The ontological argument St Anselm and Descartes both famously presented an ontological argument for the existence of God. (The word ‘ontological’ comes from ‘ontology’, the study of (-ology) of what exists or ‘being’ (ont).) Their versions of the argument are slightly different, but they both argue that we can deduce the existence of God from the idea of God. Just from thinking about what God is, we can conclude that God must exist. Because it doesn’t depend on experience in any way, the ontological argument is a priori.