An ontological argument is any one of a category of arguments for the existence of God appearing mainly in Christian theology . The exact criteria for the classification of ontological arguments are not widely agreed, but the arguments typically start with the definition of God and conclude with his necessary existence, using mostly or only a priori reasoning and little reference to empirical observation . It is widely accepted that the first ontological argument was proposed by Anselm of Canterbury in 1078 in his Proslogion . Anselm defined God as "that than which nothing greater can be conceived", and then argued that this being could exist in the mind. He suggested that, if the greatest possible being exists in the mind, it must also exist in reality. If it only exists in the mind, a greater being is possible—one which exists in the mind and in reality. Seventeenth century French philosopher René Descartes deployed a similar argument. Descartes published several var...