Full Idea
Descartes was a dualist about substance, Spinoza was a monist, and Leibniz was a pluralist (an infinity of substances).
Gist of Idea
Descartes says there are two substance, Spinoza one, and Leibniz infinitely many
Source
John Cottingham (The Rationalists [1988], p.76)
Book Reference
Cottingham,John: 'The Rationalists' [OUP 1988], p.76
A Reaction
Spinoza is appealing. We posit a substance, as the necessary basis for existence, but it is unclear how more than one substance can be differentiated. If mind is a separate substance, why isn't iron? Why aren't numbers?