I'm teaching a philosophy of religion course this term and we're beginning with a discussion of Anselm's ontological argument. The IEP entry has a pretty clear statement of this argument:
Kant famously objected to premise (3) and claimed that existence is not a "real or defining predicate." On his view, the concept of God cannot include existence. The upshot here seems to be that existence is not a "great-making property" because it's not really a property at all. According to Kant, what we do when we say something like "God exists" is to simply affirm that the entity described by the various properties belonging to the concept (omnipotence, moral perfection, creator of the universe, etc.) is actual or real. It's not as if we're ascribing to God some further property--i.e., the property of existence--on Kant's view.
Here's how Kant's main point is spelled out in the IEP entry:
"The idea here is that existence is very different from, say, the property of lovingness. A being that is loving is, other things being equal, better or greater than a being that is not. But it seems very strange to think that a loving being that exists is, other things being equal, better or greater than a loving being that doesn't exist. But to the extent that existence doesn't add to the greatness of a thing, the classic version of the ontological argument fails."
But is it true that existence doesn't add to the greatness of a thing? Consider two possible loving beings, one exists and the other does not. The real loving being could express its love and bring about various positive things in the world whereas the non-existent loving being is not able to do such things. It could be argued that the former is better--in some sense--than the latter precisely because it can do such things and this is due to the fact that it exists. It might then seem that existence is a great-making property after all.
I can think of a few objections to this move. Perhaps Kant might reply that it's not the property of existence which makes the former better, but rather it's the fact that the being engages in certain activities and that these activities have certain results. So it's not as if the former (qua loving being) is better than the latter (qua loving being). They're both the same on the "loving-being-front."
But I don't find this reply persuasive. With respect to lovingness we might say that the two beings are on par--they're both loving. But it still seems that when we compare the two beings and all of their properties taken together, the one which is actual is, in some sense, better than the one who isn't. I'd rather have the former as a friend. Imaginary friends, however loving, aren't as good to have as real, loving friends.
Of course, there's a lot to say about what we might mean by "better" here, but I'll save that for another day. For now, I wonder what you think about whether (a) existence is a property and (b) whether it's the sort of property which "adds to the greatness" of a thing or is a "great-making property."