  Although the details of Leibniz’s metaphysics are debatable, the Principle would seem to follow from Leibniz’s thesis of the priority of possibility. (See Leibniz’s remarks on possible Adams in his 1686 letter to Arnauld, in Loemker 1969, p. 333.) It does not appear to require the Principle of Sufficient Reason, which Leibniz sometimes bases it on. (See for example Section 21 of Leibniz’s fifth paper in his correspondence with Clarke (Loemker 1969, p. 699). See also Rodriguez-Pereyra 1999.) For Leibniz takes God to have created by actualising substances which already exist as possibilia. Hence there could only be indiscernible actual substances if there were indiscernible ones which were merely possible. Hence if the Principle holds for merely possible substances it holds for actual ones as well.  There is, therefore, no point in speculating as to whether there might not be a sufficient reason to actualise two of a possible substance, for God cannot do that since both would have to be identical to the one possible substance. The Principle restricted to merely possible substances follows from Leibniz’s identification of substances with complete concepts. For two complete concepts must differ in some conceptual respect and so be discernible.