  Leibniz, however, does not simply disagree with Locke about the nature of the mind and the possibility of innate ideas. It is also Leibniz's contention that human beings are capable of knowledge in a way that Locke had clearly denied. As shown above, Leibniz is convinced that our knowledge of necessary truths has a completely different foundation from that for which Locke argues. Similarly, Leibniz holds that we can have genuine knowledge of the real essences of things, something called into question by Locke. After all, Locke had argued that we ought to admit that “essence” is really just a word that we use to describe “nominal essence,” a set of sortal concepts based upon sensible qualities; we ought not to act as if “essence” means anything about the real or inner constitution of a thing, for we will remain ignorant of that. Leibniz, however, holds that we can know certain things not only about individuals but also about their species and genera. In Book IV of the New Essays, in which Philalethes – the Locke character – gives his critique of the possibility of our certain knowledge about substances qua natural kinds, Theophilus (Leibniz) says, “[L]et me tell you that there are, for example, hundreds of truths that we can be certain of concerning gold, i.e. that body whose inner essence reveals itself through the greatest weight known here on earth, or through the greatest ductility or by other marks. For we can say that the body with the greatest known ductility is also the heaviest of all known bodies” (A VI vi 400/RB 400). Earlier in the New Essays, Leibniz had said that “essence is fundamentally nothing but the possibility of the thing under consideration” (A VI vi 293/RB 293) and “essences are everlasting because they only concern possibilities” (A VI vi 296/RB 296). It would seem, then, that Leibniz has something like the following in mind: experience informs us of a certain consistent set of sensible properties in, for example, gold; that is, a certain set of properties is compossible. And, more important, we ought to be able to assert with certainty that if some object has the greatest ductility, then it also has the greatest weight.