static view of biological parthood that considers only the properties of and relations between material objects at a certain time is thus highly implausible. Biological part-whole relations essentially involve processes, even though processes	are	not	the	relata	of	part-whole	relations.11 8 There are several other examples that support this claim. For instance, the F1 subunit seems not to be a biological	part	of	a	protein	of	the	kind	"transmembrane	protein",	but	rather	a	biological	part	of	a	protein	of	the kind	"ATP	synthase". 9	Endurantists	believe	that	material	objects	have	spatial	parts	but	no	temporal	parts	and	that	material	objects are wholly present whenever they exist. By contrast, perdurantists claim that material objects are fourdimensional	space-time	worms	that	are	extended	over	time	and	that	have	also	temporal	parts. 10	It	is	unclear	whether	there	is	any	philosopher	of	biology	who	subscribes	to	the	radical	view	that	only	processes exist.	Dupré	claims	that	biological	entities,	such	as	organisms	and	genomes,	are	dynamical,	constantly	changing entities, which cannot be understood in terms of "properties of and relations between their structural constituents"	(Dupré	2012,	8). 11	Along	these	lines	one	might	add	that	a	part-whole	relation	between	biological	objects	does	not	only	depend on	the	processes	they	implement	but	also	on	the	properties	and	powers	they	possess	(Gillett	2013). 8 To conclude, I assume that part-whole relations in the biological world each exist between particular biological objects. My heart, for example, is a biological part of my circulatory	system.	Despite	this	focus	on	particulars	and	on	material	objects,	I	argue	that	we can only understand the conditions under which a certain part-whole relation exists if we broaden our perspective and take into account also other parts of the same whole, the processes	in	which	the	objects	are	involved,	and	the	kinds	to	which	they	belong.	The	criteria for biological parthood that I develop in the following sections explicate how other parts, processes	and	kinds	determine	the	existence	of	biological	part-whole	relations. 3 Spatial	Inclusion 3.1 Why	a	Primitive	Notion	of	Spatial	Inclusion	Fails A	widespread and intuitively plausible view is that the	parts of	material objects are spatial parts.	There	is	a	dispute	in	metaphysics	about	whether	material	objects	have	temporal	parts as	well,	as	the	perdurantist	believes,	but	everybody	agrees	that	material	objects	have	at	least spatial	parts.	To	be	a	spatial	part	of	a	whole	means	to	be	spatially	included	or	contained	in	the whole, that is, to be located inside the spatial boundary that continuously surrounds the whole.	The	view	that	something	is	a	part	if	it	is	spatially	included	in	the	whole	seems	to	apply to	the	biological	world	as	well (see	Craver	2007,	Clarke	2011,	Gillett	2013).	Chloroplasts	are parts of cells because they are located inside cells. My liver is a part of me because it is spatially included in me. An individual black-headed gull is a part of a certain population because	it	is	located	in	the	spatial	distribution	of	that	population.	A	green	alga	is	a	part	of	a lichen	because it is	contained in the lichen.	Bases	on these intuitions,	you	might	suggest to specify	biological	parthood	as	primitive	spatial	inclusion: Primitive	Spatial	Inclusion	(PSI) An	object	x	is	a	biological	part	of	an	object	y	if	and	only	if x	is	spatially	included	in	y,	that	is,	x	is	located	inside	the	continuous	spatial	boundary	of	y.12 Despite its initial plausibility, paradigmatic examples of part-whole relations from biological	practice	show	that	spatial	inclusion	is	neither	sufficient	nor	necessary	for	biological parthood.	Consider	first	why	it	is	not	sufficient.	In	the	case	of	lichens	green	algae	are	spatially included	in	the	fungus	but	they	are	not	conceived	of	as	parts	of	the	fungus	(rather,	they	are parts	of	the	composite	organism,	the	lichen).	Similarly,	if	a	doctor	leaves	a	cotton	ball	inside of	my	stomach	during	surgery	we	would	not	say	that	the	cotton	ball	became	a	part	of	me	just because	it	is	spatially	located	inside	of	me.	Another	example	is	the	individuation	of	the	parts of the human genome. According to ENCODE (2012), not any arbitrary DNA sequence contained	in	the	human	genome	is	a	biological	part	of	it	(Kaiser	forthc.).	Examples	like	these 12	Note	that	spatial inclusion	does	not	require	spatial	contact	or	proximity	among	the	objects	that	are	spatially included	in	Y.	Spatial	contact	or	proximity	may	have	an influence	on	part-whole	relations	(e.g.,	by	allowing	for causal	interactions)	but	it	is	not	presupposed	by	spatial	inclusion. 9 are	widespread	in	the	biological	sciences.	They	show	that	there	is	more	to	biological	parthood than spatial inclusion and that PSI specifies a criterion for biological parthood that is not sufficient. Other cases show that spatial inclusion is	not	even	necessary for	biological parthood. Take the example of a population of black-headed gulls. As an individual, it might have a specific spatial distribution (e.g., at the North Sea coast around Büsum) but this does not preclude that some	members of that population are located outside of this region (if they potentially	interbreed	with	members	located	in	that	region).	In	general,	populations	seem	not to be the kind of objects whose identity depends on having a specific spatial boundary, contrary to, for instance, cells or mitochondria that are necessarily surrounded by a membrane. Gene regulatory networks and ecosystems are further examples of biological objects	whose	parts	seem	not	to	be	held	together	by	what	I	will	call	a	natural	boundary	(see next section). PSI thus fails to	provide	even	a	necessary criterion for the	existence	of partwhole	relations	in	the	biological	world. Finally,	PSI is	problematic	because it	runs	the	risk	of	being	circular	(or	trivial).	PSI	says that biological parts of a	whole	must be spatially included in the	whole,	which	means that they	must be located inside the spatial boundary of the	whole. This raises the question of what is the spatial boundary of the whole and how can it be identified (the demarcation question, recall Section 2.1). The simplest answer is that	we demarcate an object from its context by identifying all of its parts and drawing a three-dimensional boundary that encompasses	all	parts	and	a	minimal	set	of	other	objects	(this	is	similar	to	what	Kaplan	2012 proposes). This suggestion, however, renders PSI circular because	whether x is a biological part	of	y	would	depend	on	whether	x	is	spatially	included	inside	the	boundary	around	y	and y's	other	parts. In	other	words, the individuation	of	one	part	of	a	whole	would	presuppose having individuated all of its parts. For example,	whether the case of a Caddisfly larva is a biological	part	of	the	larva	would	depend	on	whether	it	is	located	inside	the	spatial	boundary that	surrounds	all	parts	of	the	larva.	To	avoid	the	circularity	of	the	spatial	inclusion	criterion we must find a way to individuate the spatial boundary of the whole independently of individuating its	parts.	The	next	section introduces the	notion	of	a	natural	boundary,	which provides	us	with	such	an	independent	demarcation	of	biological	objects. 3.2 Natural	Boundaries The	main	idea	behind	the	notion	of	a	natural	boundary is	that	there	exist	boundaries	in	the biological	world that	are	of	particular importance to the identity	of	many	biological	objects and	to	individuating	their	parts.	Paradigmatic	cases	of	natural	boundaries	include	the	skin	of mammals, the	exoskeleton	of insects, the	cell	wall	of	plant cells,	other	membranes such	as the	blood-brain	barrier	or	the	alveolar-capillary	membrane,	and	also	things	such	as	rivers	or thick lines	of	scrub. I refer to these	boundaries	as	"natural" to	emphasize that they	exist in reality, rather than	being introduced solely through	human	demarcation. In short, they are 10 "bona fide boundaries", not "fiat boundaries" (Smith and Varzi 2000, 401).13 Not all boundaries that biologists draw – for instance, when identifying the target of their investigation – correspond to natural boundaries. Sometimes biologists are interested in questions that require studying	an	object together	with	parts	of its environment.	On	other times they ignore parts of an object because they are irrelevant to the behavior they investigate. Demarcating	a	biological	object, the	whole, from its context	by identifying its	natural boundary is independent in two respects. First, it does not require identifying all of the biological parts of the whole (i.e., answering the demarcation question does not require answering the decomposition question). Second, demarcating a biological object by identifying its natural boundary is independent from characterizing the natural boundary, itself, as a biological part (i.e., answering the demarcation question does not require answering the	parthood	question	about the	natural boundary). These independencies arise from the fact that natural boundaries typically are identified by their function as selective barriers	and	by	the	material	discontinuities	or	structural	differences	that	they	involve.	Let	me explain	this	in	more	detail. First,	natural	boundaries	that	demarcate	individuals	function	as	selective	barriers.	They bind	together	the	objects	that	are	located	inside	of	it	and	separate	them	from	what	is	outside. In	most cases, this separation will not be complete but rather selective. For instance, the nuclear membrane ensures that ribosomes stay outside the nucleus but its nuclear pore complexes	allow	some	molecules to	pass (e.g.,	mRNAs	with specific signal sequences).	As	a result,	natural	boundaries	reduce	the	causal	interactions	between	objects	inside	and	objects outside	of	them	(e.g.,	between	ribosomes	and	intranuclear	mRNAs)	and	allow	specific	kinds	of causal interactions	among the	objects they	encompass (e.g.,	between	mRNAs	and	enzymes during	mRNA	processing).	The	fact	that	interactions	between	the	parts	of	a	whole	and	parts of	its	environment	are	generally	fewer	and	weaker	than	the	interactions	among	the	parts	of the	whole is the guiding idea of so-called intensity-of-interactions approaches to parthood (e.g., Simon	1962;	Wimsatt 1972, 2007;	Haugeland	1998;	McShea 2000;	McShea and	Venit 2001). Second, natural boundaries usually involve material discontinuities or structural differences	(e.g.,	differences	in	the	chemical	structure,	texture,	or	material	constitution).	The cell	membrane,	for	instance,	is	composed	of	a	lipid	bilayer	and	transmembrane	proteins	and, as such, forms	a	unit that	has	a	different	material constitution	and	chemical structure than what is located outside and inside of it. In the case of the membrane of blood cells, for instance,	the	cytoplasm	(inside)	consists	of	a	complex	mixture	of	cell	organelles,	cytoskeleton 13	Artificially	produced	biological	objects	(e.g.,	in	synthetic	biology)	can	have	natural	boundaries	as	well	because the	term	'natural'	denotes	the	independent	existence	of	these	boundaries	in	the	natural	world,	not	their	natural origin. 11 filaments,	dissolved	molecules,	and	water,	and	the	blood	plasma	(outside)	consists	mainly	of water.14 In sum, natural boundaries exist in the natural world (independent from human demarcation), and they are individuated	by their functions as selective barriers and	by the structural differences and material discontinuities they involve. Natural boundaries play a crucial	role	in	determining	what	is	a	part	of	a	biological	individual	and	what	is	not.	In	the	next section I use the notion of a natural boundary to substantiate the condition of spatial inclusion and to turn it into a plausible criterion for biological parthood, which avoids the three objections that a primitive notion of spatial inclusion faces (recall Section 3.1). The substantial	criterion	of	spatial inclusion that I	develop in the	next	section	relies	on	the idea that natural boundaries constrain relations between biological parts and biological	wholes. However, it does not imply that any natural boundary demarcates biological individuals (because other necessary conditions might not be satisfied), nor does it imply that any biological individual must be demarcated by a natural boundary (because there can be individuals	without	natural	boundaries). 3.3 A	Substantial	Criterion	of	Spatial	Inclusion The leading idea of developing a substantial spatial inclusion criterion is that natural boundaries of biological objects constrain the individuation of their parts insofar as their biological	parts	must	be	spatially	included	in	or	inside	the	natural	boundary	of	the	object.	The dung ball that a dorbeetle rolls is not a biological part of the beetle because it is located outside	its	exoskeleton;	the	neurotransmitter	molecule	is	not	a	part	of	the	neuron	because	it is	located	in	the	synaptic	cleft	outside	the	cell	membrane	of	the	neuron;	and	a	green	alga	is discussed to be a biological part of the lichen because it is located inside the fungal membrane. Paradigmatic cases like these give rise to the following substantive notion of spatial	inclusion	as	a	criterion	for	biological	parthood. Substantial	Spatial	Inclusion	(SSI) An	object	x	is	a	biological	part	of	an	object	y	only	if (1) if	y	has	a	natural	boundary,	x	must	be	spatially	located	inside	or	in	the	region	that	the natural	boundary	occupies. This	criterion	explicates	one	way	that	the	nature	of	the	whole	–	in	this	case,	its	spatial	nature, that	is,	its	feature	of	possessing	a	specific	natural	boundary	–	constrains	the	individuation	of its	biological	parts.	The	disjunction	'inside	or	in	the	region	of'	is	important	because	it	captures also	cases	where	a	biological	part	is	located	in	the	natural	boundary	itself,	such	as	a	receptor molecule	or	an	ion	channel	that	are	located	in	the	region	that	the	cell	membrane	occupies. 14	This	is	compatible	with	the	fact	that	some	natural	boundaries	are	scattered	(e.g.,	of	the	Golgi	apparatus). 12 SSI	avoids	all	the	three	problems	that	a	primitive	notion	of	spatial	inclusion	faces	(recall Section	3.1).	First,	SSI	is	not	formulated	as	a	sufficient	condition	for	biological	parthood.	In	the next	sections,	I	will	supplement	SSI	by	an	additional	criterion	that	allows	distinguishing	cases of	mere	spatial	containment	from	cases	of	genuine	biological	parthood	(cf.	Jansen	and	Schulz 2014). Second, even though natural boundaries are of great importance to the identity of biological	objects	and	co-determine	their	parts,	this	is	not	true	for	all	cases.	Some	biological objects,	such	as	gene	regulatory	networks,	immune	systems,	certain	populations	and	certain ecosystems, do not possess natural boundaries. Nevertheless, we can retain SSI as a necessary	condition	if	we	formulate	it	conditionally,	that	is,	if	we	require	biological	parts	to	be spatially included in their wholes only if the wholes possess natural boundaries. The conditional	form	of	SSI	accounts	for	cases	in	which	spatial	inclusion	is	irrelevant	to	biological part-whole	relations.	Third,	in	the	previous	section,	I	have	characterized	natural	boundaries	as involving material discontinuities or structural differences and as functioning as selective barriers.	These	structural	and	functional	features	ensure	that	natural	boundaries	of	biological objects	can	be	individuated	independently	from	identifying	their	parts.	Hence,	by	referring	to the spatial boundary of a biological object as a natural boundary SSI avoids the circularity objection	because	where	the	natural	boundary	of	the	whole	runs	is independent	from	what its	parts	are. 4. Compositional	Relevance Paradigmatic	examples	of	biological	part-whole	relations	show	that	biological	parts	are	bound together to	a	whole	not	only	spatially	but	also	causally-functionally.	To	get	an idea	of	what this means consider the following reasoning strategies for why something is a part of a biological	object	that	can	be	found	in	biological	practice.	A	vesicle	in	the	cytoplasm	of	a	cell	is a part of the Golgi apparatus (rather than, e.g., of the endoplasmic reticulum) because it contributes	to	the	processing	and	transport	of	proteins.	A	particular	DNA	sequence	out	of	the billions	of	possible	sequences	is	a	part	of	the	human	genome	because	it	plays	a	causal	role	in the human genome as a whole, that is, because it contributes to gene expression or regulation	(Kaiser	forthc.).	Glia	cells	are	not	parts	of	the	central	nervous	system	because	they do	not	directly	participate	in	synaptic	interactions	and	electrical	signaling;	rather,	glia	cells	are parts	of	the	brain	because	they	provide	physical	support	for	neurons	and	regulate	the	internal environment	of	the	brain.	An	amino	acid	sequence	or	protein	region	is	a	part	of	ATP	synthase because it is necessary to the protein's function of synthesizing ATP through transporting protons.	A leukocyte	or	antibody is	a	part	of	an immune	system	because it is	crucial to the protection of an organism against diseases. A cotton ball left inside the stomach during surgery is not a part of the human because it does not contribute to the survival of the human,	but rather impedes it if an infection	occurs.	Harmless strains	of E. coli bacteria	are parts	of	the	human	gut	because	they	facilitate	the	survival	of	humans	by	producing	vitamin	K2 and	preventing	colonization	of	the	intestine	with	pathogenic	bacteria. 13 These	paradigmatic	examples reveal that	we	cannot	understand the conditions	under which biological part-whole relations hold if we consider biological objects and their properties	only.	We	must	also	take	into	account	the	processes	in	which	parts	and	wholes	are involved and how these processes relate to each other, such as how the binding of a particular	DNA	sequence	to	a	transcription	factor	contributes	to	the	regulation	of	genes	of	the human genome.15 My account of biological parthood thus stands in the tradition of philosophies	that	emphasize	the	importance	of	processes	and	of	activities	(which	I	take	to	be subtypes of processes; Kaiser 2017) to the biological world (Dupré 2012; O'Malley 2014; Machamer,	Darden,	and	Craver	2000).16 The	above	examples	suggest	that	biological	parts	must	be	involved	in	processes	(which	I will	refer	to	as 'parts-processes')	that	are in	a	certain	sense	relevant	to	– i.e.,	contribute	to, are	necessary	to,	play	a	causal	role	in,	are	crucial	to	–	one	or	more	processes	that	the	whole engages in (which I	will refer to as 'whole-processes'). In the following sections, I turn this rough	idea	into	a	specific,	clear	criterion	for	biological	parthood.	I	argue	that	we	should	think of the whole-processes to which parts-processes must be relevant as the characteristic behaviors	of	biological objects, such	as the characteristic	behaviors	of a	Golgi apparatus to process	and	transport	proteins	(Section	4.1).	I	then	draw	attention	to	the	parts-processes	and explain	why	we	cannot	understand	biological	parthood	by	considering	parts-processes	alone (Section	4.2).	Finally, I	use	Mackie's (1965) idea	of INUS-conditions to	specify the relevance relation	that	must	hold	between	partsand	whole-processes	(Section	4.3). 4.1 Characteristic	Behaviors	of	Biological	Wholes Plausibly,	not	any	arbitrary	process	of	the	whole	should	determine	what	its	parts	are	because the whole might be involved in some processes only accidentally or exceptionally. For example,	a	lichen	growing	on	the	wall	of	a	playground	might	be	painted	blue	by	school	kids but	this	is	not	a	process	in	which	a	lichen	is	typically	involved.	HIV	might	damage	the	immune system of a person suffering from AIDS but this process is not characteristic of human immune	systems,	in	general. Processes of this kind can be excluded by introducing the notion of a characteristic behavior of a biological object. Characteristic behaviors are processes in	which a biological object	engages	very	generally,	that is,	under	a	wide	range	of	contexts in	which	the	object is naturally found. For instance, a sunflower grows and attracts bees and other insects to promote	its	reproduction	whether	it	stands	in	my	garden,	on	a	sunflower	farm	or	at	the	edge of	a	grain	field.	Characteristic	behaviors	determine,	at	least	in	part,	the	character	or	nature	of a	biological	object.	For	example,	the	functional	nature	of	an	ATP	synthase	is	to	use	a	proton 15	Note that	my focus is still on	objects as the relata	of part-whole relations. There	might also	be	part-whole relations	between	processes	but	this	is	not	the	focus	of	this	paper.	My	claim	is	that	if	we	accept	objects	as	the relata	we	have	to	take	into	account	processes	as	well. 16	With respect to	part-whole relations,	Gillett (2013)	argues that	we	must	adopt	a	dimensioned	account that considers	not	only	individual	objects	but	also	their	properties,	powers	and	the	processes	they	engage	in. 14 gradient to synthesize ATP, which is why molecules of this kind have been named 'ATP synthase'.	Contrary to	mechanisms,	which	are	always for	a single	behavior (Glennan	2002), most biological objects are characterized by more than one typical behavior. Cells, for example, divide, synthesize proteins, and grow. Which behaviors are characteristic of a particular	biological	object	depends	also	on	the	kind	to	which	it	belongs	(recall	Section	2.2). Blood cells, for instance, exhibit other characteristic behaviors than muscle cells or than infected	host	cells. Characteristic behaviors of biological objects will often be realizations of biological functions.	The	realization	of	the	function	of	the	heart	to	pump	blood, for instance, is	also	a characteristic	behavior	of the	heart. Likewise, the characteristic	behaviors	of a stem	cell to differentiate	into	specific	kinds	of	cells,	to	synthesize	proteins,	and	to	grow	might	be	seen	as realizations of functions of a stem cell. However, I prefer to speak about characteristic behaviors rather than about functions. Many etiological theories of biological function prioritize organisms and their characteristic behaviors to survive and to reproduce (e.g., Neander	1991).	Such	a restricted focus impedes	our	understanding	of	biological	part-whole relations	because	biological	wholes	are	not	confined to the level	of	organisms	and	because even organisms display other characteristic behaviors than survival and reproduction (e.g., growth).	Cummins'	causal	role	theory	of	function	(1975)	seems	to	provide	an	understanding of	the	concept	of	a	biological	function	that	is	much	more	adequate	to	the	present	purposes. However, also Cummins' account must be applied to biological part-whole relations with caution	since	it	is	misleading	in	some	ways.	Most	importantly,	Cummins	argues	that	functions are	dispositions	or	capacities,	not	processes	(1975,	757).	But	the	mere	disposition	to	engage in	a	process	seems	not to	be	sufficient to	an	object	being	a	biological	part (more	on	this in Section	5).	A	mitochondrion,	for	instance,	is	a	biological	part	of	a	cell	because	it	produces	ATP not	because	it	has	the	disposition	to	produce	ATP,	which	may	never	be	manifested.17 4.2 Working	Parts Understanding the	conditions	under	which	part-whole	relations in the	biological realm	hold requires taking into account not only the processes in	which	wholes engages but also the processes in which parts are involved. For instance, it is the process of a specific DNA sequence	binding	certain transcription factors that is relevant to the regulation	of	genes	of the	human	genome	and	it	is	the	process	of	antibodies	recognizing	and	neutralizing	pathogens that is relevant to the protection of an organism against diseases. This is the point	where causation	enters	the	scene.	Many	of	the	parts-processes	will	be	causal	processes	that	involve 17 Furthermore, Cummins (1975) states that functions are individuated with respect to the capacity of a containing	system.	Accordingly,	Cummins'	functions	are	features	of	parts	which	they	possess	in	virtue	of	being parts	of	a	whole	(the	containing	system).	Cummins'	account	thus	applies	very	naturally	to	characterizing	partsprocesses as functions (if we ignore the capacity-process difference). By contrast, characterizing wholeprocesses (i.e.,	characteristic	behaviors	of	wholes)	as	Cummins' functions	would	require	positing	an	additional whole,	which	contains	the	original	whole	as	a	part. 15 not	a single	biological	part	but causal interactions	between two	or	more	biological	parts	of the same whole (e.g., the DNA sequence interacting with transcription factors, or the antibodies interacting with pathogens). In a similar vein, van Inwagen has argued that "parthood	essentially	involves	causation"	(1990,	81).18 The	observation	that	causal interactions	are	central	to	biological	parts	and	wholes	has led several authors to defend	what can be called the intensity-of-interactions approach to biological parthood (Simon 1962; Wimsatt 1972, 2007; Haugeland 1998; McShea 2000; McShea	and	Venit	2001).	The	basic	idea	of	this	approach	is	that	we	can	decompose	a	system into	parts	according	to	the	principle	that	interactions	among	parts	are	generally	weaker	and less	frequent	than	interactions	within	parts.	For	instance,	different	molecules	in	a	cell	can	be distinguished from another because the atoms inside of each molecule interact more frequently	and	stronger	with	themselves	(e.g.,	they	form	certain	kinds	of	chemical	bindings), rather than for instance with atoms that belong to different molecules. The intensity-ofinteractions	approach	has	initial	plausibility	but	it	encounters	some	serious	objections.	Most importantly, the approach overlooks that biological parts are not only determined by the intensity of causal interactions in and among parts but also by specific features of the whole.19 In	Section	3, I	argued	that	biological	wholes	spatially	constrain	the individuation	of their	biological	parts	through	the	natural	boundaries	they	possess. In	the	next	section, I	will argue	that	biological	parts	must	be	involved	in	causal	interactions	that	are	relevant	at	least	to one of the characteristic behaviors of the whole. If we consider the strength of causal interactions	only,	we	overlook	this	directedness	of	causal	interactions	to	the	behaviors	of	the whole. To	conclude, considering	only the	processes in	which	parts	engage is	not sufficient to understanding	biological	part-whole	relations.	Instead,	we	must	analyze	how	parts-processes relate	to	whole-processes	and	what	makes	biological	parts	to	"working	parts"	(Mellor	2008, 68)	that	work	to	bring	about	one	of	the	characteristic	behaviors	of	the	whole. 4.3 Specifying	the	Relevance	Condition Biologists	conceive	of	a	vesicle	as	a	biological	part	of	the	Golgi	apparatus	if it	contributes	to the	processing	and transport	of	proteins.	E. coli	bacteria	are regarded	as	biological	parts	of the	human	gut	if	they	facilitate	the	survival	of	humans,	for	instance,	by	producing	vitamin	K2 and	preventing	colonization	of the intestine	with	pathogenic	bacteria.	Neurobiologists treat glia cells	as	biological	parts	of the	brain rather than	of the	central	nervous system	because they	play	no	direct role in	electrical signaling	but	provide	physical support for	neurons	and 18 Van Inwagen thinks about causal interactions between simples, rather than between biological objects of various	kinds. 19 Other major objections are that the intensity-of-interactions approach requires specifying a threshold of number	and	intensity	of	interactions	above	which	they	are	included	and	under	which	they	are	excluded.	Such	a threshold seems arbitrary or highly context-dependent (Wimsatt 1972). Furthermore, the intensity-ofinteractions approach fails to exclude non-parts or background conditions that possess relatively many and strong	interactions	to	other	parts	(e.g.,	Craver	2007,	143f). 16 regulate	the internal	environment	of	the	brain.	What	unifies	all these	paradigmatic	cases	of part-whole	relations	is	that	biological	parts	engage	in	processes	(e.g.,	a	vesicle	budding	off	the Golgi	apparatus	and	moving	towards	the	cell	membrane)	that	are	relevant	to	at	least	one	of the characteristic behaviors of the whole (e.g., the Golgi apparatus transporting proteins). Under	which	conditions,	exactly,	is	the	relation	of	relevance	satisfied?	What	does	it	mean	that a part-process	must contribute to, play a role in, or be crucial to one of the characteristic behaviors	of	the	whole	(a	whole-process)?	The	philosophical literature	yields	different ideas that	one	could	pursue	further.	In	this	section,	I	briefly	introduce	the	major	ideas	and	point	out their shortcomings, before I then present	my own relevance condition,	which I refer to as compositional	relevance. One	idea	is	that	biological	parts	must	have	significant	causal	effects	on	the	properties	of the	whole such that they give the	whole a "causal unity" (Mellor 2008, 67). This is not an implausible idea	but it	does	not	help to specify the relevance condition	because it remains unclear what makes a causal effect significant and under which conditions causal unity is reached.	Gillett	argues	that	a	part-whole	relation	between	individual	objects	exists	only	if	the part	is	a	"member	of	a	spatiotemporally	related	team	of	individuals	many	of	whose	members bear powerful and/or productive relations to each other" (2013, 321). Gillett nicely draws attention	to	the	fact	that	biological	parts	are	not	isolated	but jointly	working	parts	(see	also Fagan 2012). However, the parthood criterion he specifies – namely that a biological part must	bear	powerful	and/or	productive	relations	to	other	parts	of	the	same	whole	–	runs	the risk	of	becoming	circular	(at	least	it	requires	that	other	parts	of	the	whole	are	already	fixed).	It also seems too	weak	because it does	not restrict the kinds	of	dispositions that a	biological part	must	have	and	the	kinds	of	processes	it	must	engage	in	(also	the	cotton	ball left inside my stomach during surgery, e.g., has some dispositions and engages in some processes). Moreover, to	capture	cases, such	as inactive	parts,	Gillett	backs	away from	formulating the parthood criterion as a necessary condition. His account thus fails to provide a satisfying answer	to	the	central	question	under	which	conditions	a	biological	object is	a	member	of	a jointly	working	team. Craver's	account	of	"constitutive	relevance" (2007,	139)	applies	to	the	components	of mechanisms	but	one	might suggest that its	mutual	manipulability condition	can	be	used to specify	also	the	sense	in	which	biological	parts	must	be	relevant	to	their	wholes.	The	mutual manipulability condition requires that the components of a mechanism (which are acting entities, i.e.,	material	objects	engaging in	processes	of	a specific	kind;	Kaiser	2017)	and the phenomenon that a	mechanism is responsible for (e.g., an object-involving process; Kaiser and Krickel 2016) must be	mutually manipulable. That is, some change in the component must change the phenomenon and vice versa. Craver's mutual manipulability condition is criticized,	for	instance,	for	being	epistemic	rather	than	metaphysical	because	it	specifies	how researcher	get	evidence	about	the	components	of	mechanisms,	rather	than	specifying	which 17 features	in	the	world	determine	component-mechanism	relations	(Couch	2011).20	Even	more importantly,	Craver's	account	of	constitutive	relevance	requires	components	and	mechanistic phenomena	not	only	to	be	mutually	manipulable	but	also	to	be	related	as	parts	and	wholes.21 Using the mutual manipulability criterion to specify biological parthood thus introduces a vicious	circularity	into	Craver's	account	of	constitutive	relevance. In	the	early	literature	on	functional	explanation	and	functional	analysis,	Hempel	(1965) and	Nagel	(1961)	put	forward	the	idea	that	a	biological	part	must	be	a	bearer	of	a	function	in the	sense	that	it	must	have	certain	effects	that	contribute	to	some	activity	of	the	containing system,	the	whole	(cf.	Cummins	1975,	741).	Much	of	the	discussion	focuses	on	the	question of whether the effects that biological parts have on their wholes can be understood as necessary	conditions,	such	as	the	heart	circulating	blood	might	be	a	necessary	condition	for the	proper	working	of	the	organism.	However,	counterexamples	such	as	artificial	pumps	show that a heart pumping blood is not necessary for an organism's survival, and even if one excludes	things	such	as	artificial	pumps	by	adding	the	phrase	'under	normal	conditions'	one still	has to face	counterexamples such	as redundant	parts (e.g., the second	kidney	which is not	necessary	for	the	organisms	survival	but	is	a	biological	part	of	it)	and	relevant	non-parts (e.g.,	the	case	of	the	caddisfly	larva	which	seem	to	be	necessary	for	the	larva's	survival	but	is not a biological part of it). Still, the idea that biological parts must in a certain sense be necessary	to	their	wholes	is	appealing	because	it	may	give	rise	to	a	criterion	of	compositional relevance	that	is	specific	and	strong	enough. Mackie	(1965)	claims	that	we	should	understand	causes	as	INUS-conditions,	that is,	as being insufficient but necessary elements of overall conditions that are themselves unnecessary	but	sufficient	for	their	effects.	Mackie's	idea	provides	us	with	a	promising	tool	to maintain	the	idea	that	biological	parts	must	be	necessary	to	their	wholes,	while	it	allows	us	to weaken	the	relevance	criterion in	a	way	that it	accounts for	cases,	such	as	redundant	parts (e.g.,	the	second	kidney)	and	collective	parts	(e.g.,	the	calcium	ions	in	a	muscle	fiber	that	only collectively	have	a	significant	effect	on	muscle	contraction).22 I	have	argued in the	previous section that	we cannot individuate biological parts by considering their causal interactions only,	as	the	intensity-of-interactions	approach	proposes.	Instead,	the	processes	in	which	parts engage (which comprise causal interactions) must be examined with regard to how they affect	the	characteristic	behaviors	of	the	whole.	The	contribution	that	a	part-process	makes to	one	of	the	characteristic	behaviors	of	the	whole	need	not	be	necessary	because	it	can	be redundant	(e.g.,	the	second	kidney	filtering	blood	is	not	necessary	for	the	human's	survival)	or only collectively significant (e.g., the transport	of	a single calcium ion into the sarcoplasmic reticulum	does	not	raise	the	electrostatic	potential).	Instead,	a	part-process	must	be	an	INUS- 20	Other objections can be found, for instance, in	Harbecke 2010; Couch 2011; Leuridan 2012;	Harinen 2014; Baumgartner	and	Gebharter	2016. 21	More	specifically,	the	component-objects	X	must	be	parts	of	the	system	S	that	is	involved	in	the	phenomenon that	the	mechanism	is	responsible	for	(Craver	2007,	153;	Kaiser	and	Krickel	2016). 22 In a similar vein, Harbecke (2010) and Couch (2011) use	Mackie's account to specify the conditions under which	an	acting	entity	is	a	component	of	a	mechanism. 18 condition	for	at	least	one	of	the	behaviors	that	the	whole	characteristically	displays.	That	is, biological	parts	must	engage	in	processes	that	are	necessary	members	of	a	minimal	subset	of parts-processes, which are jointly sufficient to a characteristic behavior of the whole.23 Applying	Mackie's	idea	of	INUS-conditions	to	the	question	of	biological	parthood	gives	rise	to the	following	second	criterion	for	biological	parthood. Compositional	Relevance	(CR) An	object	x,	which	engages in	biological	process	p, is	a	biological	part	of	an	object	y,	which shows	characteristic	behaviors	b1,...bn,	only	if (2) p is relevant to at least	one	of	b1,...bn, that is, p	makes	a	necessary contribution to a condition	that	is	minimally	sufficient	to	one	or	more	of	b1,...bn. CR is a clearly ontological criterion (contrary to, e.g., Craver's mutually manipulability condition)	because it specifies	how the	processes in	which	putative	biological parts engage must	relate	to	the	characteristic	behaviors	of	the	whole.	CR	is	also	more	specific	than	other approaches because it specifies precise conditions under which a (causal) process of a putative	part	is	relevant	to	a	whole.	In	other	words,	CR	specifies	the	conditions	under	which causal	effects	of	putative	parts	are	significant	(Mellor	2008)	and	the	conditions	under	which putative	parts	bear	powerful	and/or	productive	relations	to	each	other	(Gillett	2013). 5 The	Hard-Cases	Challenge	to	Monism The	goal	of this	paper is to specify the	general conditions	under	which	particular	biological objects	stand	in	a	part-whole	relation	to	each	other.	To	sum	up,	my	analysis	of	paradigmatic examples	from	biological	practice	reveals	two	criteria	for	biological	parthood. An	Account	of	Biological	Parthood An	object	x,	which	engages in	biological	process	p, is	a	biological	part	of	an	object	y,	which shows	characteristic	behaviors	b1,...bn,	if	and	only	if (1) Substantial Spatial Inclusion (SSI): if y has a natural boundary, x must be spatially located	inside	or	in	the	region	that	the	natural	boundary	occupies	and (2) Compositional	Relevance	(CR):	p	is	relevant	to	at	least	one	of	b1,...bn,	that	is,	p	makes	a necessary contribution to a condition that is minimally sufficient to one or more of b1,...bn. 23	Compositional	relevance	thus	turns	out	to	be	a	subtype	of	causal	relevance	(if	one	accepts	Mackie's	claim	that causes	are	INUS-conditions	for	their	effects).	Note,	however,	that	this	does	not	imply	that	biological	part-whole relations are special kinds of causal relations. Biological parts	must satisfy further criteria (e.g., they	must be spatially	included	in	the	whole	in	a	substantial	way)	and,	thereby,	violate	conditions	that	many	authors	regard	as characteristic	of	causal	relations	(e.g.,	that	cause	and	effect	are	distinct). 19 This is a	monistic account of biological parthood because it specifies two criteria that are individually	necessary	and	jointly	sufficient	for	a	biological	part-whole	relation	to	exist.24	That is,	my	account	identifies	a	single	set	of	criteria	that	is	supposed	to	hold	for	any	object	in	the biological	world. My	analysis	also	shows	that	there	exist	certain	kinds	of	part-whole	relations	that	can	be referred to as 'hard cases' because they place requirements on a monistic account of biological	parthood	that	pull	in	different	directions.	Understanding	what	these	hard	cases	are and	why	it is	so	difficult	to	account	for	them	at	once	sheds	light	on	the	merits	and	limits	of monism. Hard	Cases	for	an	Account	of	Biological	Parthood (1) Redundant parts (e.g., kidney of humans removing waste products from the body): Some	biological	parts	are	redundant	because	there	are	other	token	objects	that	engage in	the	same	kind	of	process,	only	one	of	which	is	necessary	to	a	behavior	of	the	whole. (2) Irrelevant	parts	(e.g.,	appendix	of	humans):	Some	biological	parts	engage	in	processes that	are	irrelevant	to	any	characteristic	behavior	of	the	whole	(e.g.,	because	they	have become	non-functional	during	evolutionary	history). (3) Inactive	parts	(e.g.,	members	of	a	population	not	interbreeding):	Some	biological	parts do	not	actually	engage	in	relevant	processes;	they	merely	have	the	disposition	to	do	so, which	is	not	manifest. (4) Collective parts (e.g., calcium ion in a muscle fiber being released into the cytosol): Some	biological	parts	only	collectively	have	a	significant	effect	on the	behavior	of the whole. (5) Relevant	non-parts	(e.g.,	case	of	a	caddisfly	larva	that	protects	it	from	predators):	Some biological	objects	engage	in	processes	that	are	relevant	to	the	behavior	of	the	whole	but are	treated	as	background	conditions	or	as	parts	of	the	context. How	does	my	account	of	biological	parthood	cope	with	these	hard	cases?	The	Compositional Relevance	(CR)	criterion	accounts for	redundant	parts	because	the	processes in	which	parts engage	need	not	be	necessary	to	one	of	the	characteristic	behaviors	of	the	whole,	they	must only be necessary	members of a set of biological parts,	which are sufficient to one of the characteristic behaviors of the whole. Each kidney of a human, for example, is an INUScondition for the	human's	survival	–	even	though	they	are	not	necessary	parts	of the	same condition. Cases of collective parts do not present a problem for CR because even if a biological	part,	such	as	a	calcium	ion,	on	its	own,	lacks	a	significant	effect	on	the	contraction of a	muscle fiber it still belongs to an INUS-condition for	muscle contraction. CR is only a necessary	but	not	a	sufficient	condition	for	biological	parthood.	Cases	of	relevant	non-parts, 24	In	what	follows	I	provide	support	for	the	claim	that	these	criteria	are	jointly	sufficient.	I	consider	and	dismiss additional	criteria	(e.g.,	common	origin/history	or	genetic	identity)	for	biological	parthood	because	they	are	not necessary. 20 such	as	the	blood	that	is	transported	to	the	heart	and	thus	is	an	INUS-condition	for	the	blood pumping of a heart, pass CR but are excluded by the Substantial Spatial Inclusion (SSI) criterion	because	only the	blood inside the	ventricles	and	atria is located inside the	natural boundary	of	the	heart.	This	is	a	major	reason	for	why	SSI	is	a	separate	criterion	for	biological parthood	that	cannot	be	reduced	to	CR. Inactive	parts present a	much stronger challenge to	CR	because they	only potentially engage in	processes that are INUS-conditions for	one	of the characteristic	behaviors	of the whole and thus call into question that CR is a necessary condition. For example, not all organisms	that	belong	to	a	particular	population	actually interbreed	with	other	members	of the population. The mere capacity to interbreed suffices to be a biological part of the population.	One	might	argue	that	examples like	this	require	that	CR	must	be	weakened,	for instance,	by	replacing	'x	engages	in	biological	process	p'	by	'x	has	the	disposition	to	engage	in biological	process	p'.	A	weakened	CR	criterion,	however,	invites	more	counterexamples	than it copes with because several non-parts would turn out to be relevant. T-helper cells, for example,	have	the	disposition	to	recognize	antigens	and	thus	would	count	as	biological	parts of	our immune	system. In	addition,	a	dispositionally	formulated	relevance	criterion	neglects the	fact	that	processes	are	central	to	biological	parthood	(recall	Section	4). Cases	of irrelevant	parts	also	challenge the status	of	CR	as	a	necessary condition. For example,	our	appendix	does	not	engage	in	processes	that	are	INUS-conditions	for	one	of	our characteristic behaviors, such as our survival, reproduction, reasoning or cooperation with others	but	we	typically	consider	our	appendix	to	be	a	biological	part	of	us.	Again,	one	might argue	that	examples	like	this	show	that	we	must	replace	CR	by	a	weaker	relevance	criterion. But if we weaken the relevance criterion we will face many new counterexamples of biological	objects	which	pass	the	weaker	relevance	criterion	but	which	we	do	not	regard	as biological	parts	(i.e.,	relevant	non-parts),	such	as	the	cotton	ball	that	is	left	inside	our	stomach during	surgery. Alternatively,	one	might	argue	that	the	two	criteria	that	I	present	in	this	paper	are	not sufficient	to	biological	parthood	and	must	be	supplemented	by	additional	necessary	criteria. In the case	of our appendix, for example, referring to the common	history	or origin	of the appendix and the human body as a	whole or to the genetic identity of	most of their cells might	provide	us	with	a	criterion	that	individuates	the	appendix	as	a	biological	part,	but	not the	cotton	ball inside	my	stomach (cf. Jansen	and	Schulz	2014).	The impression that	spatial and	causal-functional	relations	are	only	part	of	the	story	about	biological	part-whole	relations is confirmed	by	examples that show	how	some	biological	wholes	also structurally constrain their	parts.	For	example,	it	follows	from	the	chemical-structural	nature	of	the	human	genome (i.e., from the fact that it is a nucleic acid) that its biological parts must be sequences of nucleotides rather than, for instance, proteins.25 Each of these putative additional criteria, 25 Likewise, the structure of the Golgi apparatus implies that its parts must be membrane-enclosed compartments or vesicles, and a population of black-headed gulls consists only of organisms of the species Chroicocephalus	ridibundus	(not	of	the	beach	or	of	other	kinds	of	birds). 21 however, captures	only a small subset	of biological part-whole relations and thus	does	not constitute a necessary criterion that is universally applicable to the biological world. For instance,	only	some	biological	wholes	have	a	structural	nature	that	constrains	their	parts	in	a non-trivial	manner.26	The	criterion	of	genetic	identity	is	violated	in	many	cases	(e.g.,	genetic chimera, symbionts, ecosystems, etc.), and also the requirement that biological parts	must originate together with or inside the whole does not hold for the majority of part-whole relations	because	most	biological	objects	continuously	loose	and	acquire	particular	parts. 6.	Conclusions:	On	the	Merits	and	Limits	of	Monism My analysis of hard cases reveals at which point monism reaches its limit. For a	monistic account of biological parthood, it is impossible to cope	with all hard cases at once. If you weaken the Compositional Relevance (CR) criterion to account for irrelevant parts and inactive	parts, you	will not be able to exclude several cases	of relevant non-parts (and the other	way	round).	You	might	solve	the	problem	by	introducing	additional	criteria	that	capture other kinds of constraints on biological part-whole relations (e.g., historical, genetic, or structural	constraints).	These	additional	criteria,	however,	will	apply	only	to	certain	kinds	of biological	objects	and	not	hold	universally.	The	goal	of	universality	thus	runs	contrary	to	the goal	of	descriptive	adequacy	(Kaiser	2015,	Chapter	2)	and it	seems	as if	we	need	to	make	a principled	decision	at	this	point	– in	favor	of	more	descriptive	adequacy	and	pluralism	or in favor	of	more	universality	and	monism. Among	the	virtues	of	my	monistic	account	is	that	it	gives	a	clear	and	precise	answer	to the general question under which conditions one biological object is a part of another biological	object.	Monism	highlights	the	commonalities	of	part-whole	relations	from	different levels	of	organization	(e.g.,	a	vesicle	being	a	part	of	a	Golgi	apparatus	and	an	organisms	being a	part	of	a	population)	and	it	reveals	how	biological	wholes	in	general	constrain	their	parts.	A monistic	account	of	biological	parthood	thus	provides	unification.	On	the	other	hand,	monism runs the risk of yielding an overly simple and descriptively inadequate view of biological parthood,	which	fails	to	account	for	the	diversity	of	biological	part-whole	relations,	reflected in	the	various	approaches	that	biologists	adopt	to	individuate	part-whole	relations	(cf.	Kellert, Longino,	and	Waters	2006). The goal of this paper is not to defend my monistic account against pluralistic alternatives.	Rather, I	presuppose the	goal	of	monism,	defend	my	account	against	monistic alternatives,	and	use	my	analysis	to	point	out	the	limits	that	monism	faces.	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