Philosophers have asked two different questions about the identity of objects through time: 1) What criteria guide our tracing of an object’s career through time, and; 2) What are necessary and sufficient conditions for the identity of an object picked out as existing at one time with an object picked out as existing at another time? At the opening of this chapter, Ray distinguishes between the two sorts of projects. She says, “I do not want to account for the sameness of the tree. I take the ‘sameness’ of the tree as a given in most cases. What is interesting is why I am even inclined to say that this is the same tree.” [Ray 99]
Ray is interested in the second question. She is interested in the criteria that we actually use in tracing the careers of objects through time. Why are we inclined to identify as the same tree a tree picked out as existing at one time with a tree picked out at a later time, she asks?
She says in reply,
I want to say that this is the same tree because of the simple fact of spatiotemporal continuity. I assume that this is the same tree, because it is in the same place I planted it. Since the soil is not disturbed, I assume that, even when I was not watching it, it did not move to another spot and then move back. Since I have never had any reason to judge that things sometimes cease to exist, and then later come back into existence, I assume that the existence of the tree was temporally continuous as well.. [Ray 99]
Later in this chapter she continues:
Spatiotemporal continuity , then, is a criterion of identity on which I rely for most of my judgments of identity. Is it a metaphysically sufficient condition for the persistence of an object? Obviously not; a tree can burn until it is just a pile of ash, though there were no leaps through space or time. Is it a necessary condition? Apparently so, in my experience, and in the experience of a vast number of people who have expressed their opinions on the subject either implicitly or explicitly. [Ray 99]
I wish to argue that spatiotemporal continuity is not a necessary condition for identity, and further, to ask why we would think that the criteria which guide our individuations and identifications through time would constitute either necessary or sufficient conditions?
The first difficulty in discussing this issue is that Ray does not give us a clear definition of ‘spatiotemporal continuity.’ The term is absent from the elegant and useful glossary that Ray provides her readers. Quite different definitions have been offered by various philosophers, including those discussed by Ray, most importantly Hirsch and Wiggins. Hirsch himself offers a number of different suggestions. The matter of definitions matters here because it determines which counterexamples are apt. Which counterexamples are appropriate depends upon precisely which notion of spatiotemporal continuity is being put forward (though I believe that all the numerous definitions proposed by philosophers fall prey to one or another counterexample.)
We begin then with a somewhat “loose’ conception of spatiotemporal continuity. I will try in this discussion not to stray from what I take to be the core of a number of different ideas about the appropriate definition of this concept.
Is spatiotemporal continuity necessary for the identity of objects through time, in addition to being a criterion which guides our identifications through time? First, consider the issue of spatial continuity. What are we to make of objects such as trombones, which spend much of their existence in a disassembled state? The trombone doesn’t go out of existence when it is disassembled and put into its case. The separation of the trombone into spatially separate parts, no one of which could fairly be called a trombone (only the collection of parts, or some significant subset of the set of parts could be so-called) does not result in the destruction of the trombone. The trombone in this case is temporally continuous. We have the same trombone with which we began, and will continue to have the same trombone when we take its parts from the case and reassemble them. Would this trombone’s career exhibit spatiotemporal continuity? If none of the individual separated parts of our disassembled trombone are fairly characterized as a trombone, then there is not a continuous succession of trombone-stages which connects our original with our ultimate trombone. If one dislikes the temporalism of “trombone stages’, one can simply ask whether there is a spatially continuous trombone which exists at every time in the process described. It seems clear to me that there is not--again, none of the separated parts is a trombone, but only a part of a trombone.
Is Ray’s conception elastic or liberal enough to allow her to claim that the spatiotemporal continuity criterion has been met in this case? It seems to me that the trombone case shows that the core or ordinary sense of spatiotemporal continuity is not a necessary condition for identity through time. More generally, any case of an object which persists through disassembly and reassembly and where none of its disassembled parts are covered by the same sortal as the object as a whole would seem an apt counterexample to the claim of that spatiotemporal continuity is necessary.
But let us press the case further. Ray says she has never had any reason to judge that things sometimes cease to exist and then later, and come back into existence. Is temporal continuity necessary? Can objects have an interrupted existence? Can an object exist at one time, cease to exist , and then come back into existence as the numerically identical object?
Consider the following case: I take my watch, (call it A) to a jeweler, who disassembles it. The mere act of disassembly would not seem to result in the destruction of the watch. But suppose that the jeweler has ten watches, (call them B, C, D,...K) of the same make and model as mine, and that my watch has exactly ten parts into which it might plausibly be disassembled. The jeweler takes each of the ten parts of A and replaces the similar part of each one of his ten watches. At the end of this part-replacement we have ten watches before us, each of which has a part which began as a part of mine, and a pile of watch parts discarded from the jeweler’s original ten.
To eliminate the potential distraction of this pile of parts, let’s have it that the jeweler destroys them. We have before us ten watches. In fact we have the same ten watches B, C, D, ...K with which the jeweler began. It is true that watch B has a new crystal, and watch C a new spring, and so forth. But as watches are among the sorts of objects which normally persist through a simple replacement of any one of their parts, we must agree that the ten watches before us are the jeweler’s original ten, B, C, D ...K. It seems that Ray’s endorsement of persistence through part-loss and part-addition would lead her to the (correct) view that each of the jeweler’s watches has survived the loss of one of its original parts and the incorporation of a part from mine.
Does my watch exist at this time? Do we have eleven watches before us (the jeweler’s ten plus mine) sharing the parts of ten? Clearly it seems that we have only ten watches and that mine is not among them (just as Geach has only one cat). My watch A has ceased to exist, and each of its parts has been incorporated into pre-existing watches, each of which is distinct from mine.
Yet, were our playful jeweler to now remove the original parts of my watch from his ten and reassemble these pieces in their original order, isn’t the resultant watch identical with the one I originally gave him? A existed, ceased to exist temporarily, and then came back into existence. Such cases would show that spatiotemporal continuity, even generously conceived, could not constitute a necessary condition for identity. They would further show that spatiotemporal continuity is not even always the criterion by which we reidentify objects.
It is puzzling that Ray involves herself in this claim of necessity. Ray is asking after the criteria which guide our tracing of objects’ careers. At several points, she emphasizes correctly that our individuations are to be explained by reference to purposes for which those individuations are made. This is the key insight, I believe. But then why would Ray think that the criteria which we actually use would in addition constitute either necessary or sufficient conditions? As she notes, several of the famous puzzle cases depend upon the fact that different players in the stories have different purposes in mind.
Our concepts of artifacts and the criteria for their application have evolved to handle a range of fairly typical cases involving the objects designated by those concepts, not to handle the sometimes odd cases produced by philosophers. Why would we expect, in other words, that criteria created for largely practical purposes would be either necessary or sufficient?