  As we have seen, rationalist epistemology is grounded in a metaphysical commitment to substance. The concept of substance allowed the rationalists to reduce all complexity and plurality to an underlying simplicity and unity, versus the empiricists, who, in their skepticism about substance, were committed to regarding reality as fundamentally plural and complex. Spinoza’s metaphysics marked the culmination of this rationalist momentum. In Leibniz, the last great continental rationalist, we see its final movement. Leibniz, like other rationalists before him, regarded quotidian things as phenomena that ultimately reduce to perfectly simple substances. However, for Leibniz, there is an infinite number of these simple substances, each of them causally and perceptually isolated from all of the others. Leibniz reasoned that this is the best of all possible worlds because it balances the maximal possible complexity with the maximal possible order. In thus privileging neither unity nor plurality, neither simplicity nor complexity, and in striking the balance that he did on purely rational principles, Leibniz exemplified a more complex, more comprehensive and, ultimately, more mature rationalism than that of his predecessors.