Some of us may be quite familiar with the Ontological Argument under St. Anselm (1033-1109), the archbishop of Canterbury. This argument, simply understood, is Anselm’s formulation of an argument which he sought to unravel God in all of his majesty. However, in this argument, Anselm start’s off with a basic assumption that (1) God exists in the understanding and not in reality.
God, according to Anselm, is “the being by which none greater can be conceived” or simply, “the greatest conceivable being”. Thus, Anselm’s argument in full can be stated as:
- God exists in the understanding but not in reality
- Existence in reality is greater than existence in the understanding alone.
- A being having all of God’s properties plus existence in reality can be conceived.
- A being having all of God’s properties plus existence in reality is greater than God – from (1) and (2)
- A being greater than God can be conceived (3), (4).
- It is false that a being greater than God can be conceived – by definition of “God”
- Hence it is false that God exists in the understanding but not in reality – (1)-(16), reductio ad absurdum.
Seems like an interesting argument doesn’t it? In fact, the argument appears to be “downright irritating; it smacks too much of word magic” . We have defined God as “the greatest conceivable being”, and thus stipulate that a being of this kind, must exist in reality rather than merely in the understanding; since, according to (2), existence in reality is greater than existence in the understanding alone. Therefore, God exists in reality.
Surely to most, in light of the literature associated with this argument, the objection surfaces to be of akin type. One contemporary of Anselm, Gaunilo, objected to Anselm’s argument by saying that in the same way he could conceive of a perfect island, and thus stipulate that it must exist in reality and not the understanding alone. However, Anselm replied back to Gaunilo by saying that his argument is not in line faithfully with his. For, a perfect island is not perfect in the same sense as a perfect being is.
Immanuel Kant’s objection goes something of a similar taste. In his Critique of Pure Reason (A598/B626), Kant argues that stipulating existence as a predicate to its subject does not give the subject actual existence. For instance, supposing the statement “all traingles have three sides” is true (which it is), a contradiction emerges once I deny the predicate in respect to its subject. In other words, I cannot be consistent in suggesting that “triangles do not have three sides”, which would then no longer constitute a triangle.
However, a denial of the subject (triangle) in respect to its predicate is not a contradiction. It is in the same way with God – you cannot stipulate the predicate of existence to the subject “God” and then suppose that it exists in reality. Saying I have 100$ in my pocket doesn’t put 100$ in my pocket.
Both Kant’s and Gaunilo’s objection are bad because they straw man Anselm’s position. Barry Loewer says that Anselm’s argument is “surely false”  because of the inherent inconsistencies of the argument brought up by Gaunilo. As E.J. Lowe once wrote:
Nothing in… the ontological argument implies that… existence… must be a divine attribute or property, in the way that omniscience or omnipotence are… [T]he Kantian objection… is just a red herring with no real bearing on the soundness of the ontological argument. (2007, 337)
You can see my other essay, ‘The Ontological Argument and the Irrelevance of Kant’ for more on this particular criticism of Kant. To summarize the paper, it is quite simple to remember that Anselm’s main contention is that “in rejecting the existence of a thing we cannot be contradicting ourselves” (Plantinga, 31), so, since that would be the case with God, we have seemed to do so. However, Kant critiques:
If… I reject the predicate while retaining the subject, contradiction results; and I therefore say that the former belong necessarily to the latter. But if we reject subject and predicate alike, there is no contradiction; for nothing is then left that can be contradicted.
To posit a triangle, and yet to reject its three angles, is self-contradictory; but there is no contradiction in rejecting the triangle together with its three angles. The same holds true of the concept of an absolutely necessary being. If its existence is rejected, we reject the thing in itself with all its predicates; and no question of contradiction can then arise. 
This was a point stated before. E. J. Lowe gave an appropriate response to Kant (as well did Plantinga) that it is simply a “red herring with no real bearing on the soundness of the ontological argument.” Since Kant might be right in saying that “by whatever and however many predicates we may think a thing… we do not make the least addition to the thing when we further declare that the thing is” , this was not the case Anselm was making. To finish with Plantinga:
Unfortunately, it seems to have no particular bearing on Anselm’s argument. For Anselm can certainly agree, so far as his argument is concerned, that existence is not a real predicate in the explained sense. Anselm maintains that the concept the being than which none greater can be conceived is necessarily exemplified;
that this is so is in no way inconsistent with the suggestion that the whole concept of a thing diminished with respect to existence is equivalent to the undiminished whole concept of that thing. Anselm argues that the proposition God exists is necessarily true; but neither this claim nor his argument for it entails or presupposes that existence is a predicate in the sense just explained .
-  Alvin Plantinga, God and Other Minds (1967) p. 26
-  30-Second Philosophies, ed. by Barry Loewer (Metro Books, 2009) p. 102
-  Immanuel Kant, Critique of Pure Reason, tr. N.K. Smith (London, 1929), pp. 502-503
-  Kant, A600/B628)
-  Plantinga, 36