  Both of these perspectives raise interesting and distinct metaphysical issues (for a useful introduction see Castellani 1998b).  Let us consider, first, the objects-as-individuals ‘package’. How is the relevant notion of individuality to be articulated? One option would be to take one of the traditional lines and ground it some form of primitive thisness or Lockean substance. However, this kind of metaphysics is anathema to many of a naturalistic persuasion, not least because it lies beyond the physical pale, as it were. Alternatively, one might take individuality to be primitive but then assuage any naturalistic tendencies by tying it to the idea of ‘countability’ – in the sense that we can always count how many quantum objects are in a given state – and take the latter to be both physically significant and capable of being ‘read off’ from the theory (Dorato and Morganti 2013). Nevertheless, it may be felt that naturalism is better satisfied by eschewing such primitivist moves and taking the individuality of the objects to be reducible to their discernibility and ground the latter in their properties, as presented by the theory (a feeling that may be further supported by doubts as to the physical plausibility of possible worlds containing only one object, as mentioned above). Of course, for this to work, we need some assurance that no two objects are indiscernible (or indistinguishable) in the relevant sense. Traditionally this assurance has been provided by Leibniz’s famous Principle of the Identity of Indiscernibles, so let us consider the status of this Principle in the context of modern physics.