  PC and PSR may seem innocent enough, but Leibniz's other well-known principle, the Principle of the Identity of Indiscernibles (PII), is more controversial. (See also the entry on  identity of indiscernibles.)  In one of Leibniz's typical formulations, PII states that “it is not true that two substances can resemble each other completely and differ only in number [solo numero]” (A VI, iv, 1541/AG 42). In other words, if two things share all properties, they are identical, or   (∀F)(Fx ↔ Fy) → x = y.  What is particularly important to note, however, is that Leibniz is adamant that certain kinds of properties are excluded from the list of properties that could count as difference-making properties, chief among these spatio-temporal properties. This is what Leibniz means (in part) when he asserts that there can be no purely extrinsic (i.e., relational) determinations. Therefore, it is not the case that there could be two chunks of matter that are qualitatively identical but existing in different locations. In Leibniz's view, any such extrinsic difference must be founded on an intrinsic difference. As he puts it in the  New Essays,