I’ve always thought the ontological argument was invalid, and I’ve known what the problem is, but I’ve never really been able to express it. Today, I found a book at the library that took the problem right out of my head and expressed it perfectly:
“St. Anselm says: If the most perfect being that can be conceived did not exist, it would be possible to conceive of a being which has all the qualifications of the former, plus existence, so that this latter being would then be more perfect than the most perfect being that can be conceived. I admit that if this being did not exist, and was not conceived as self-existing, it would be possible to conceive one more perfect. But I deny the assertion that if it did not exist, though it was at the same time conceived as self-existing, then it would be possible to conceive of a more perfect being. Hence it is not logical to conclude: ‘Therefore God exists’; all that can be logically concluded is: Therefore, God must be conceived as self-existing, and in truth does so exist, and is entirely independent of any other being, if He exists.” (Reginald Garrigou-Lagrange, God: His Existence and His Nature, vol. 1, ch. 2)
The problem is that the argument fails to distinguish between whether God is conceptualized as existing and whether God actually exists. The argument shows that God conceptualized as actually existing is greater than God conceptualized as not existing. But just because one must conceptualize God as existing if one is to conceptualize Him at all, doesn’t mean God has to exist independently of a person’s concept of Him.