 Given that ordinarily we do appeal to intentions in explaining our physical actions and to bodily states in explaining sensations, what would lead Spinoza to such a prima facie implausible doctrine? The reason ultimately has to do with how Spinoza understands the nature of the most fundamental entity in his metaphysics, “substance”. Building on philosophical tradition that goes back to Aristotle, Spinoza associates being a “substance” with existential and explanatory self-sufficiency or independence (E1def3). Each essential quality or “attribute” of substance also must, Spinoza believes, manifest this independence proper to substance: “Each attribute of a substance must be conceived through itself” (E1p10). As a result, we cannot look for an explanation of the fact that substance thinks, or of how it thinks, anywhere else but in substance’s thinking nature (for example, we cannot appeal to the fact that it is also a material thing, i.e., a substance with the attribute of “extension”). In this sense substance as thinking is “conceived through itself”. The same reasoning will be true of any other substantial attribute.