An ontological argument for the existence of God defines God as the greatest perfect being and states that He must exist because He would not be perfect or the greatest, if He remained only in thought. Anselm and Descartes proposed it, and Kant pointed out that this argument was wrong in deducing a synthetic judgment from an analytic one, but the real problem of this argument is that the greatest perfect being whose existence it proves is quite different from what Christians regard as God, namely the omniscient and omnipotent supreme being.

A Cartesian coordinate system is named after René Descartes. But Descartes did not first invent the Cartesian coordinate system nor did he first establish analytic geometry on the ground of the Cartesian coordinate system. Still it deserves its name, because the method of the Cartesian coordinate system is similar to that of Cartesian philosophy.