  Setting aside the historical issue of Leibniz’s own attitude towards his Principle (see, for example, Rodriguez-Pereyra 2014), supporters of it have tended to retreat from the claim that it is necessary and have adopted the alternative view that it is at least contingently true (in the face of apparent counter-examples such as possible worlds containing just two indistinguishable spheres).  There is the further issue as to how the Principle should be characterised and, in particular, there is the question of what properties are to be included within the scope of those relevant to judgments of indiscernibility. Excluding the property of self-identity (which, again, we’ll come back to below), three forms of the Principle can be broadly distinguished according to the properties involved: the weakest form, PII(1), states that it is not possible for two individuals to possess all properties and relations in common; the next strongest, PII(2), excludes spatio-temporal properties from this description; and the strongest form, PII(3), includes only monadic, non-relational properties. Thus, for example, PII(3) is the claim that no two individuals can possess all the same monadic properties (a strong claim indeed, although it is one way of understanding Leibniz’s own view).