  Descartes’ ontological (or a priori) argument is both one of the most fascinating and poorly understood aspects of his philosophy. Fascination with the argument stems from the effort to prove God’s existence from simple but powerful premises. Existence is derived immediately from the clear and distinct idea of a supremely perfect being. Ironically, the simplicity of the argument has also produced several misreadings, exacerbated in part by Descartes’ tendency to formulate it in different ways.