DISTINGUISHABILITY
At last we come to the issue of distinguishability. As it is not susceptible to the problem of describability noted above, distinguishability is obviously easier
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to characterize. We recall question (2) above: what is it that confers distinguishability on individuals? Answers have typically involved either (i) one or more of the set of properties of the entity or (ii), spatio-temporal location. If PII is secure, then option (i) will guarantee individu 13113g621n ality in the sense of position (a) above. If impenetrability holds, then (ii) likewise guarantees individuality in the sense of STI. Our conceptual distinction between individuality and distinguishability can then only be maintained in practice under view (b) above; that is, by adopting some form of T I. In this case, it can be said that either spatio-temporal location or a certain set of properties allow one to infer individuality, but that what confers it, metaphysically speaking, is something like the Lockean substratum or primitive thisness, underlying, or 'transcending', the properties of the individual. It is by means of the distinguishability or determinate distinctness of an object that we become aware of it as an individual, in a negative fashion as Gracia puts it. However, ontologically and positively, as an individual it is one thing, or, as Suarez said, it has an 'individual unity'. In terms of these distinctions we can understand the process of naming or labelling an individual as involving two aspects: the first concerns our epistemic access by means of which we become aware of the individual, the second is to do with the act of denoting, or giving the individual a name. The former involves description, whereas the latter may not. 37 On this view, the primary function of (proper) names is to refer and they are established through an act of baptism. On the other hand, we learn to use these names effectively via description. Thus, a description may play an epistemic role in distinguishing an individual from others, in terms of some set of properties, and in fixing reference through baptism, but may not be necessarily tied to the name. 38
With this distinction between distinguishability and individuality in hand, we can understand the different senses in which individuals can be indistinguishable. As we have indicated, two, or more, individuals are regarded as indistinguishable if they possess the same (sub)set of properties. The extent of this subset will give us different degrees of indistinguishability and also, as we have already set out, different forms of PII. Here we can usefully introduce yet another distinction, that between intrinsic and extrinsic properties. Again, this is hotly debated in the philosophical literature but for our purposes we can articulate it by introducing the notion of the state of a thing. In the vernacular, we might comment on the 'state' of our umbrella, referring to its battered
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and torn appearance for example. Physical systems can be in certain states, typically characterized by such properties as velocity, energy etc. Such properties can be called 'state-dependent' and may be spatio-temporal, although not necessarily. We shall find it useful to identify the intrinsic/extrinsic distinction with that between state-independent and state-dependent properties. 39
On the STI view, then, two individuals can be indistinguishable in terms of their intrinsic properties but not in terms of their spatio-temporal ones, if impenetrability is assumed. Whether physics allows for such individuals and whether PII is violated, and in what form, is something we shall consider in subsequent chapters. If we accept some form of TI, then there would appear to be no metaphysical impediment to having individuals which are 'absolutely' indistinguishable in the sense of possessing both their intrinsic and extrinsic properties in common. This may appear peculiar, since we could not distinguish such individuals, not even by their different states, yet it is permitted in terms of our conceptual distinction above. Again, whether physics allows for individuals in this sense is something to be discussed below.
The metaphysical peculiarity of TI is further revealed if we consider that such individuals may be in possible states and, indeed, it is in these terms that the impact of physics has been cast. Bringing the arcane machinery of possible worlds to bear, TI would appear to support the doctrine of haecceitism.
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