Clarke's reputation rested largely on his effort to demonstrate the existence of God, and his theory of the foundation of rectitude. The former is not a purely "a priori" argument, and it was not presented as such. The intelligence, for example, of the self-existence and original cause of all things is, he says, "not easily proved "a priori"," but "demonstrably proved "a posteriori" from the variety and degrees of perfection in things, and the order of causes and effects, from the intelligence that created beings are confessedly endowed with, and from the beauty, order, and final purpose of things." The theses maintained in the argument are: