The presocratic philosopher Parmenides, who is regarded as the founder of western metaphysics, believed that all material things, and their changing forms and motions, are a reflection of the same eternal reality, the “Being”—he proposed the monistic principle “all is one”. In his Physics, Aristotle rejects Parmanedian monism by noting that a thinker who denies the multiplicity of things, and all the changing forms and motions, is not engaging in natural philosophy. In the thirteenth century, Aquinas assumed that, with Aristotle’s assistance, he could appropriate the “all is one” Parmanedian god while avoiding the pitfalls of monism. In his Compendium of Theology, Aquinas writes: "If we gather together the various points established thus far, we perceive that all perfections in God are in reality one. We have shown above that God is simple. But where there is simplicity, there can be no distinction among the perfections that are present. Hence, if the perfections of all things are in God, they cannot be distinct in Him. Accordingly they are all one in Him.” (Translation by Cyril Vollert). But Aquinas, it seems, was unable to bring Parmenides and Aristotle together and the duel between the two Ancient Greek philosophers continues.