Which philosopher proposed that a necessary being's non-existence is logically contradictory?

The philosopher who proposed that a necessary being's non-existence is logically contradictory is St. Anselm. Anselm's argument for the existence of God, especially in his ontological proof, hinges on the concept of necessity. He posits that if we can conceive of a being greater than which nothing can be conceived, this being must exist necessarily; otherwise, it would not be the greatest conceivable being. Therefore, the idea of a necessary being not existing would be a logical contradiction, as a truly necessary being must exist in all possible worlds, including our own.

In contrast, Aquinas does not argue from the perspective of necessity but rather from contingent beings and the existence of a first cause. René Descartes presents his own form of the ontological argument but does not focus explicitly on the logical contradiction of non-existence in the way Anselm does. William James, known for his pragmatic approach, questions the necessity of proof for metaphysical claims, rather than defending the notion of necessary existence.