Locke's principle is an applicable criterion of identity

According to what has been called ‘Locke’s Principle’, two material objects of the same kind cannot occupy the same place at the same time. Put otherwise,(Locke’s Principle) Necessarily, material objects are identical if and only if they are of the same kind and once occupy the same place at the same time. There is disagreement about whether this principle is true (see, among others, Locke 1976[1689], Leibniz 1996[1765], Simons 1985, Hughes 1997, Simons 1997,Fine 2000, De Clercq 2005, and Johnston 2005). To some extent, t he disagreement may be due to different interpretations of ‘kind’, one of the terms needed to state the principle. However, what is seldom disputed is that, even if true (under some reasonable precisification of ‘kind’), the principle fails to constitute a useful or applicable criterion of identity.1More specifically, it is generally agreed that the principle cannot be used to decide identity problems, for instance, ‘Is the person I met yesterday the same as the person I met the day before yesterday?’. This broad agreement on the practical uselessness of Locke’s Principle may help to explain why several prominent philosophers (for example, Kripke 1981 and Merricks 1998)have expressed skepticism about the possibility of finding a criterion of identity through time for ordinary material objects, even if they do not explicitly mention the principle or the objections that have been raised against it. However, in what follows, I will attempt to show that skepticism about the usefulness of Locke’s Principle is ill-founded. At least, there are no arguments in the literature to justify it. More positively, I will attempt to show, first, that we can have knowledge of past whereabouts, and so verify the right hand side of Locke’s Principle, without having to assess concrete identity claims first; second, that even under conditions of incomplete knowledge of past whereabouts we can use Locke’s Principle to decide concrete identity problems concerning material objects. (Here, and elsewhere in the text, I am using the phrase ‘knowledge of past whereabouts’ as short for ‘knowledge of which objects have occupied the same place at the same time’.