 Spinoza argues that substances are conceptually, and hence causally, isolated from one another (Ip2–Ip5). He infers from this that substances cannot be caused to exist or be prevented from existing by any other substance (Ip6). But since all existing substances must nonetheless have causes and reasons for their existence, the fact that a substance exists must be explained entirely by the substance itself. That is, all existing substances must be self-caused and hence self-explained (Ip7). For parallel reasons, the non-existence of a non-existing substance must be explained solely through facts about the non-existing substance. Spinoza infers that the concepts of non-existing substances must include the explanation for their own non-existence. What about the concept of a non-existing substance could explain that substance’s non-existence? Spinoza’s answer: a self-contradiction. Non-existing substances do not exist for the same reason that squared circles do not exist in Euclidean space: they are conceptually impossible (Ip11d). From the fact of non-existence plus the PSR, Spinoza concludes that non-existing substances do not exist because it is impossible for them to exist. They cannot cause themselves to exist because their very concepts contain a contradiction, and facts about causation track facts about conceptual involvement and explanation.