 So, returning to our opening questions, could there have been more substances than there in fact are? Spinoza’s answer is no, which means that there are no merely possible substances. A substance either exists or its existence is impossible. Could an existing substance have failed to exist? Again, Spinoza’s answer is no, since the only available explanatory basis for its possible non-existence would be facts about the substance itself. But if an existing substance is completely causally isolated, what could bring about its non-existence? Only itself, Spinoza answers, which would again amount to the concept of that substance containing an internal contradiction that would prevent such a substance from existing in the first place (Ip7). Could an existing substance exist for a certain length of time and then go out of existence? Spinoza’s answer is again no, since that would mean that an existing substance causes its own self-destruction, a violation of Spinoza’s doctrine that “No thing can be destroyed except through an external cause” (IIIp4). In reaching all of these conclusions, Spinoza relies on the self-explaining or self-causing nature of substances and the explanatory demands of the PSR. Spinoza concludes that for any existing substance, it could not have failed to exist nor can it cease to exist. Hence, if a substance exists, it necessarily exists (Ip7d). On the other hand, if a substance does not exist, its existence is impossible. Combining these conclusions, we reach (1): every possible substance necessarily exists.