St. Anselm's "Ontological" Argument for God's Existence

    (1) God is a being greater than which none can be conceived — or, for short, the greatest conceivable being or the most perfect conceivable being (from the meaning of the term 'God' even as atheists use it)

    (2) Suppose, then, for the sake of argument, that this greatest conceivable being existed only as a concept in the mind and not in reality (assumption for reductio ad absurdum)

    (3) In that case, we could conceive of a greater being, namely one that existed both as a concept in the mind and also in reality (premise)

    (4) But then the being we began with (the greatest conceivable being) wouldn't be the greatest conceivable being after all — a contradiction (from 2 and 3)

    (5) Therefore, if a being is (to qualify as) the greatest conceivable being, it must exist not only as a concept in the mind but also in reality (from 2, 3, and 4)

    (6) Therefore, the greatest conceivable being exists in reality.

    (7) But the greatest conceivable being is God (from 1)

    (8) Therefore, God exists (from 6 and 7)