Having discussed the empirical arguments by St, Thomas Aquinas I saw it fit to continue with the discussion on the Existence of God by introducing the Ontological argument which was hugely critisized by Aquinas.
The ontological argument was formulated by a group of philosophers including St. Anselm and René Descartes. The argument proposes that God’s existence is self-evident. The logic, states as follows: Whatever is contained in a clear and distinct idea of a thing must be predicated of that thing; but a clear and distinct idea of an absolutely perfect Being contains the notion of actual existence; therefore since we have the idea of an absolutely perfect Being such a Being must really exist.
Thomas Aquinas criticized the argument for proposing a definition of God which, if God is transcendent, should be impossible for humans. Immanuel Kant criticized the proof from a logical standpoint: he stated that the term "God" really signifies two different terms: both idea of God, and God. Kant concluded that the proof is equivocation, based on the ambiguity of the word God. Kant also challenged the argument’s assumption that existence is a predicate (of perfection) because it does not add anything to the essence of a being. If existence is not a predicate, then it is not necessarily true that the greatest possible being exists.
A common rebuttal to Kant’s critique is that, although "existence" does add something to both the concept and the reality of God, the concept would be vastly different if its referent was an unreal Being. Another response to Kant is attributed to Alvin Plantinga who explains that even if one were to grant Kant that "existence" is not a real predicate, "Necessary Existence", which is the correct formulation of an understanding of God, is a real predicate, thus according to Plantinga Kant’s argument is refuted.