The tenth proposition of Spinoza’s Ethics reads: ‘Each attribute of substance must be conceived through itself.’ Developing and defending the argument for this single proposition, it turns out, is vital to Spinoza’s philosophical project. Indeed, it’s virtually impossible to overstate its importance. Spinoza and his interpreters have used EIp10 to prove central claims in his metaphysics and philosophy of mind (i.e., substance monism, mind-body parallelism, mind-body identity, and finite subject individuation). It’s crucial for making sense of his epistemology (i.e., Spinoza’s (...) account of knowledge and response to skepticism) and in resolving puzzles within the Ethics (i.e., explaining human ignorance of all but two attributes). Even those who do not attribute some of the above claims to Spinoza need EIp10 to defend much of what they believe about Spinoza’s system. This paper locates a previously unnoticed argument for this proposition in Spinoza’s Short Treatise on God, Man, and His Well Being. There, Spinoza shows himself concerned with a powerful and underappreciated form of philosophical skepticism, one with surprising echoes in the work of his contemporary Leibniz as well as in the later Wittgenstein. Spinoza’s introduction of E1p10 in the Ethics circumvents this form of skepticism, solving the problem the Short Treatise envisions while also explaining that text’s argument’s absence from the explicit justificatory structure of the Ethics. (shrink)