Mereological Commitments
Achille C. Varzi
Department of Philosophy, Columbia University, New York
(Published in Dialectica 54 (2000), 283–305)
Abstract. We tend to talk about (refer to, quantify over) parts in the same way in which wetalk about whole objects. Yet a part is not something to be included in an inventory of theworld over and above the whole to which it belongs, and a whole is not something to be
included in an inventory over and above its own parts. This paper is an attempt to clarify away of dealing with this tension which may be labeled the Minimalist View: an element in
the field of a part-whole relation is to be included in an inventory of the world if, and only if,
it does not overlap any other element that is itself included in the inventory. As it turns out,a clarification of this view involves both a defense of mereological extensionality and an
account of the topological distinction between detached and undetached parts (and the parallel
opposition between scattered and connected wholes).

1. Introduction
My finger is touching the top of the table. How many things am I touching?
We’d be likely to count just the table and answer: One. But if we are given a list
including the table, its top, its four legs, and other things, and if somebody asks
“How many things on this list are you touching?”, we’d be likely to count thetable as well as the top and answer: Two.
I take this ambiguity to reflect a general concern about the ontologicalstatus of proper parts, a concern that lies in the background—though rarely inthe foreground—of much part-whole theorizing. Typically one treats parts asobjects of quantification on a par with the wholes to which they belong. This is
so in formal treatments of the parthood relation, such as Leśniewski’s
Mereology or Leonard and Goodman’s Calculus of Individuals. And it is so
also in much ordinary speech. We often talk about parts with the same easinesswith which we talk about whole objects; we name parts, describe them, comparethem with one another. (The top of the table is not as scratched as its legs.
Tibbles’ tail is longer than Pluto’s.) However, this way of speaking goes handin hand with the thought that an object’s parts are things that do not quite have alife of their own—things that do not count except as parts of the whole to which
1they belong. This is why we would only count the table in the first place. I’m
touching the top of the table, so it is the table that I am touching. And if we hadto draw up an inventory of the world—a catalogue of all there is—we’d belikely to include the table and not its parts.
This tension between countenancing and counting is, of course, particularly
striking when we think of parts that are not physically separated from the rest of
their wholes, such as the top of a table or the tail of a cat. Such parts do not havea complete outer boundary, so they do not have the thingy character that we
expect from ordinary objects. By contrast, there seems to be nothing peculiar
about the ontological status of those parts that do have a complete boundary,parts that are fully separated from the rest of the wholes to which they belong.
Consider the scattered whole consisting of Tibbles and the table: their
mereological sum, or fusion. Tibbles and the table are parts of this sum—theyare proper parts. But that is no reason to question their individuality.
This also illustrates a dual worry concerning the ontological status ofwholes, or mereological complexes broadly understood. If we do indeed include
Tibbles and the table in our inventory of the world (and not their proper parts),
shall we also include their mereological sum? This is not the question ofwhether the sum should be countenanced at all. We may agree that any
collection of objects, however disparate and gerrymandered, forms a legitimate
whole and yet ask: Shall we list this whole in an inventory of the world that
already includes its constituent parts? Again, the tension is between a tendency
to treat such things as objects of reference and quantification, on the one hand,
and their being nothing over and above their proper parts, on the other. If
Tibbles and the table are already in the inventory, adding their sum would be
redundant. And as the redundancy is especially striking in the case ofundetached parts, it is striking when it comes to disconnected wholes.
2. The Minimalist View
I am not, of course, suggesting that there is only one proper way of drawing up
an “inventory of the world”, nor that the best way of drawing up an inventory
is by listing all and only connected wholes. We may want to count a bikini (adisconnected aggregate) as one item, and we may want to count a bunch of fivebananas as five items even though each one is undetached from the rest. We
know since Frege that counting is no simple business, and an inventory of the2

world—in the intuitive sense in which I am using the phrase—is closely related
to a count policy. We cannot just ask—How many things? We must countthings of a sort—How many bunches? How many bananas?1 So we may thinkof an inventory as the result of sorting out the items in our domain of discourse,which is often a matter of context. The cashier will count five bananas; we may
count them as one item when it comes to sorting out the things to be carried
upstairs. There are many ways of drawing up an inventory. The question I am
interested in is: How does mereology enter the picture? How does discourse
about parts and wholes constrain our freedom to sort things out?
David Lewis sees this concern as one of ontological commitment:
Given a prior commitment to cats, say, a commitment to cat-fusions is not a further
commitment. The fusion is nothing over and above the cats that compose it. It just isthem. They just are it. Take them together or take them separately, the cats are thesame portion of Reality either way. [1991: 81]

Thus, the ‘are’ of mereological composition—the many-one relation of the parts
to a whole—is for Lewis a sort of plural form of the ‘is’ of identity. This viewis also defended by Donald Baxter (among others), who actually holds that
mereological composition is more than analogous to ordinary identity. It is
identity:
The whole […] is just the parts counted loosely. It is strictly a multitude and loosely asingle thing […] The whole is just the n parts collectively on the strict count, or is asingle thing on the loose count, and in neither count are there n+1 things. [1988: 580f]

Now this view offers a simple and appealing way of resolving the tension
alluded to above. Mereology distinguishes a whole from its parts. But the whole
and the parts encompass the same amount of reality and should not, therefore,
be listed separately in an inventory of the world. More generally, we should notinclude entities that overlap, i.e., share common parts. If we include the table weshould not include its top and legs. If we include its top and legs, we ought todisregard the whole table as well as every other table part. For instance, we
ought to disregard the right half of the table, consisting of the two right legs andthe right half of the top.
1
See Frege [1884: §§ 46ff]. For some classic discussion see, e.g., Dummett [1973, Ch.
16], Geach [1980: §§ 31ff], Wiggins [1980], and Lowe [1989].

3

Call this the Minimalist View. The Minimalist View says nothing specificabout mereology, about what entities are part of what. But, given a mereologicaltheory and a corresponding domain of quantification, the view tells us how toweigh our ontological commitments:(M) An inventory of the world is to include an entity x if and only if x
does not overlap any other entity y that is itself included in thatinventory.
Thus, a good inventory must be complete: everything in the domain ofquantification must show up somewhere.2 But a good inventory must also avoid
redundancies: nothing should show up more than once.
This is of course not a definition. Rather (M) is a postulate on what it takes
to be included in a good inventory of the world. On the Minimalist View, it does
not matter what things we actually include, as long as the completeness andnon-redundancy conditions in (M) are both satisfied.3 Moreover, in saying thatthe variables range over a domain that is fixed by the underlying mereologicaltheory, I am formulating (M) in a way that is neutral with respect to issues ofmetaphysical realism. On the Minimalist View it does not matter whether thedifferent domains that we can get by employing different mereological theories
represent different descriptions of the same world, or different worlds
altogether.4 The point is that once we have got the pieces, we may start drawing
up an inventory. And a good inventory—one that reflects our ontologicalcommitments—is not simply a list of all the pieces that we have got, a list of allthat can enter our parthood relation. On the Minimalist View, a good inventory
is more selective: it must meet the constraint in (M).
To put it differently, the Minimalist View draws a distinction between twosenses in which a thing can be object of reference or quantification—two
2

I am assuming here that the underlying mereology satisfies the remainder principle,
so that if x properly overlaps y, then x has two parts, one included in y, and one disjoint
from y.
3
If we think of the world as the mereological fusion of everything in it, then thisamounts to the requirement that any inventory be a list of things of which the world isstrictly made up, in the sense of Chisholm [1973: 587], or of which the world is composed ,in the sense of van Inwagen [1987: 22]. A first formulation of (M) is presented and briefly
discussed in Casati and Varzi [1999b: 111ff].
4
See Putnam [1987: 16ff].

4

notions of individual existence. In one sense, those items exist that are includedin the domain of quantification of our part-whole theory, whatever it is (call thisthe Quinean notion of existence5 ). Obviously, different part-whole theories may
have different domains, so the choice of one theory rather than another makes adifference with respect to this notion of existence. For instance, a standard Leśniewskian mereology will countenance the mereological sum x+ y whenever itcountenances x and y, whereas a mereology deprived of the sum axiom may justcountenance x and y. On the other hand, there is a sense in which just those
items exist that are listed in the relevant inventory of the world, depending on
how this is drawn up. These items will all be included among those things thatenjoy Quinean existence, since they will have to be recognized as objects ofreference and quantification by the part-whole theory; but not everything thatexists in the Quinean sense must exist in this restricted sense (the selective
notion of existence). It may actually be the case that different inventories are
drawn up on the basis of one and the same mereological theory. One
mereologist may go for a fine-grained inventory including x and y but not x+ y;
another mereologist may go for a coarser inventory including x+ y but not x or
y. It may also be that the same inventory is grounded on different part-whole
theories. (The mereologist who goes for the fine-grained inventory may end up
with the same inventory as the alternative mereologist who does notcountenance mereological sums: both would only list x and y.) Be it as it may,the Minimalist View says that the Quinean notion of existence sets the background for the selective notion, but does not exhaust it; the rest must be done incompliance with (M).
So this is how the Minimalist View takes the fundamental tension of mereology to be explained. We quantify over everything, since the meaning of‘everything’ is set by the domain of the quantifiers; yet counting is selective.
And we may set different standards for counting, but we must avoid omissions
and repetitions. My aim in the following is to test and attempt a more complete
account of this explanation. I shall especially focus on three questions, all ofwhich seem to me crucial for a proper understanding of the Minimalist View.
The first concerns the assumption—implicit in Lewis’s and Baxter’s remarks
as well as in my formulation of (M)—that, when it comes to counting things,

5

Quine [1939].

5the alternative is really between the parts and the whole. This is essentially theassumption that mereology is extensional—that there is only one whole for any
given number of parts (i.e., equivalently, that two distinct wholes may not becomposed of exactly the same parts). I shall address this question and some of
its ramifications in Sections 3 and 4. The second question is addressed in Section 5 and concerns the distinction between connected and disconnected entities,
and the related notion of a maximal, one-piece entity. This notion—I shallargue—plays some role in the proper account of certain puzzles that arise inrelation to extensionality and is therefore needed for a proper understanding of(M). Moreover, a clarification of what it means for something to be maximallyconnected is needed if the intuitive count policy described in the introduction(according to which undetached parts and disconnected wholes should not becounted) is to make any sense at all. Finally, the third question that I wish toaddress concerns the possibility of formulating other count policies—otherviable ways of drawing up inventories of the world. This question will be taken
up in Section 6 and will lead to an examination of various reasonable
constraints that can be added to (M), and of their interaction with the underlying
mereology.
3. Extensionality
Let us begin with the question of extensionality. Take again the case of Tibbles,
the cat. Call her tail ‘Tail’ and the rest of her body ‘Tib’. According to the
Minimalist View, when we set ourselves to drawing up an inventory of theworld we have two main options: either we count the whole cat, Tibbles (this is
the loose count, in Baxter’s terminology), or we count an exhaustive collectionof disjoint parts, such as Tib and Tail (strict count)6 . (There are other options
corresponding to other ways of carving out objects, for instance by taking
objects that properly overlap both Tibbles and Pluto, but let us ignore these possbilities now.) Indeed, since this is a case where the whole object is not
scattered, the loose count might be more natural: there is only one thing—awhole cat. Yet this is precisely where the presupposition of extensionality lies.
6

I shall use the loose/strict opposition always in Baxter’s sense. It has nothing to dowith Bishop Butler’s opposition (taken up by Chisholm [1969]) between a ‘strict and philosophical’ and a ‘loose and popular’ sense of the ‘is’ of identity.

6

What are the grounds for saying that there is only one whole? Let Tib+Tail bea7 mereological sum of Tib and Tail—an entity which is overlapped exactly by
those things that overlap either Tib or Tail. What are the grounds for saying thatTibbles is the same as Tib+Tail? This identity reflects the very idea that a wholeencompasses the same portion of reality as its constituent parts. Yet a certain
familiar argument suggests that it is not unproblematic.
Here is one way of putting this familiar argument.8 Suppose that one day
there is an accident and Tibbles’ tail is cut off. Every cat can survive the loss ofher tail, or so we may suppose, hence Tibbles will continue to exist but will nolonger include Tail among its parts. A sum of Tib and Tail, however, must bydefinition consist of both those parts, Tib and Tail, so it will include Tail also
after the accident, if the tail is not destroyed. And if the tail is destroyed, thenafter the accident Tib+Tail will just no longer exist. Either way, Tibbles and
Tib+Tail seem to have different persistence conditions, different properties.
Ergo—the argument goes—by the principle of the indiscernibility of identicals
Tibbles and Tib+Tail must be two different things. Schematically:(1) After the accident, Tail will not be part of Tibbles.
(2) After the accident, Tail will be part of Tib + Tail.
(3) Thus, Tibbles and Tib+Tail have different persistence conditions
and must be kept distinct (even before the accident).
This is the version where the tail is not destroyed in the accident, but only cut
off. Otherwise the argument goes like this:(1') After the accident, Tibbles will still exist.
(2') After the accident, Tib + Tail will no longer exist.
(3') Thus, Tibbles and Tib+Tail have different persistence properties
and must be kept distinct (even before the accident).
7

Above I spoke of the sum of two given entities x+y, but of course the uniqueness of a
mereological sum presupposes extensionality.
8
The puzzle has been introduced to contemporary philosophical discussion by Wiggins[1968], apparently drawing on Peter Geach (who attributes it to William of Sherwood: see thesophisma in [1968: 60-61]). It actually goes back at least to the Stoics (see e.g. Sedley[1982], Sorabji [1988: §1.6]). For a selection of recent relevant literature, see Rea [1997].

7

Nor is the actual occurrence of the accident a necessary condition for this sortof argument to be formulated. One could just argue that Tibbles can, whereas
Tib+Tail cannot (by definition), cease to include Tail as part. For this would
imply that Tibbles and Tib+Tail have different properties—different modal
properties—and that is all one needs to distinguish them. Schematically:(1") Tail is not necessarily part of Tibbles.
(2") Tail is necessarily part of Tib + Tail.
(3") Thus, Tibbles and Tib+Tail have different modal properties
and must be kept distinct (even in this world).
If these arguments are accepted, then, in front of us there would now be notone entity, a cat, but two entities, a cat and a mereological sum comprising her
body and her tail. Indeed, by the same pattern we could distinguish many otherentities in front of us: a sum consisting of the left ear and the remainder, a sum
consisting of the right ear and the remainder, and so on. I will come back to thislater. For the moment let me just emphasize that this sort of argument, if sound,would introduce a serious problem for the Minimalist View. For suppose weare drawing up an inventory of the world at time t0 , before the accident. Suppose
we are doing this according to the loose count: we want to list only connected
wholes, and no undetached parts. What shall we include—Tibbles or Tib+Tail?
If these are not the same whole, then we have to choose. For they have the same
parts, hence they overlap, and so by (M) only one should count. But which one?
(The problem would also arise if we preferred a stricter count, counting for
instance two things, Tib and Tail, rather than one, Tibbles, or Tib+Tail. For then
one could run the argument with regard to the parts. Let ‘T’ denote an initial bitof Tail, and let ‘Ail’ denote the rest . . . In the following I shall focus on theloose count for simplicity.)
Of course, if one believes in contingent identity the problem dissolves: one
can just say that Tibbles and Tib+Tail are different at certain times or worlds,
where they have different parts, but not at this time in this world, where they
have the same parts.9 That is good enough to save the Minimalist View. On theother hand, suppose we reject contingent identity. Then there are various ways
9
This is the strategy urged by Gibbard [1975] and Myro [1985] (with respect to worlds
and times, respectively) and recently defended by Gallois [1998].

8

of saving mereological extensionality vis à vis these arguments, but they are oflittle help for our purposes—at least to the extent that we want our inventoriesof the world to represent our ontological commitments in agreement with the
Minimalist View. One could, for example, subscribe to a four-dimensional
ontology and insist that Tibbles and Tib+Tail are indeed distinct because they
have different temporal parts.10 Or one could take extensionality to mean that
distinct things must have different parts at some time or other or in some worldor other (though not necessarily at every time and in every world). In that casethe conclusion of the arguments is unproblematic as long as we give up thetraditional (Lockean) principle to the effect that distinct bodies can neversimultaneously occupy the same spatial region.11 (And there are certain kindsof entity for which this principle seems to fail anyway: holes, shadows, perhaps
ghosts.12 ) Whatever their merits, however, these are not ways out for the
Minimalist View. For these temporal and modal variants of extensionality are
not what the Minimalist View presupposes. If we accept the conclusion of theargument (in any of its versions) on account of temporal or modal differences
between Tibbles and Tib+Tail, it simply becomes impossible to draw up
inventories of the world except with reference to the full history (past, present,
and future) of everything included in the domain (let alone its history in otherpossible worlds). And for the Minimalist View this would be a meager
achievement: one would not avoid double counting; rather, one would findreasons to make sense of it.
If we do not want to accept the conclusion, however (and if we do not want
to give up the principle of the indiscernibility of identicals), then we must reject
one of the premises. And again the question is: which one? A denial of the firstpremise (the proposition that Tail will or would cease to be part of Tibbles after
the accident), amounts to a form of mereological essentialism that does not
10

As urged e.g. by Heller [1984] and, more recently, Sider [1997].
This is the position of Thomson [1983].
12
The case of holes is discussed at some length in Casati and Varzi [1994: Ch. 7];
shadows were suggested by Leibniz in his commentary of Locke [New Essays, II-xxvii-1],
van Inwagen [1990: 81] mentions ghosts, and David Lewis [1991: 75] has two angels dancing
forever on the head of a pin. Chisholm [1973: 590] also considers shadows and holes ascounterexamples to Locke’s principle. For another example, J. M. Shorter [1977] mentions
the intriguing case of intersecting clouds produced by two distinct “cloud projectors”. For ageneral discussion, see Casati and Varzi [1996].
119seem to be implied by the Minimalist View. And how can we possibly deny thesecond premise (the premise that Tib+Tail will or would still include Tail after
the accident, if it survives at all)? Perhaps we could say that after the accident
Tib+Tail ceases to exist insofar as its parts get separated, and we could
therefore treat the second premise as false or truth-valueless. But this would
seem to imply an equally delicate form of essentialism—a form of topologicalessentialism whereby the identity and existence conditions of a mereologicalsum depend on the arrangement of its constituent parts.13
I don’t know which of these premises must be rejected. Still, maybe we
don’t need to choose in order to resist the argument and the threat that it
represents for the Minimalist View. For let us be careful here. There are two
ways of reading the premises, depending on whether the terms occurring therein
are understood de dicto or de re. And I think the prima facie plausibility of theargument ultimately rests on this ambiguity. Consider first the de dicto reading,and focus on the first version of the argument, (1)–(3). I agree that on thisreading the two premises can hardly be rejected. The first premise reads:(1dd ) The entity picked out by the term ‘Tibbles’ at time t1 (after theaccident) will not include Tail as part,and this simply reflects our way of using the term ‘Tibbles’ in the envisaged
situation. The second premise reads:(2dd ) The entity picked out by the term ‘Tib+Tail’ at time t1 (after theaccident) will include Tail as part,and again this simply reflects our use of the term ‘Tib+Tail’, which must pickout a mereological sum of Tib and Tail. So, on a de dicto reading both premisesof the argument are naturally accepted. However, this is clearly beyond thepoint. For if we are interested in the temporal properties of the entity or entities
that are now in front of us, then we should not look at the future referents of ournames and descriptions. Obviously, if those names and descriptions havedifferent senses (as in our examples), they may have different referents at
different times. Obviously, the terms ‘Bill Clinton’ and ‘the US president’ may
13

The first option—mereological essentialism—is Chisholm’s favored strategy [1973],
while the second option—topological essentialism—may be associated with those who feel
uncomfortable with scattered fusions (see Casati and Varzi [1999a] for discussion).

10

pick out different people at different times. But that is not the issue. The issue is
not whether our terms will have different referents. It is whether they do havedifferent referents, whether they have different referents at this time. So, on a dedicto reading the premises may well be true, but the argument is not valid. The
conclusion does not follow.
It is the de re reading that matters, then. But on a de re reading (1) and (2)are in the same boat:(1dr ) The entity picked out by the term ‘Tibbles’ now—that particular
entity—is such that at time t1 (after the accident) it will not include
Tail as part.
(2dr ) The entity picked out by the term ‘Tib+Tail’ now—that particular
entity—is such that at time t1 (after the accident) it will include Tail aspart.
If Tibbles is not Tib + Tail, then fine, we are talking about two different entities,
and perhaps we can say that both (1dr ) and (2dr ) are true. But this opposition
would be prior to our thought experiment—it cannot be inferred from it andcalls for independent grounds. We can’t distinguish two entities by looking attheir properties unless we already know what they are, at least unless we alreadyknow whether they are distinct. On the other hand, if Tibbles is Tib + Tail—andto rule that out would be to beg the question—then that particular entity is thesame in both cases, so (1dr ) and (2dr ) cannot be both true. (And which one we
reject will reflect our views on mereological essentialism. Maybe we want to saythat Tibbles cannot exist without Tail, making (1) false; maybe we want to denythat, making (2) false.) In short, on a de re reading this sort of non-identity
argument is valid, but it is either unsound or question begging: one can havereasons to accept the two premises only if one already has reasons to
distinguish between Tibbles and Tib+Tail in the first place.14 Since the de rereading is the only one that matters (and since on a de dicto reading the14

My point here is similar to that of Della Rocca [1996], though my concern is withmereological rather than material constitution. In a similar fashion, Neale [1990, §4.6] argues
that this sort of de re / de dicto ambiguity is responsible for a certain confusion surrounding
the discussion on events and event identity. I shall come back to this below. The basic point,of course, owes much to Smullyan [1948] and Kripke [1972] (see e.g. Kripke’s discussion of
the identity of heat and molecular motion, pp. 129ff).

11argument is fallacious anyway), this means that the argument fails to establish
the distinction.
Of course, the identification of Tibbles and Tib+Tail may give rise to someawkwardness when it comes to the way we speak. If we are not essentialists
about Tibbles, and if Tibbles = Tib+Tail, then we must accept premise (1) andreject premise (2). Yet the denial of premise (2), i.e., the statement that after theaccident Tib + Tail will not include Tail, sounds awkward. Indeed. But what
follows from this? Not that we are forced to accept (2) instead and reject (1). It
simply follows that the negation of (2) is not a good way of expressing the
proposition that the one entity we are talking about will cease to include Tail aspart—the proposition that after the accident Tail will no longer be part of that
entity. If we think that this proposition is true (if we are not essentialists about
tails), then sentence (1) is a much better way of expressing it than the negationof (2). Certainly we shall speak this way after the accident—we shall say ‘Now
Tail is no longer part of Tibbles’ rather than ‘Now Tail is no longer part of
Tib+Tail’—for then the term ‘Tib+Tail’ does not even refer to the entity weintend to talk about.
Bennett [1973] and Anscombe [1979] actually pointed out that this sort ofawkwardness may arise all the time: a man cannot truly be called a husband
before the wedding, or after a divorce, and a person cannot be called a killer
before the death of the victim. It would have been wrong to refer to Brutus as
the murderer of Caesar before Caesar had died. Yet this is not to say that when
we now speak of Brutus and when we speak of the murderer of Caesar we are
speaking of two different entities. We are speaking of a person who—as things
turned out—murdered Caesar. On a de re reading, the sentence ‘The murdererof Caesar had already left when Caesar died’ may sound awkward, but it isperfectly meaningful. And it may well be true.
In short, then, the non-identity argument (1)–(3) is either invalid (on a dedicto reading) or question begging (on a de re reading). And the same goes forthe other versions of the argument. Take for instance the modal version, (1")–(3"). On the de dicto reading, the premises simply assert that the terms ‘Tibbles’
and ‘Tib+Tail’ have different senses, from which it certainly does not followthat they have different referents. On the de re reading, the argument says that
we can distinguish the entities picked out by ‘Tibbles’ and ‘Tib+Tail’ by looking at their modal properties. But we simply cannot look at their properties
unless we already know what they are, at least whether they are distinct. We
12must know how many passengers we are taking along before we can embark inother-worldly philosophical excursions. Indeed, why should we be able to settle
identity issues in this world by looking at other worlds? Why should identity
across possible worlds be easier to handle than identity within this world?
This diagnosis, I think, applies to a whole family of non-identity arguments, even arguments that have nothing to do with mereological sums. Take
Tibbles and Sitting-Tibbles, the material body arranged in the shape of Tibbles
now that she is sitting on the mat. Or take Tibbles and the lump of feline tissueof which she is presently constituted. Or take Tibbles’ accident (the event when
she lost her tail) and her painful accident (when she painfully lost her tail). In
each of these cases, some philosophers would distinguish between the first andthe second member of the pair precisely on account of their different temporalor modal properties. Tibbles can get up, but Sitting-Tibbles cannot. Tibbles
could be made of different tissue, but the lump of feline tissue that constitutes
her could not. Tibbles’ accident could have been painless, but her painful
accident could have not.15 In each of these cases, I submit, the argument iseither invalid (if read de dicto) or question begging (if read de re). Of course
the terms ‘Tibbles’ and ‘Sitting-Tibbles’ have different senses; but it does notfollow that they have different referents.16 And if their referent is the same, ifthey name the same thing now that Tibbles is sitting on the mat, then either thatthing can get up or it cannot. Of course the terms ‘Tibbles’ and ‘that lump of
feline tissue’ have different senses. But if they have the same referent, if Tibbles
is that lump of feline tissue, then Tibbles and the lump have exactly the same
properties, here and in every possible world. And of course the predicates ‘accident’ and ‘painful accident’ pick out different properties, which entails that the event descriptions ‘Tibbles’ accident’ and ‘Tibbles’ painful accident’ have different senses. It does not, however, follow that the two descriptions have different referents: if the accident was painful, both phrases may referto the same event (though with different, increasing degree of precision).
15

On material constitution (the cat vs. the lump of feline tissue), see e.g. Fine [1994]
and Thomson [1998]. On events (the accident vs. the painful accident), see e.g. Brand [1977:334] or Yablo [1987]. The latter case is discussed in some detail in Pianesi and Varzi [1999].
16
In Aristotle’s terms, “the ‘accidental’ is, in a way, but a name” [Metaphysicsb1026 14]. Brentano [1933] took this to imply that the accident is a modal extension of thesubstance, but that would resuscitate the puzzle. For a discussion, see Chisholm [1978] andSmith [1994: §§ 3.3–3.5].

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4. Cats-in-waiting
With all this, I have not given any argument for mereological extensionality. But
I hope to have shown that the prima facie plausibility of the familiar arguments
against extensionality founders upon the necessary disentanglement ofsemantic and ontological issues. Language involves a lot of doubling up. But
why should we take that to imply a corresponding doubling up in the ontology?
Why should a plenitude of nomina correspond to a plenitude of nominata? To
me this is sufficient to establish the meaningfulness of extensionality, hence toprovide grounds for the Minimalist View.
It is worth pointing out that this line of reasoning applies also to otherversions of the non-identity argument. For instance, consider the following
version, which concerns the number of entities at time t1 (after the accident)
rather than t0 (before the accident). We know that Tib and Tibbles have thesame parts at t1 . But we also know that Tib and Tibbles have different parts at t0 :before the accident Tail is part only of Tibbles, not of Tib. So isn’t this enoughto conclude that Tibbles is different from Tib at t1 , even though they have thesame parts?17 Schematically:(4) Before the accident, Tail was part of Tibbles.
(5) Before the accident, Tail was not part of Tib.
(6) Thus, Tibbles and Tib have different persistence conditions
and must be kept distinct (even after the accident).
As I see it, this argument is perfectly similar to the previous one(s), except thatits question-begging character (on the de re reading) is now reversed. We arelooking at Tibbles at t1 —that tailless thing—and we are saying that at t0 it
included Tail as part; we are looking at Tib—that tailless thing—and we aresaying that it did not include Tail as part. If Tibbles and Tib are distinct, then we
may well be right. But if Tibbles and Tib are one and the same (as I should liketo think), then one of our statements (de re) is simply false. That one thing
could not have been such as to both have and not have Tail as part. To put it
differently, we may assume the premises to be true; but then the conclusion
17
It is mostly in this form that the puzzle has been discussed in the literature. See
Simons [1987, §3.3] and the introduction to Rea [1997] for comparisons and overviews.

14follows only on the supposition there are still two entities after the accident,
which is exactly what the argument is supposed to establish. (In the argumentsof Section 3, the conclusion followed only on the supposition that there are
already two entities before the accident.)
Of course, one can find good reasons in support of this supposition.
Common sense wants Tibbles to survive the loss of Tail. And why should Tib
not survive? As already Philo of Alexandria put it (in his discussion of Chrysippus’ suggestion that only Tibbles would survive the accident):
How can it be that [Tib], who has had no parts chopped off, has been snatched away,
while [Tibbles], whose [tail] has been amputated, has not perished? [Aet. mundi 48] 18

I don’t know how that can be. But this is now a different issue and there seemto be many ways of dealing with it. For instance, it is true that Tib has had noparts chopped off in the accident. But that does not mean that Tib was notaffected by the accident. On the contrary, the detachment of Tail was a major
topological change that may well have had drastic effects on Tib: the boundaryaround that lump of feline tissue is now complete, whereas it was not at t0 .19
Likewise, the survival of Tibbles is itself a complicated business. We are
inclined to say that it survives, I suppose, insofar as Tibbles is a cat, and we are
inclined to say that cats can survive small mereological changes. If instead of areal cat Tibbles were a china cat, say, one would reason differently. When thestatue breaks into two pieces, Tib and Tail, we might want to say that it does notsurvive the accident. Or, if it survives, we might want to say that after theaccident the statue still includes Tail among its parts. “Look, Tibbles is broken.
One part of it is on the floor near the sofa, the rest is near the coffee table.” I
simply don’t think our intuitions about survival through time are clearer andmore basic than our intuitions about synchronic identity. So I don’t find thenon-identity argument in this form any more compelling than in the forms
considered earlier.
18

I quote from Sedley [1982: 268] (altering Philo’s original to fit our example.) Burke[1994] is to be credited for bringing Chrysippus’ suggestion back into current discussions.
19
This is how I would reply to Philo’s prima facie intuitive remarks. Contra Denkel[1995], we are not talking about a Cambridge change that mysteriously results in a
substantial change—for topological relations are not Cambridge relations. A similar point ismade in Burke [1996: 66], though towards a different conclusion. See also Smith [1992] for a
pertinent discussion of the Tib+Tail puzzle from this perspective.

15

One could still consider a derivative problem, though. If Tibbles (or Tib, or
whatever is left after the accident) truly deserves being called a cat after the lossof Tail, wouldn’t it be correct to call Tib a cat before the accident? After all,before the accident Tib comprises exactly the same lump of feline tissue as
Tibbles does after the accident (or so we may suppose). Indeed, since we canraise the same question for any proper part of Tibbles that is similar enough todeserve being called a cat upon detachment of the remainder (any part, forinstance, that entirely coincides with Tibbles except for her left ear, or except fora single hair), what prevents one from saying that at t0 we have a plethora ofcats, not one? (This way of describing the situation is sometimes referred to as
the “problem of the many”.20 ) This is not immediately a problem for the Minimalist View, since (M) rules out at the outset the possibility that overlapping
entities (Tibbles minus hair x and Tibbles minus hair y, for instance) be included in the same inventory of the world. Still one can raise the question: If thesmaller parts truly deserve to be called cats—albeit only potential cats, or “catsin-waiting”21 —for what reason should we count only the maximal one (on theloose count)? Alternatively, on what grounds could we ever pick out one ofthose proper parts as the only cat to be included in the inventory (on a stricter
count)?
This question is legitimate, but I think it reflects a concern that is more
appropriately regarded as semantic, not ontological. If there is a sense in which
Tib and countlessly many other proper parts of Tibbles have a claim to being a
cat at t0 , it is because the rules of language do not set forth any rigid restrictions
on the use of the word ‘cat’ with regard to mereological composition (thepossession of a tail or of a hair in a certain spot.) The word ‘cat’, we may say,is vague, like ‘table’ or ‘heap’. But that goes hand in hand with a very precise
convention governing the use of these words, namely that cats, like tables orheaps, are maximal entities. As Quine put it, pondering upon the graded multitude of nested or overlapping tables-in-waiting that are contained in a table:
Each of these physical objects would qualify as a table, that is, if cleared of the overlying and surrounding molecules, but should not be counted as a table when still embed20

In this version the puzzle comes from Geach [1980: § 110]. Unger [1979, 1980], uses
it to argue toward the nihilist conclusion that ordinary objects don’t exist. Kim [1976: 170]
draws an analogy with the problem of event individuation.
21
The terminology is from Michaels [1983: 102].

16

ded in a further physical object that so qualifies in turn; for tables are meant to be exclusive. Only the outermost, the sum of this nest of physical objects, counts as a table.
[1981: 92–93]

So surely there are many cats-in-waiting contained in Tibbles at t0 , andeach of them would deserve to be called a cat upon removal of the remainder.
But none of them is a cat. So none of them should be counted if we arecounting cats (on the loose count). And none of them would be a lucky cat
randomly chosen from a plethora of equally good candidates, if we are counting
more strictly; it would be a randomly chosen cat-in-waiting, and that is perfectly
unproblematic.
5. Connectedness
One could still point out a loose end in the foregoing. Briefly put: just what is a
maximal cat (table, heap)? More generally, what is a maximally connectedbody? The intuition seems clear when we speak of Tibbles and Tib+Tail after
the accident. But how can that intuition be formalized? Moreover, at somedeeper level of description we know very well that ordinary material bodies are
not topologically connected (in one piece). So how can we speak of connectedthings at all?
Here the answer comes in two steps. First, the existence of connected
wholes (and of the relevant boundaries) depends on the underlying mereology—more precisely, on the topological features of the domain fixed by themereological theory that we are assuming (the theory that delivers the ontological commitments according to the Quinean notion). And these topologicalfeatures are to be thought of as lying at the same conceptual level as every otherpart-whole relation. Of course, if our mereology reflects the microscopicorganization of the physical world, if the solid bodies of common sense are
analyzed as intricate systems of subatomic particles, then no such body will
have a topologically connected boundary, except in a loose manner of speaking.
In that case, the relevant notion of connectedness is not strictly topological. For
instance, one could speak of tables and cats as being causally connected, orfunctionally connected, or perhaps also teleologically connected. There might bevagueness and other difficulties involved in these notions; but that need not be aconcern for the Minimalist View. On the other hand, if our set of mereological17

cookie cutters is not so fine-grained—if our mereology reflects the organizationof the physical world at a macroscopic level—then the relevant notion of connectedness may well be topological in a strict sense. It may not be the topologyof the world from the physicist’s point of view, but it will be the topology ofour domain as it is fixed by our mereological theory. And for the Minimalist
View that is all that matters.
The second part of the answer concerns maximality. Let us focus on thecase of maximal topological connectedness. We may define connectedness asfollows, paralleling the ordinary point-set-theoretic definition:22(7) x is self-connected (i.e., in one piece) if and only if any two parts that
add up to x are connected to each other (i.e., either touch or overlap).
This is not unproblematic, since a lot depends on the relevant notion of touching, which is itself a source of philosophical puzzlement. (How can two things
be exactly next to each other, if space is dense?) However we need not go into
the details here: any notion of genuine topological contact should do—such as
the relation of contact that holds between the tail and the rest of Tibbles, as
opposed to the relation of mere physical closeness that may obtain between
Tibbles and the mat she is sitting on.23 Given (7), the relevant notion of maximality is then to be understood as a means for expressing the intuitive distinction between self-connected parts (such as Tib) and self-connected wholes(Tibbles). As a preliminary characterization, this can be obtained simply by
exploiting the fact that parthood is a partial ordering:24(8) x is maximally self-connected if and only if (i) x is self-connected and(ii) every self-connected y is either disjoint from x or included in x.
This says why Tib should not, while Tibbles should, count as maximal in
the relevant sense. For, intuitively, Tib does not—while Tibbles does—satisfy
the condition in (8)(ii). More generally, this characterization captures the intui22

This notion was first given philosophical content by Cartwright [1975: 174]. The
mereotopological definition adopted here follows in the footsteps of Tiles [1981: 56], Hirsch[1982: 97], Chisholm [1992/3], and Smith [1996], inter alia .
23
On these issues I refer to Varzi [1996a, 1997] and to Casati and Varzi [1996, 1999b].
See also Zimmerman [1996] for the historical background.
24
See Smith 1996, §9.

18

tive difference between Tibbles and the many cats-in-waiting properly included
in Tibbles at t0 . But there is still one problem. As it stands, (8) quantifies over
all entities included in the domain of our mereological theory. And since it isplausible to assume that everything is connected to its mereological complement, it may well be that nothing is maximally connected strictu sensu, except
for the entire universe. In other words, (8) will not make Tibbles maximallyconnected after all, because (8) is not about cats, or about physical objects; it isabout everything. To obtain the desired formulation, we must instead rely on therelevant categorical distinctions. We must rely, not on a predicate of absolute
maximal self-connectedness, but on a parametrized predicate of maximal selfconnectedness relative to entities of a kind φ, or φ-maximal self-connectedness
for short:(9) x is φ-maximally self-connected if and only if (i) x is a φ, (ii) x is selfconnected, and (iii) every y that is a φ and is self-connected is either
disjoint from x or included in x.
In the passage from Quine quoted above, φ was just the property of being aphysical object, but we can hardly leave it at that. Any body of air surrounding
Tibbles may qualify as a physical object in the relevant sense, if Tibbles does. It
is, rather, properties or conditions of qualitative homogeneity (of material
composition, density, texture, electric charge, etc.) that are relevant to the point at
issue. Qualitative discontinuities introduce physical boundaries; so being
maximal with respect to a condition of qualitative homogeneity is to be demarcated by a complete physical boundary. What these conditions are may bematter of controversy and may depend heavily on context and cognitive factors.
But suppose we take ‘made of feline tissue’ to be such a condition φ. Then
Tibbles is, and Tib is not, φ-maximally self-connected. In fact Tibbles is a cat
insofar as it is made-of-feline-tissue-maximally-self-connected: there exists nobigger self-connected entity made of feline tissue of which Tibbles is part.25
Tib, on the other hand, is not so connected (at t0 ), since it is properly included in
Tibbles.
25

Of course, there are groups of physical objects that properly include Tibbles and thatappear to form a connected whole, e.g., the sum of Tibbles and the mat she is sitting on.
However this is not a case of topological connection in the sense presupposed by (7) but, as
already mentioned, a case of mere physical closeness.

19

With the help of (9), then, the semantic argument of Section 4 is complete.
And we are at last in a position to clarify the intuition behind the counting policy described in the introduction—the intuition that an inventory of the worldmust include only things of the garden variety (things with a complete, continuous boundary). Call this the “garden count policy”. It is the result of supplementing (M) with the maximality condition:(G) An inventory of the world is to include an entity x only if x is φmaximally self-connected with respect to some condition φ of qualitative homogeneity.
6. Ways of counting
Let us, finally, go back to our options. The Minimalist View solves our initialquestion on the ontological status of parts by drawing a distinction between aprimordial, Quinean notion of existence (to be is to be included in the domainof our mereological theory, whatever it is) and a more selective, cognitively
relevant notion (to be is to be included in an inventory of the world, however
this is drawn up). The Minimalist View says that, based on the Quinean notion,
the selective notion is subject to the additional condition (M). And (G) gives us
a further condition, corresponding to the garden count policy.26
Now, the garden count policy is only one among many options. One canimagine stronger or weaker conditions corresponding to stricter or looser countpolicies. For instance, the strictest count policy is atomistic—never count anentity that has proper parts:(A) An inventory of the world is to include an entity x only if x is mereologically atomic.
Of course, this would presuppose that everything in the domain of quantification is ultimately constituted by mereological atoms; otherwise the left-to-rightcondition of (M) (the non-redundancy condition) could be violated. And ifeverything is constituted by mereological atoms, then the right-to-left conditionof (M) (the completeness condition) entails the converse of (A): every atom is
26
One way of implementing (G) is explored in chapter 6 of Casati and Varzi [1999b],
where the following options are also briefly considered.

20

to be included in the inventory. So this count policy actually amounts to therequirement that the only way to draw up an inventory of the world is to make alist of all and only the mereological atoms in the domain. Everything else is justan aggregate of such primary entities.
By contrast, the loosest possible count is holistic—never count an entity if
it is part of something else:(H) An inventory of the world is to include an entity x only if x is mereologically maximal.
If the underlying mereology is standard—in particular, if the domain is closed
under arbitrary sums—this implies that there is only one entity to be included in
an inventory, namely, the sum total of all there is in the Quinean sense. On theother hand, suppose that the sum axiom fails to hold. Suppose the domain includes three things x, y, and z along with x+ y and x+ z, but not x+ y+ z. If, say,x+ y is included in the inventory, then x+ z cannot be included, owing to theleft-to-right condition of (M). But by the right-to-left condition of (M), thismeans that z must be included, which contradicts (H). Thus, if the domain is not
closed under arbitrary sums, then (H) may not be available at all as a supplement to (M).
What intermediate policies are there? The garden count policy (G) is one,but it is itself a rather extreme position: it prevents an inventory from including
undetached parts or disconnected wholes of any sort. Are there any ways of
weakening this conception? I think so. Counting as one is a function of possessing a certain unity. But the relevant notion of unity may vary, and the general
notion of φ-connectedness does not capture every interesting case. There arecircumstances where a stricter criterion than (G) (though not as strict as (A))
seems appropriate; and there are circumstances where it is a looser criterion (butnot as loose as (H)) that seems appropriate.
A circumstance of the first sort was illustrated in Section 2 with the bananas example. Though they are topologically connected and qualitatively
homogeneous, the cashier may want to count a bunch of five bananas as five
distinct items. Here it is the pragmatics of the context that sets the relevant
policy, removing the appearance of arbitrariness from the partition. For a different example, consider a simple perforated object such as a doughnut. The
doughnut is sure to be connected to its hole—yet one may want to list these
entities separately in an inventory of the world. Within the constraints set by21

(G) one can rely on the qualitative difference (in terms of material constitution)
between the hole and the doughnut. But the hole must also be listed separatelyfrom the rest of the doughnut’s complement, and that cannot be handled interms of qualitative discontinuities. A hole is naturally regarded as a homogeneous and yet distinguished undetached proper part of the airy complement of
its material host; and one may have reasons to include such a part in an inventory of the world rather than the whole complement, contrary to (G).27 Moregenerally, (G) will be violated whenever we include things that are demarcated,
not by a complete bona fide, physical boundary, but by a fiat, cognitively
induced boundary to which there corresponds no physical discontinuity.28 For
a further example—from the ontology of geography—two territories may beconnected through a common fiat border, like the territories of Maryland andPennsylvania; yet that does not deprive them from the salience needed to countthem separately.29 So if we are willing to put geographic territories in ourinventory of the world, then again (G) seems to be violated.
As for the looser counts, these seem appropriate in those cases where ascattered aggregate is bound together in virtue of its being unitary in somerelevant sense, as with the bikini example of Section 2. Thus, we are likely tocount a pen as one item, even if the cap is not topologically connected to thewriting instrument. And an encyclopedia may count as twelve if we are making
a list of volumes, but it counts as one if we are setting up a library catalogue.
The selective factor may be purely cognitive, or it may be explained in terms ofsome sort of non-topological (e.g., functional) connectedness, as mentioned in
Section 5. In fact, if the mereotopological structure of the domain of reference is
as fine-grained as demanded by the physical sciences, then physical objects ofthe garden variety can only be included in an inventory of the world as a resultof such a loosening of (G): no bona fide boundary would individuate them.
27

On the ontology and mereotopology of holes see Casati and Varzi [1994] and Varzi[1996b].
28
For the notion of a fiat boundary I refer to Smith [1995] and Smith and Varzi [1999].
29
We can speak of the Mason–Dixon line as the border between Maryland and Pennsylvania, but this border is best construed as being made up of two coinciding fiat boundaries,
one for each state. (See Smith and Varzi [1999] for details.) The coincidence, however, ispurely spatial and involves no mereological overlap, which is why Maryland and Pennsylvania can be counted separately without violating (M). For the same reason, Tib and Tail canbe listed separately also before the accident: their fiat boundaries coincide, but share no parts.

22

Speaking of the table’s boundary would be like speaking of the flat top of a
fakir’s bed of nails,30 and the relevant discontinuity would involve the same
degree of arbitrariness as that of any fiat demarcation.
I’m going to leave this part of the story open. The spectrum of available
counting policies is presumably very large and to some extent underdetermined.
Nonetheless the initial question—the question of how we can talk about parts
and wholes while at the same time regarding the former as nothing over andabove the latter, or vice versa—that question has at this point a clear enoughanswer. Our inventory of the world will depend on the underlying mereology(the theory of parts) and on the way we set our standards for unity (the theoryof wholes). Not any standard will do, though. And the Minimalist View sets thenecessary constraints. Any additional criterion may require a story of its own.31
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