Theodore Sider

André Gallois’s Occasions of Identity injects a refreshing new perspective into an old debate. Actually, what is new is the advocacy of the perspective: Gallois takes up a view that many consider a non-starter, and shows this reaction to be premature. The debate is over the right way to understand the traditional puzzles involving two things being in the same place at the same time; the perspective is that identity can hold temporarily. Suppose an amoeba, name it AMOEBA, divides in two. One of the resultant amoebas, POND, lives in a pond; the other, SLIDE, is examined on a slide in a laboratory. Does AMOEBA survive this process, and if so, does it survive as POND or SLIDE? If we stipulate that POND and SLIDE are symmetrically related to AMOEBA then it seems arbitrary to identify AMOEBA with exactly one of POND and SLIDE. But we cannot identify AMOEBA with each, for then by the transitivity and symmetry of identity we would wrongly identify POND and SLIDE. We are left with the conclusion that AMOEBA is identical to neither. But this seems wrong too; surely fission does not result in death. So just what does happen to AMOEBA? How to respond to this and related cases has been much discussed.1 There are many proposals, each with distinctive strengths and weaknesses. To these Gallois adds his own, which runs as follows. After division, there are two amoebas, POND and SLIDE, each of which existed before division. But it does not follow that there were two amoebas before division. Though POND and SLIDE are numerically distinct after division, they were numerically identical before division. The identity relation can hold temporarily, or occasionally, as Gallois puts it. My sense is that this sort of claim is regarded by most metaphysicians as downright wacky. And yet there is something very natural about it. Why distinguish POND and SLIDE today because they will differ tomorrow? I suspect the “wackiness” reaction has two sources, one based on Leibniz’s Law..

In , Peter van Inwagen asked a good question. (Asking the right question is often the hardest part.) He asked: what do you have to do to some objects to get them to compose something---to bring into existence some further thing made up of those objects? Glue them together or what?1 Some said that you don’t have to do anything.2 No matter what you do to the objects, they’ll always compose something further, no matter how they are arranged. Thus we learned of the fusion of the coins in our pockets and the Eiffel tower.

In order to perfectly describe the world, it is not enough to speak truly. One must also use the right concepts - including the right logical concepts. One must use concepts that "carve at the joints", that give the world's "structure". There is an objectively correct way to "write the book of the world". Much of metaphysics, as traditionally conceived, is about the fundamental nature of reality; in the present terms, this is about the world's structure. Metametaphysics - inquiry into the status of metaphysical questions - turns on structure. The question of whether ontological, causal, or modal questions are "substantive" is in large part a question of whether the world has ontological, causal, and modal structure - whether quantifiers, causal relations, and modal operators carve at the joints.

Exotic ontologies are all the rage. Distant from common sense and often science as well, views like mereological essentialism, nihilism, and fourdimensionalism appeal to our desire to avoid arbitrariness, anthropocentrism, and metaphysical conundrums.1 Such views are defensible only if they are materially adequate, only if they can “reconstruct” the world of common sense and science. (No disrespect to the heroic metaphysicians of antiquity, but this world is not just an illusion.) In the world of common sense and science, bicycles survive changes in their parts, billiard balls strike one another, and nothing travels faster than light. The mereological essentialist denies the rst, but offers this replacement: “there exist successions of numerically distinct, but appropriately related, bicycles with different parts” (Chisholm, , chapter ). The nihilist denies the second, but offers this replacement: “there exist X s and Y s such that the X s are arranged billiard-ball-wise, the Y s are also arranged billiard-ball-wise, and the X s strike the Y s”.2 The four-dimensionalist denies the third, but offers this replacement: “no sequences of matter-stages that are related by genidentity travel faster than light”.3 There is room for disagreement over what exactly “reconstruction” amounts to, but at a minimum: when a metaphysical theory reconstructs ordinary sentences φ1. . . as replacement sentences ψ1. . . , ordinary and scienti c evidence must not refute the view that, strictly speaking, it is ψ1. . . rather than φ1. . . that are true. The metaphysician needs reconstruction in order to face the tribunal of experience. An intriguing newcomer to the contemporary scene is the ancient doctrine of monism, the claim that “reality is one”.4 I will argue that, contrary to..

Some say that presentism precludes time travel into the past since it implies that the past does not exist, but this is a bad argument. Presentism says that only currently existing entities exist, and that the only properties and relations those entities instantiate are those that they currently instantiate. This does in a sense imply that the past does not exist. But if that precluded time travel into the past, it would also preclude the one-second-per-second “time travel” into the future that is ordinary persistence, for presentism accords the future the same ontological status as the past. Instead of quantifying over past and future objects and events, presentists speak a tensed language, regimented with primitive sentential tense operators. For a presentist, a persisting person is one who did exist, and who will exist. Regimented, these claims become: it was the case that she exists, and it will be the case that she exists. The presentist may then apply the same strategy to time travel proper. Suppose Katy travels back to the time of the dinosaurs. The presentist can say that it was the case two hundred million years ago that Katy exists. This claim, which consists of a present-tense statement “Katy exists” embedded within the past tense operator it was the case two hundred million years ago that, is exactly the sort of statement about time that a presentist is free to accept. This has all been made clear by Simon Keller and Michael Nelson ( ). In addition to rebutting the bad argument against the consistency of presentism and time travel, Keller and Nelson argue positively in favor of consistency by showing how to translate David Lewis’s ( ) account of time travel into the presentist’s tensed language. The appearance of con ict between presentism and time travel, they argue, is due only to the fact that most defenders of time travel (for example Lewis) have tended to phrase their defenses in nonpresentist terms. As much as I applaud their rebuttal of the bad argument, I wish to sound a note of caution..

Some philosophers believe that everyday objects are 4-dimensional spacetime worms, that a person (for example) persists through time by having temporal parts, or stages, at each moment of her existence. None of these stages is identical to the person herself; rather, she is the aggregate of all her temporal parts.1 Others accept “three dimensionalism”, rejecting stages in favor of the notion that persons “endure”, or are “wholly present” throughout their lives.2 I aim to defend an apparently radical third view: not only do I accept person stages; I claim that we are stages.3 Likewise for other objects of our everyday ontology: statues are statue-stages, coins are coin-stages, etc. At one level, I accept the ontology of the worm view. I believe in spacetime worms, since I believe in temporal parts and aggregates of things I believe in. I..

Many otherwise reasonable philosophers are impatient with ontology. These philosophers will probably have little time for Objects and Persons, which claims that while there do exist “atoms arranged statuewise”, there do not exist statues; while there do exist atoms arranged tablewise and atoms arranged chairwise, there exist no tables and chairs. Though I join these philosophers, at the end of the day, in rejecting Merricks’s claims, that day is long, whereas they want a quick verdict. But why? Do our impatient friends think that Merricks’s claims are contradictory, analytically false, or otherwise conceptually incoherent? They may say that the conventional meaning of “there exists a statue” is “there exist atoms arranged statuewise”, but this does not stand up to scrutiny. Someone could, of course, just decide to mean such a thing by “there exists a statue”, but then her pronouncements would be irrelevant to Merricks, who intends to be using a..