We argue in this paper that Leibniz's characterization of a substance as "un être" in his correspondence with Arnauld stresses the per se unity of substance rather than oneness in number. We employ two central lines of reasoning. The first is a response to Mogens Lærke's claim that one can mark the difference between Spinoza and Leibniz by observing that, while Spinoza's notion of substance is essentially non-numerical, Leibniz's view of substance is numerical. We argue that Leibniz, like Spinoza, qualifies the substance as "one" primarily in a non-numerical sense, where non-numerical means per se unity or qualitative uniqueness. The second line of reasoning suggests that the term "one" should be understood as a-unity-presupposed-by-multiplicity in two senses: A) externally, in the sense of being presupposed by higher complex structures, such as aggregates, and, b) internally, in the sense of having itself a complex structure. We develop an analogy along these lines between the role the notion of a fundamental unity plays in Leibniz's view of numbers and his view of substance. In other words, we suggest that looking at the role units play in Leibniz's view of mathematics can shed some light on the role they play in his metaphysics.