This article discusses Leibniz’s claim that every substance is endowed with the property of perception in connection with Platonism, rationalism and the problem of substance monism. It is argued that Leibniz’s ascription of perception to every substance relies on his Platonic conception of finite things as imitations of God, in whom there is ‘infinite perception’. Leibniz’s Platonism, however, goes beyond the notion of imitation, including also the emanative causal relation and the logical (i.e. definitional) priority of the absolute over the (...) limited. It is proposed that Leibniz’s endorsement of Platonism, in conjunction with some rationalist elements of his philosophy, implies a monistic conception of particulars as modifications of a single substance. Following some scholars and opposing others, the article offers evidence that Leibniz accepted this implication during the last years of his Paris period. However, it is further argued that it was precisely the idea of perception as a property of every substance that allowed Leibniz to find a way out of monism after that period. More specifically, the article defends the view that what made room for ontological pluralism within Leibniz’s rationalist and Platonic outlook was the idea that perception is the property which constitutes the very being of substances: substances are their perceptions. (shrink)