The quest for knowledge and the desire for certainty are two of the concerns most characteristic of Western philosophy. This attitude toward the goal of philosophy is perhaps given its clearest expression by Descartes when he sets forth as one of his five rules for the direction of the mind to refuse to believe anything that is subject to doubt. Descartes thinks that his version of the ontological argument offers proof of God’s existence, and he accordingly accepts God as one of the proven principles of philosophy. It is clear to us, though perhaps it was not obvious to Descartes, that all such arguments for God’s existence are less than absolutely convincing; at least we have to admit that they are not completely coercive in their effect. If we cannot have rational certainty of the existence of God, are we to conclude that the only philosophically defensible attitude is to doubt that God exists? Some philosophers have argued for this view, based on the principle that we have no right to extend belief beyond the available evidence. Other thinkers have argued that we are justified in believing in God even though we cannot claim absolute certainty for these beliefs.