Anselm of Canterbury, answered from the texts and cited to the page.
The argument turns on a distinction that Gaunilo, for all his intelligence, pressed past too quickly. Any being whose existence is merely contingent — any being that happens to exist but might not have — can be conceived not to exist. This much is common ground. But that than which nothing greater can be conceived cannot be such a being, for a being whose nonexistence is inconceivable is greater than one whose nonexistence can be conceived.1
So if the greatest conceivable being could be conceived not to exist, it would not be the greatest conceivable being — a plain contradiction. The sharper formulation runs thus: that than which a greater is inconceivable cannot be conceived except as without beginning.2 Whatever can be conceived to exist and yet does not exist could have come into existence through a beginning — it is the kind of thing that might start.
But a being that might start, or might stop, or might simply never have been, is not the being than which nothing greater can be conceived. Hence if such a being can be conceived at all, it must exist; and if it exists, its nonexistence is not merely false but inconceivable.3 This is what distinguishes God from every other thing. Of whatever else exists — of me writing these words, of the world spread out beyond this window — it can be said that its nonexistence is possible, even when its existence is presently known.4
The contingent thing exists, but the negation of its existence remains thinkable. Of God alone it can be said that it is impossible to conceive of his nonexistence.5 He exists not as one being among others who happens to be present, but as the being whose very nature excludes the possibility of absence. Gaunilo raised the case of his own existence: he is most certainly aware of it, yet he knows his nonexistence is possible.6
Precisely so — and this is the mark of a contingent being, not a necessary one. He then wondered whether, if he cannot conceive of his own nonexistence while knowing himself to exist, God would not be the only being of whom necessary existence holds.7 The reply is that the inability to conceive of one's own nonexistence in the moment of self-awareness is a psychological limit, not a metaphysical one; the nonexistence of Gaunilo remains genuinely possible, whereas the nonexistence of that than which nothing greater can be conceived is not merely unconceived but inconceivable in principle.
The difference is between a mind that happens not to form the thought and a nature that excludes the thought altogether. Necessary being, then, is not a stronger grade of the same kind of existence that contingent things have. It is existence of a wholly different order — existence that cannot be otherwise, that has no beginning through which it might be conceived to have come about, and whose negation is not a thinkable alternative but a contradiction.
it is possible to conceive of a being which cannot be conceived not to exist; and this is greater than one which can be conceived not to exist. Hence, if that, than which nothing greater can be conceived, can be conceived not to exist, it is not that, than which nothing greater can be conceived.Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo
For that than which a greater is inconceivable cannot be conceived except as without beginning. But whatever can be conceived to exist, and does not exist, can be conceived to exist through a beginning.Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo
Therefore if a being than which a greater is inconceivable can be even conceived, it cannot be non-existent.Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo
we can conceive of the non-existence of an object, although we know it to exist, because at the same time we can conceive of the former and know the latter.Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo
So, then, of God alone it can be said that it is impossible to conceive of his non-existence; and yet many objects, so long as they exist, in one sense cannot be conceived not to exist.Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo
I am most certainly aware of my own existence; but I know, nevertheless, that my non-existence is possible.Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo
If, however, I cannot, God will not be the only being of which it can be said, it is impossible to conceive of his non-existence.Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo