Works of St. Anselm, tr. by Sidney Norton Deane, , at sacred-texts.com
How the fool has said in his heart what cannot be conceived. ‑‑A thing may be conceived in two ways: (1) when the word signifying it is conceived; (2) when the thing itself is understood As far as the word goes, God can be conceived not to exist; in reality he cannot.
BUT how has the fool said in his heart what he could not conceive; or how is it that he could not conceive what he said in his heart? since it is the same to say in the heart, and to conceive.
But, if really, nay, since really, he both conceived, because he said in his heart; and did not say in his heart, because he could not conceive; there is more than one way in which a thing is said in the heart or conceived. For, in one sense, an object is conceived, p. 9 when the word signifying it is conceived; and in another, when the very entity, which the object is, is understood.
In the former sense, then, God can be conceived not to exist; but in the latter, not at all. For no one who understands what fire and water are can conceive fire to be water, in accordance with the nature of the facts themselves, although this is possible according to the words. So, then, no one who understands what God is can conceive that God does not exist; although he says these words in his heart, either without any or with some foreign, signification. For, God is that than which a greater cannot be conceived. And he who thoroughly understands this, assuredly understands that this being so truly exists, that not even in concept can it be non‑existent. Therefore, he who understands that God so exists, cannot conceive that he does not exist.
I thank thee, gracious Lord, I thank thee; because what I formerly believed by thy bounty, I now so understand by thine illumination, that if I were unwilling to believe that thou dost exist, I should not be able not to understand this to be true.