 Leibniz’s opposition to Cartesian dualism stems not from a rejection of unextended substance, but from his denial of the existence of genuine extended material substance. To begin with, Leibniz held the Scholastic thesis that “being” and “one” are equivalent. He writes to Arnauld: “To be brief, I hold as axiomatic the identical proposition which varies only in emphasis: that what is not truly one being is not truly one being either” (30 April 1687; G II, 97/LA 121). For Leibniz, in order for something to count as a real being—a substance—it must be “truly one,” an entity endowed with genuine unity. And, as we saw above, in order for something to be a genuine unity, it must be a simple, indivisible entity. “Substantial unity,” he writes, “requires a complete, indivisible and naturally indestructible entity” (to Arnauld, 28 November 1686; G II, 76/LA 94). But matter is extended, and thus, Leibniz believes, infinitely divisible. Hence, there is no such thing, for Leibniz, as material substance.