One of the more interesting topics debated by Leibniz and Locke and one that has received comparatively little critical commentary is the nature of essences and the classification of the natural world.1 This topic, moreover, is of tremendous importance, occupying a position at the intersection of the metaphysics of individual beings, modality, epistemology, and philosophy of language. And, while it goes back to Plato, who wondered if we could cut nature at its joints, as Nicholas Jolley has pointed out, the debate between Leibniz and Locke has very clear similarities to the topic that has dominated the philosophy of language from the 1970s on: namely, the challenge mounted by Kripke, Kaplan, Putnam, and others against Russellian and Fregean descriptivist accounts of meaning. Yet, this topic is also, as Jolley writes, one of the “most elusive” in the debate between Leibniz and Locke.2 The purpose of this paper is to examine in detail Leibniz’s critique of Locke’s distinction between real and nominal essences. In doing so, I hope to show certain virtues in Leibniz’s account of metaphysics and philosophy of language that usually escape notice. While I wish to provide a general account of Leibniz’s disagreement with Locke, I also plan to focus on the nature of species and natural kinds. In my opinion, those who have treated this topic have not paid sufficient attention to Leibniz’s claims that “Essence is fundamentally nothing but the possibility of the thing under consideration” and “essences are everlasting because they only concern.