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 is an unreal Being. Another response to Kant is attributed to Alvin Plantinga, who says that even if one were to grant that existence is not a real predicate, "necessary existence", which is the correct formulation of an understanding of God, "is" a real predicate.