The definition of cause of itself (causa sui) is the first definition given in the Ethics: “By cause of itself I understand that whose essence involves existence, or that whose nature cannot be conceived except as existing” (E1def1). Because God is the only possible substance in Spinoza’s metaphysics, there is only one causa sui, that is, God. God is his own cause. Interestingly, Spinoza holds that causa sui is a key to understanding how God causes all the things there are: “God must be called the cause of all things in the same sense in which he is called the cause of himself” (E1p25s). That God is the cause of himself is important for Spinoza because he wanted to argue for the total intelligibility of the world, and for Spinoza understanding consisted in knowing the causes.