Faced with strong arguments to the effect that Leibniz''sPrinciple of the Identity of Indiscernibles (PII) is not a necessary truth, many supporters of the Principle have staged a strategic retreat to the claim that it is contingently true in this, the actual, world. The purpose of this paper is to examine the status of the various forms of PII in both classical and quantum physics, and it is concluded that this latter view is at best doubtful, at worst, simply wrong.

In considering the possibility that the fundamental particles of matter might violate Leibniz's Principle, one is confronted with logical proofs that the Principle is a Theorem of Logic. This paper shows that the proof of that theorem is not universal enough to encompass entities that might not be unique, and also strongly suggests that photons, for example, do violate Leibniz's Principle. It also shows that the existence of non-individuals would imply the breakdown of Quine's criterion of ontological commitment.

In this paper it is argued that The Principle of the Identity of Indiscernibles can be justified as a concrete application of Ockham’s Razor, the maxim which enjoins us not to multiply entities beyond necessity. First, a statement of the Principle is presented, according to which the Principle, while interesting enough, is not logically necessary. It is then argued that the assumption of the falsity of the Principle prescribes an epistemological situation where it seems to be impossible to find grounds (...) for thinking that the Principle is indeed false. Hence it is to be accepted as an epistemological necessity of sorts, one that recommended by the desire not to multiply entities beyond need. (shrink)

In this paper it is argued that The Principle of the Identity of Indiscernibles can be justified as a concrete application of Ockham’s Razor, the maxim which enjoins us not to multiply entities beyond necessity. First, a statement of the Principle is presented, according to which the Principle, while interesting enough, is not logically necessary. It is then argued that the assumption of the falsity of the Principle prescribes an epistemological situation where it seems to be impossible to find grounds (...) for thinking that the Principle is indeed false. Hence it is to be accepted as an epistemological necessity of sorts, one that recommended by the desire not to multiply entities beyond need. (shrink)