Identity and distinction in Spinoza's ethics

In Ethics 1p5, Spinoza asserts that “In Nature there cannot be two or more substances of the same nature or attribute”. This claim serves as a crucial premise in Spinoza’s argument for substance monism, yet Spinoza’s demonstration of the 1p5 claim is surprisingly brief and appears to have obvious difficulties. This paper answers the principle difficulties that have been raised in response to Spinoza’s argument for 1p5. The key to understanding the 1p5 argument lies in a proper understanding of the substance-attribute relationship and the principles of metaphysical individuation that Spinoza accepts.