 Edwards' mental phenomenalism is a natural extension of his occasionalism and views on substance. If God is the only real cause of spatio-temporal phenomena, he is the only real cause of “thoughts” or “perceptions.” If a substance is what “subsists by itself,” “stands underneath,” and “keeps up” a set of properties, then a mental substance can only be what subsists by itself, stands underneath, and keeps up mental properties. It follows that the concept of mental substance either denotes nothing or refers to God's causal activity. “What we call spirit,” then, “is nothing but a composition and series of perceptions [mental events]…connected by…laws” (“Notes on Knowledge and Existence”; Edwards 1957–, vol. 6, 398).