One of the most famous (and supposedly devastating) criticisms of Anselm’s ontological argument comes from Immanuel Kant. It is virtually undisputed by those who mention the argument. One hears this criticism even on the Internet. In nearly every instance in which I have encountered this objection, an explanation is never provided. What is this criticism, and what does it mean?
Kant claimed that “existence is not a predicate.” To illustrate what this means, consider an apple (or a horse, or a pencil, or any other object). One may describe it as “red,” and “sweet,” and any number of things. All of these are in the predicate position in a sentence. They translate into properties of the object like being red or being sweet. Kant held that in order for something to count as a property, it had to tell us something about the object that added to its description. Kant’s argument is that two apples will be identical where they have all of the same properties, even if we stipulate that one of them exists. If that is correct, then existence is not a property after all. But if existence is not a property, then Anselm cannot be correct when he says it is greater for God to exist in reality than merely in the intellect (since the difference between the two would be only in existence). So, is Kant right?
It seems that there is good reason to doubt that he was. First, Stephen T. Davis points out that actually-existing things do have properties, by virtue of their existence (although they are accidental ones), that they would not have were they not to exist. For example, the concept of a hundred dollars does not possess the accidental property of having purchasing power in the real world.  So, there are, or at least can be, relevant differences between identical concepts brought about by existence. Therefore, existence does, in at least some cases, add something to a concept.
Second, Davis gives an example of the perfect chancellor.  The idea can be extrapolated to the perfect person, or ruler, or whatnot. Take two conceptual persons who embody this perfect X. Suppose one person, A, satisfies all of the criteria for being a perfect X. However, A is a fictional character in a story. B has an identical list of attributes, but as it turns out, lives in northwestern Montana. What sense does it make to say there is no difference between fictional A and actually-existing B? But then it follows existence can be a real property or predicate.
“But wait!” I hear an objector say. “That doesn’t show that the concept of God is such a concept that allows for the predication of existence!” Perhaps, perhaps not. However, at the very least, it has been shown that existence can function in some instances as a predicate, so that doubt upon Kant’s criticism has been cast. It will no longer do merely to quote Kant. One will have to show that the concept of God is such that existence cannot be properly predicated of it.
Many have abandoned Anselm’s formulation of the argument for this criticism. I do not see the need to do so. While there are other ontological arguments I prefer (such as Plantinga’s modal ontological argument), Kant’s criticism does not damage Anselm’s nearly as much as many think.
2 It just does not matter that the conceptual hundred dollars has the counterfactual property of having purchasing power in the real world, where we would say “If this hundred dollars were actual, then it would possess the property of having purchasing power in the real world.” This is because, whatever the grounding for counterfactual properties, it nonetheless remains that the concept does not, in fact, possess this actual property.
3 Davis, 35.