The paper is concerned with the problem of individuation in Spinoza. Spinoza's account of individuation leads to the apparent contradiction between, on the one hand, the view that substance (God or Nature) is simple, eternal, and infinite, and on the other, the claim that substance contains infinite differentiation - determinate and finite modes, i.e. individuals. A reconstruction of Spinoza's argument is offered which accepts the reality of the contradiction and sees it as a consequence of Spinoza's way of posing the problem of individuation: it is argued that Spinoza's ontology is constructed on the basis of his methodology, rather than conversely. The contradiction appears in its acutest form in Spinoza's discussion of number. The paper explains the philosophical motivation behind Spinoza's Problematik and reflects on the historical context of the contradiction to which that Problematik gives rise.