Rani	Lill	Anjum	&	Stephen	Mumford	What	Tends	to	Be:	the	Philosophy	of	Dispositional Modality.	London	&	New	York:	Routledge,	hbk	pp.	x+193 (published	in	Notre	Dame	Philosophical	Reviews:	https://ndpr.nd.edu/about/	) Stathis	Psillos	&	Stavros	Ioannidis,	University	of	Athens,	Greece There	seems	to	be	widespread	agreement	that	there	are	two	modal	values:	necessity	and possibility.	X	is	necessary	if	it	is	not	possible	that	not-X;	and	Y	is	possible	if	it	is	not	necessary that	not-Y.	In	their	path-breaking	What	Tends	to	Be:	the	Philosophy	of	Dispositional	Modality, Rani	Lill	Anjum	&	Stephen	Mumford	(henceforth	A&M)	defend	the	radically	new	idea	that there	is	a	third	modal	value,	weaker	than	necessity	and	stronger	than	possibility.	This	third value	is	dubbed	'dispositional	modality'	(DM)	or	'tendency'	and	is	taken	to	be	an	irreducible and	sui	generis	worldly	modality:	"the	modality	that's	everywhere"	(5).	The	source	of	DM	is the	causal	powers	of	particulars;	hence,	DM	is	constitutively	involved	in	causation:	"a	cause tends	or	disposes	towards	its	effect,	and	can	sometimes	succeed	in	producing	it"	(9). Accordingly,	DM	is	involved	in	all	causal	processes,	from	fundamental	physics	to	the	social and	moral	realm.	This	"deeply	tendential	view"	(11)	of	the	metaphysics	of	nature	is	advanced as	distinct	from	both	extant	neo-Aristotelian	and	Humean	views.	Its	key	feature	is	that	there	is neither	pure	contingency	nor	necessitation	in	nature. The	idea	of	DM	has	already	been	introduced	by	A&M	in	previous	publications. However	not	only	does	this	book	offer	a	systematic	exposition	of	DM,	but	also	it	uses	it	as	a tool	to	address	(and	claim	accounts	for	and	solutions	of)	a	number	of	central	philosophical concepts	and	problems:	causation,	chance,	the	logic	of	conditionals,	conditional	probability, perception,	inductive	inference,	ethics	and	the	problem	of	free	will. Here	is	the	structure	of	the	book. Chapter	1	outlines	the	two	basic	planks	of	the	deeply	tendential	view:	the	'external	principle of	tendency'	(there	is	no	causal	necessitation,	period)	and	the	'internal	principle	of	tendency' (the	modal	nature	of	the	cause	is	internally	tendential,	meaning	that	the	effect	of	a	cause	may be	produced	even	if	it	is	not	necessitated	but	it	may	not	be	produced	even	if	the	conditions	are right	(18-19)).	Chapter	2	offers	a	brief	(and	sketchy)	narrative	of	possible	historical predecessors	of	DM	and	attempts	a	diagnosis	as	to	why	DM	has	not	been	hitherto	adopted even	by	friends	of	tendencies.	In	chapter	3,	it	is	argued	that	powers	come	in	various	degrees of	strength	and	they	can	even	overdispose	towards	an	outcome;	yet,	it	is	stressed	that	the degrees	of	strength	should	not	be	taken	to	be	probabilities.	Chapter	4	(co-authored	with Fredrik	Andersen)	argues	that	since	causation	does	not	involve	necessitation,	there	is	no problem	with	causation	in	quantum	mechanics.	The	main	claim	of	chapter	5	(co-authored with	Johan	Arnt	Myrstad)	is	that	realism	about	dispositions	requires	an	intensional conditional,	whereas	in	chapter	6,	A&M	outline	a	dispositional	account	of	conditional probability.	In	chapter	7,	it	is	argued	that	perception	is	a	mutual	manifestation	between	the perceiver	and	the	object	perceived.	Chapter	8	turns	to	causal	inference.	It	is	argued	that	since tendencies	may	fail	to	produce	their	outcome,	inductive	inference	cannot	be	certain	and	will only	"tend	to	track	the	truth"	(142).	Finally,	chapters	9	(co-authored	with	Svein	Anders	Noer Lie)	and	10	are	about	value	and	the	problem	of	free	will.	Their	key	point	is	that	since causation	does	not	require	necessitation,	one	can	be	compatibilist	about	free	will	without being	committed	to	determinism. Let	us	now	take	a	closer	look	at	some	of	the	central	arguments/contentions	of	A&M.	As noted	already,	the	two	planks	of	DM	are	the	'external'	and	the	'internal'	principles	of	tendency. Capitalising	on	earlier	work,	A&M	argue	that	there	is	no	causal	necessitation,	since	if	causes necessitate	their	effects,	"it	should	be	impossible	to	have	the	cause	without	the	effect"	(11); but	it's	possible	that	something	might	interfere	with	the	cause	and	block	the	occurrence	of	the 2 effect.	So,	while	C	typically	causes	E,	C-plus-I	might	not	cause	E.	A&M	argue	that	a	hallmark	of necessitation	is	the	so-called	antecedent	strengthening	(AS)	test,	viz.,	if	A	necessitates	B,	then whenever	A,	then	B,	which	they	take	it	to	mean	that	"if	A-plus-φ,	for	any	φ,	then	still	B"	(13). Alleged	causal	necessitation	fails	this	test,	so	A&M	argue	there	is	no	causal	necessitation. Causes,	however,	do	produce	their	effects.	On	A&M,	"The	external	principle	has	the implication	that	causes	did	not	necessitate	their	effects	even	on	the	occasions	where	they succeeded	in	producing	them"	(11). This	claim,	we	think,	makes	mysterious	how	effects	are	produced	when	they	are produced.	But	we	shall	come	back	to	this	worry.	For	the	time	being,	two	others	worries	have precedence.	The	first	is	that	it	is	not	clear	what	notion	of	possibility	is	involved	in	the	claim that	it	is	possible	that	something	might	interfere	with	the	cause	and	block	the	effect.	If	it	is allowed	as	possible	that	Morgana	casts	a	spell,	then	this	notion	of	possibility	is	too	weak.	If there	is	some	restriction	to	possibility,	a)	it	is	not	clear	what	this	is;	but	b)	if	it	is	based	on some	idea	of	natural	possibility	which	is	itself	based	on	dispositional	modality	(as	is insinuated	on	pp.	22-23),	then	the	account	is	circular.	The	second	worry	is	this.	That	A causally	necessitates	B	means	that	A	is	(causally)	sufficient	for	B.	But	then	the	AS-test	is	moot vis-à-vis	causal	necessitation.	To	see	this	(briefly),	consider	that	a	cause	is	(at	least)	an	INUScondition	for	an	effect.	Hence,	it	is	part	of	a	cluster	of	factors	which	are	jointly	sufficient	but typically	unnecessary	for	the	effect.	It's	not	hard	to	see	then	that	when	there	is	an	additive interference,	one	or	more	of	the	INUS-conditions	either	change	or	get	eliminated;	hence,	there is	a	different	cause	and	(no	surprise)	the	original	effect	does	not	follow. The	real	bite	of	DM,	what	yields	a	'deeply	tendential'	view,	is	the	more	radical	'internal principle	of	tendency'	(17).	A&M	insist	that	though	causes	do	not	necessitate	their	effects, they	nevertheless	can	produce	them.	When	this	happens,	it	is	not	simply	a	matter	of contingency:	when	a	fine	china	falls	to	the	ground,	there	are	many	possibilities	besides breaking	(it	may	evaporate!),	but,	A&M	think,	one	of	those	possibilities	(the	breaking)	is	in some	sense	'stronger'	or	'more	privileged'	than	others	(4).	Beware:	this	does	not	mean	that this	one	possibility	is	more	likely	to	happen;	tendencies,	we	are	told,	should	not	to	be understood	in	terms	of	probabilities.	What	privileges	some	of	the	infinitely	many	possibilities is	that	there	are	powers	that	tend	towards	them,	where	these	tendencies	(of	the	powers)	are "real	modal	features	that	are	stronger	than	mere	possibilities"	(20).	As	they	put	it,	"[o]f	all	the infinite	possibilities,	only	some	of	them	are	those	towards	which	there	are	tendencies"	(19). So,	A	causally	produces	B	by	having	a	tendency	towards	B,	where	this	tendency	is	something more	than	mere	possibility. What's	really	striking	is	not	that	tendencies	can	produce	without	necessitating	an effect,	but	rather	that	the	effect	may	not	happen,	even	if	all	"non-trivially	right"	conditions	for it	are	present	(18).	They	couldn't	be	more	explicit:	"With	the	internal	principle,	there	is nothing	that	prevents	the	effect	from	occurring,	but	still,	it	need	not	occur,	just	because	the modal	feature	of	the	cause	is	internally	tendential"	(18).	This	move	seems	to	pile	a	new mystery	on	the	old.	But	perhaps	there	is	no	mystery	here.	A&M	seem	to	model	the	tendential view	on	the	case	of	radioactive	decay	(19).	But	some	caution	is	needed	here.	That	a radioactive	nucleus	is	unstable	and	will	eventually	decay	by	emitting	a	particle,	transforming the	nucleus	into	another	nucleus,	or	into	a	lower	energy	state,	does	not	mean	that	this	nucleus has	a	tendency	to	decay.	At	any	given	time,	an	atom	will	be	stable	or	will	decay.	What	is	true	of the	atom	is	that	it	has	a	probability	e-At	of	remaining	stable	for	an	interval	t	(depending	on	the decay	constant	A).	If	we	were	to	think	of	this	case	in	terms	of	tendencies	we	would	have	to ascribe	to	the	atom	two	conflicting	tendencies:	to	decay	and	to	remain	stable! Be	that	as	it	may,	the	result	is	that	no	causal	sequence	of	events	is	determined-not even	mundane	sequences	like	boiling	an	egg.	A&M	allow	that	particulars	A	and	B	have	the same	powers	and	are	under	exactly	the	same	conditions,	and	hence	they	have	exactly	the same	tendencies	and	yet	"it	is	possible	that	one	does	and	one	does	not	[manifest	this 3 tendency]–not	because	of	any	external	interference	but	because	of	the	internal,	dispositional nature	of	power"	(19). The	motivation	for	the	internal	principle	seems	to	be	the	thought	that	it	shows	that events	can	be	caused	without	there	being	a	sufficient	reason	for	their	occurrence:	the tendency	of	A	to	cause	B	can	never	necessitate	the	occurrence	of	B;	still	it	causes	(produces)	B when	B	does	occur.	But	note	the	irony.	On	this	view,	every	event	has,	after	all,	a	cause-a tendential	one,	but	still	a	cause. Though	it	seems	tempting	to	think	that	A&M	subscribe	to	generalised	indeterminism, they	confuse	matters	by	drawing	a	distinction	between	deterministic	and	indeterministic tendencies:	"Indeterministic	propensities	in	exactly	the	same	circumstances	need	not	produce the	same	outcome,	whereas	deterministic	propensities	do"	(62).	But	haven't	we	been	told	that things	that	possess	the	same	deterministic	propensities	may	behave	differently	in	exactly	the same	circumstances?	Such	behaviour	would	be	indistinguishable	from	what	would	result from	indeterministic	propensities.	But	then,	we	end	up	with	a	case	of	metaphysical underdetermination. Take	the	law	of	inertia:	a	body	with	no	net	force	acting	on	it	will	continue	to	move	with uniform	motion	or	remain	at	rest.	If	this	results	from	a	tendency	of	a	body,	then,	on	the tendential	view,	it	is	possible	that	without	any	external	interference	whatsoever,	the	tendency might	fail	to	produce	its	outcome.	Wouldn't	then	the	internal	principle	imply	that	the tendency	leads	to	the	violation	of	the	law	it	gives	rise	to? A&M	argue	that	although	tendencies	may	have	degrees	of	strength	(they	say	that	"a power	that	has	such	a	degree	is	what	we	call	a	propensity"	(56)),	they	should	not	be	viewed	in terms	of	probabilities,	since	tendencies,	unlike	probabilities,	can	overdispose.	As	they	explain, "[o]verdisposing	is	where	there	is	a	stronger	magnitude	than	what	is	minimally	needed	to bring	about	a	particular	effect"	(50).	For	example,	if	a	stone	of	a	certain	weight	can	break	a particular	window,	a	stone	that	weighs	twice	as	much	and	is	thrown	towards	the	window, overdisposes	towards	its	breaking.	This	leads	to	a	main	difference	between	tendencies	and probabilities:	in	contrast	to	probabilities,	which	do	not	take	values	>1,	the	intensity	of	a tendency,	precisely	because	of	overdisposing,	can	be	'unbounded'	(52). If	"tendencies	are	more	fundamental	than	probabilities",	and	they	"can	ground	the	facts of	probability"	(56),	what's	the	connection	between	the	two?	It's	not	clear.	But	there	seems	to be	a	deeper	problem.	Suppose	there	is	a	tendency	A	with	the	minimal	strength	required	to bring	about	effect	B.	A&M	argue	that	even	then	the	probability	of	the	effect	given	the	cause	is not	1.	They	actually	argue	that	overdisposing	(increasing	the	strength	of	the	tendency)	is	a way	to	raise	the	probability	of	the	effect	happening,	which	might	approach	1	but	will	never	be 1	(57-58).	Given	this,	it	is	totally	unspecified	what	'minimal	strength'	is.	If	'A	has	the	minimal strength	required	to	bring	about	B'	does	not	licence	the	claim	that	the	probability	of	B	given	A is	1,	there	is	no	way	we	can	link	tendencies	with	probabilities.	For	if	the	probability	of	the effect	B	given	a	sufficiently	strong	cause	A	is	≠1,	then	there	is	no	fact	of	the	matter	about	what this	probability	is	-it	could	be	anywhere	in	(0,1).	It	couldn't	even	be	OK	to	say	that	the probability	of	the	outcome	is	more	likely	to	happen	than	not	(i.e.	p>0.5),	since,	due	to	the nature	of	dispositional	modality,	nothing	guarantees	that	it	is	more	likely	that	a	tendency	will manifest	itself	than	not. The	connection	between	probabilities	and	tendencies	gets	even	more	complicated, when	A&M	distinguish	between	non-probabilistic	and	probabilistic	tendencies.	Probabilistic tendencies	are	tendencies	"towards	a	distribution	within	a	whole	that	is	constituted	by	results of	a	sequence	of	trials"	(61).	But	consider	a	fair	die;	since	only	6	outcomes	are	possible,	there is	something	that	is	necessitated	after	all:	that	every	throw	of	the	die	will	result	in	one	of	the six	possible	outcomes.	A&M	deny	this:	in	actual	situations	(as	opposed	to	the	idealised	model) the	die's	probabilistic	tendency	does	not	require,	nor	does	it	guarantee,	that	only	these	6 outcomes	are	possible.	Since	the	die	"might	break	apart,	land	in	a	crack"	and	so	on	(61),	the 4 disjunction	(either	1	or	2	or	3	or	4	or	5	or	6)	is	not	necessitated.	Like	the	case	of	nonprobabilistic	tendencies,	tendencies	towards	a	distribution	might	fail	to	manifest	themselves. This	account	leaves	unsettled	how	probabilities	are	fixed.	Why	assume	that	the	die	has any	tendency	towards	a	distribution,	if	any	outcome	is	possible?	Besides,	given	that	to	have	a probabilistic	tendency,	for	A&M,	is	to	have	multiple	tendencies	with	mutually	excluding manifestations	(50),	how	many	tendencies	are	there	in	a	die?	Only	6	or	indefinitely	many?	It seems	there	is	a	genuine	difficulty	to	go	from	an	actual	distribution	to	the	actual	probabilistic tendencies	(and	conversely). Surprisingly,	A&M	say	on	p.	61	that	radioactive	decay	is	a	non-probabilistic	propensity: "There	is	only	one	manifestation	type,	which	either	occurs	or	not,	so	this	is	not	a	probabilistic propensity	(...)	because	it	doesn't	tend	towards	multiple	outcomes".	But	coins	have probabilistic	propensities.	What's	the	difference?	In	the	coin	case,	it	either	lands	heads	or tails;	in	the	atom	case	"it	either	decays	or	it	doesn't"	(61). Why	does	all	this	pan	out	in	scientific	methodology?	A&M	argue	that	the	existence	of tendencies	is	the	best	explanation	for	both	the	extent	to	which	the	world	is	regular,	and	the extent	to	which	every	regularity	can	fail.	But	given	the	internal	principle,	A&M	end	up	facing	a version	of	the	so-called	Inference	Problem	for	laws	of	nature:	how	can	we	infer	from	the worldly	tendencies	the	behaviour	of	worldly	things?	It	is	the	distinctive	mark	of	the	deep tendential	view	that	the	same	tendencies	are	compatible	with	many	different	possible scenarios	concerning	the	extent	of	regularity	observed	in	the	world.	Conversely,	it	is	not	clear how	we	can	have	statistical	evidence	(136)	for	what	tendencies	there	are	in	the	world.	"A tendency	could	be	evident,	for	instance,	in	a	raised	incidence	of	some	phenomenon,	where	the incidence	is	less	than	universal	but	detectably	above	levels	of	pure	chance",	A&M	claim	(136). But	then	again,	given	what	has	been	said,	a	raised	incidence	of	some	phenomena	is	fully consistent	with	no	relevant	tendency	since	the	latter	might	not	lead	(or	might	not	have	led)	to the	raised	incidence.	Once	tendencies	have	been	cut-off	from	worldly	(probabilistic	or universal)	distributions,	it's	hard	to	re-connect	them	again.	For	all	we	know,	'less-thanuniversal'	regularities	(136)	observed	in	the	world	might	be	attributed	to	pure	luck.	Similarly, scientific	explanation	cannot	only	rest	on	what	tendencies	are	operating,	but	also	on	the	brute fact	that	those	tendencies	succeed	in	producing	their	manifestations.	Needless	to	say	that,	on this	view,	all	prediction	(even	based	on	tendencies	with	sufficient	strength)	should	be	made with	'fingers	crossed'. What	Tends	to	Be	is	an	admirable	book.	Anjum	&	Mumford	advance	a	novel philosophical	thesis,	they	defend	it	with	rigour	and	ingenuity	and	explore	uncharted territories.	Given	that	the	map	of	the	area	is	currently	under	drawing,	there	are	bound	to	be inaccuracies	and	infelicities.	But	that's	the	fate	of	all	explorers,	philosophical	or	otherwise. Despite	our	critical	points,	Anjum	and	Mumford	have	succeeded	in	producing	a	thorough, deep	and	challenging	exploration	of	matters	metaphysical.