ABSTRACT Leibniz's metaphysics appears to go a long way towards monism: it supports a strong dependence of limited things on the absolute or God and understands this dependence not only as causal dependence but also as a pervasive ontological dependence which involves the communality of nature between absolute and limited. Yet, Leibniz stops short of affirming monism. Why? This paper takes a fresh look at Leibniz's reasons for opposing monism through the lens of a virtually unknown text of 1698 on the metaphysical foundations of the infinite. Against the backdrop of the present-day monistic proposals of Jonathan Schaffer and Michael Della Rocca, the paper identifies and evaluates four different types of monism: 1. Whole-Part Monism; 2. World-Animal Monism; 3. God-Nature Monism and 4. Eleatic Monism. It argues that Leibniz's opposition to Whole-Part Monism, World-Animal Monism, and God-Nature Monism, is due to his conceptions of the infinite and of what it is to be “an absolutely absolute Being”. Furthermore, it argues that the only type of monism which could preserve Leibniz's demanding notions of real infinite and absolute is Eleatic Monism. The latter, however, is also rejected by Leibniz due to our first-hand experience of what it is to be a substance.