 What is Spinoza’s argument for this rather implausible claim, that we simply cannot get certain properties (such as extension, thought, motion and rest) wrong? As we have seen, a lot hinges on this doctrine: nothing less than the very possibility of intellectual emendation, of transcending the confusion and inadequacy of mere imagination. The answer has to do with how Spinoza thinks about what is responsible for the absence of adequacy. As we saw above (3.2.2), in his view, any ideas of external bodies that I form through mere sense experience will in fact be ideas of how those bodies affect me. (For example, my empirical idea of the sun in fact only represents one way the sun can affect my body in its current state.) But common properties like extension are “equally in the part and in the whole”: wholly and without distinction in each thing. So for the adequacy of my idea of the sun as extended it does not matter if I’m representing extension as it is in the sun, or as it is in my body, or as it is in the “confused” amalgam of the two. Extension is extension is extension. Because common notions are insulated against error in this way, and because no idea inferable from an adequate idea can itself be inadequate (E2p40), these notions can indeed form a “foundation” of reasoning, and so be our entry point into the realm of the intellect.