“Thus when I think a thing, through whichever and however many predicates I like (even in its thoroughgoing determination), not the least bit gets added to the thing when I posit in addition that this thing is. For otherwise what would exist would not be the same as what I had thought in my concept, but more than that, and I could not say that the very object of my concept exists” (Critique of Pure Reason, A600/B628).
It is important to note the context of Kants rejection of existence as a predicate, which is his criticism of the ontological argument. Kant, as he says above, took Anselm to be arguing that predicating the concept ‘being’ of anything added something to the concept of a thing. This is not entirely correct, however, when we look at Anselm, who says that something which exists only in the understanding is not as great as something which exists in reality. Thus, a god which actually exists is greater than one which exists merely in the understanding.
So having noted that Kant is not a terrific reader of Anselm, is he in fact wrong? Lots of folks have argued that being is not a predicate or a property of individuals. Russell is probably the most well known – he argued that existence or being is a second-order concept. So, to say that X doesn’t exist is just to say that the property of being X is not instantiated. This makes more sense if you look at his debate with Meinong, which really turns out to be a debate over whether existence is equivocal or univocal. Meinong held to the former, Russell the latter (Frege as well). Russell took it to be the case that everything exists, while Meinong too it to be the case that not everything that exists exists. If existence is a predicate, then the problem of negative existentials really looms large, which is probably the main reason Russell held to his view. That is, if existence is a predicate, it becomes quite easy to argue from, say the existence of donkeys to the existence of, say, Eeyore (there’s actually issues here, but bracket those for the moment).
The fundamental disagreement between Aquinas and Anselm, IMO, occurs in the SCG, where Aquinas says that
‘No difficulty, consequently, befalls anyone who posits that God does not exist. For that something greater can be thought than anything given in reality or in the intellect is a difficulty only to him who admits that there is something than which a greater cannot be thought in reality.’
Obviously, this is in direct conflict with Anselm’s invocation of the Fool, but to me it also shows that the Ontological Argument is more logical than metaphysical. Anselm is basically interpreting negative existentials as being both about something ‘in the understanding’ that does not exist in reality: Anselm is trying to derive a logical contradiction or absurdity here. In other words, Anselm is trying to show that ‘There is no God’ or ‘God doesn’t exist’ is a contradiction. But this is easily avoided if we employ something like Russell theory of definite descriptions: we can say that ‘God’ = ‘something than which nothing greater can be conceived’. The fool can be taken to be saying that ‘there is nothing which fits the description ‘‘something than which nothing greater can be conceived’. To avoid the contradiction, all we have to do is translate that to ‘For any given thing, in the understanding or in reality, a greater than it can be conceived’, and, since Anselm’s argument doesn’t require the Fool to know that his statement is true but only to state it without contradiction, we have avoided Anselm’s contradiction. Aquinas’s quote above is basically the same as what I just laid out.