  Leibniz's point is that, since the Adam of the actual world (call him “Adam@”) has a complete concept containing all of his properties and since Adam@ (or any substance) expresses the entire world of which he is a member, Adam@ could not exist in a different world. Moreover, if any of Adam@'s properties were somehow changed, then the resulting being would not be Adam@.  This is indeed an important consequence of his Principle of the Identity of Indiscernibles. When we speak of the Adam who brought sin into the world and an Adam who did not, we cannot, strictly speaking, be referring to the same individual. And that failure of identical reference is the key to seeing that all of Adam's properties are essential to him. Leibniz makes this point about as clearly as he makes any point in §30 of the Discourse on Metaphysics in his discussion of Judas' sin: “But someone else will say, why is it that this man will assuredly commit this sin? The reply is easy: otherwise he would not be this man.” (A VI iv 1576/AG 61) In other words, Leibniz seems to deny the possibility of “transworld-individuals” and asserts instead that every individual is “world-bound.” Nevertheless, Leibniz argues that there are “vague Adams,” that is, quasi-individuals which exist in more than one world. And when we imagine other possible worlds in which “Adam” does not sin or eats the forbidden donut, we are imagining only an incomplete or vague “Adam” who is the individual in that world – not the same individual. That is, the “vague Adams” are only quasi-individuals because their individual concepts are not complete. Indeed, when Leibniz speaks of the “vague Adams,” what he has in mind is some set properties that is common to a number of individuals: Adam@, Adamw′ (i.e. Adam in world′), Adamw″, Adamw′′′, and so on. In the terminology of David Lewis's modal theory, they are the “counterparts” of Adam in other possible worlds. Strictly speaking, however, only individuals with complete individual concepts are denizens of other possible worlds; quasi-individuals per se cannot be said to exist. But an incomplete concept can be used to delimit a set of counterparts that do exist with complete individual concepts in different possible worlds. And lest it be thought that talk of the idea of a “vague Adam” is limited to his discussion in the 1680s with Arnauld about modal matters, it should be noted that Leibniz makes much the same point 20 years later in the concluding sections of the Theodicy discussed and quoted above when he writes of the “several Sextuses” that are to be found in the palace containing the books of fates. All of this speaks very strongly for attributing the doctrine of superessentialism to Leibniz throughout his life as well as the denial of the doctrine of transworld identity.