 Why does Spinoza think that an account of an object’s causes explains that object’s existence? His most revealing answer traces back to the PSR as well. Because facts about the nature of causation are themselves facts, by the PSR, facts about causation require explanation. In a case in which x causes y, the PSR demands an explanation of what it is in virtue of which the causal relation between x and y obtains. Although one may try to answer this question by appealing to other causes (say, z’s causing x to cause y), the PSR also demands an account of what it is in virtue of which x and y are causally related, as opposed to being merely sequentially ordered (or an account of why mere sequential ordering is sufficient for causal relatedness). That is, the PSR applies not only to facts about existence, but also to facts about relations between existents. (This point will be extremely important for correctly understanding Spinoza’s modal theory in section 2 below.) The PSR asks not only for an explanation of the fact that x causes y in terms of prior first-order causes, but also for an explanation of why there is a causal relation of dependence between x and y and why it is not some other kind of relation. As it would not get us very far to answer by appealing to yet further first-order causal relations, the PSR is really asking us to give an account of causation itself. Causation, if the PSR is true, cannot be a primitive metaphysical relation. So, to echo a question that lingers today, just what is causation? (For more on the contemporary discussion, see the entry on the  metaphysics of causation. For more on the PSR and causation, see Della Rocca (2010)).