The paper offers a new account of Spinoza's conception of “substance”, the fundamental building block of reality. It shows that it can be demonstrated apriori within Spinoza's metaphysical framework that (i) contrary to Idealist readings, for Spinoza there can be no substance that is not determined or modified by some other entity produced by substance; and that (ii) there can be no substance (and hence no being) that is not a thinking substance.