Anselms argument By definition, God is a being greater than which cannot be conceived. I can conceive of such a being. It is greater to exist than not to exist. Therefore, God must exist.
Gaunilos objection You could prove anything perfect must exist by this argument! I can conceive of the the perfect island, greater than which cannot be conceived. And so such an island must exist, because it would be less great if it didnt. But this is ridiculous, so the ontological argument must be flawed.
Anselms reply An island wouldnt cease to be what it is – an island – if it wasnt perfect. Islands arent perfect by definition or essentially. God wouldnt be God if there was some being even greater than God. Being the greatest conceivable being is an essential property of God. Gods existence is therefore necessary, while even a perfect island exists only contingently.
Descartes argument (Med. V) It is certain that I… find the idea of a God in my consciousness, that is the idea of a being supremely perfect: and I know with… clearness and distinctness that an [actual and] eternal existence pertains to his nature.
Descartes argument God is a supremely perfect being. (Necessary) existence is a perfection. Therefore, God (necessarily) exists.
A first objection There is a difference between thinking God exists and God actually existing: though I conceive God as existing, it does not seem to follow on that account that God exists. Reply: unlike any other thing, because I cannot conceive of God unless as existing, it follows that existence is inseparable from him, and therefore that he really exists. –the necessity of the existence of God determines me to think in this way: for it is not in my power to conceive a God without existence
A second objection Gassendi: I can conceive of God without existence. Reply: it can be hard to realize, but all divine perfections entail each other – e.g. –If God is omnipotent, then God must not depend on anything else. –Therefore, God must not depend on anything else to exist. –Therefore, God must have necessary existence.
Pressing the second objection Johannes Caterus: Descartes argument only works if God exists, because only if God exists, is God omnipotent, etc. The interdependence of perfections shows only that the concept of existence is part of the concept of God. Reply: that was all it was intended to show – that we cant conceive of God without existence. So God must exist.
A third objection Kant: existence is not a perfection, because it is not a predicate at all. –To say x exists is not to describe x at all or explain what x is. Existence is not part of the concept of anything. –To say x exists is to say that some real object corresponds to the concept of x.
Diagnosis: necessary existence necessarily, God exists v. God exists necessarily necessarily, God exists = it must be true that God exists. God exists necessarily = the type of existence God has (if God exists at all) is necessary, i.e. not contingent, without dependence on anything else. The ontological argument confuses the two.