A Discourse Concerning the Unchangeable Obligations of Natural Religion and the Truth and Certainty of the Christian Revelation: Being Eight Sermons Preached at the Cathedral Church of St. Paul, In the Year 1705

Clarke's reputation rests to a large extent 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, nor is it presented as such by its author. It starts from a fact and it often explicitly appeals to facts. The intelligence, for example, of the self-existence arid 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."