What, in summary, was Kant's objection to the Ontological argument for the existence of God.

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The Ontological Argument, in its simplest form, runs as follows: God, by definition, is infinitely perfect in every respect. One respect in which something can be perfect is by existing -- it's better to exist than not to exist -- therefore God must possess this perfection alongside all the others (omnipotence, omniscience, etc.). If God lacked existence, He would thereby qualify as imperfect, but that would contradict the definition of what we mean by 'God'.

Now, Kant makes a number of points about this argument: but, at the heart of his critique, his main objection is that it's wrong to think of existence as a perfection comparable to those others. The point is sometimes expressed by saying that the term "exists" is not a predicate: its logical role in sentences and thoughts is very different from that of genuine predicates like "is omnipotent" or "is omniscient". Existence is not a quality that we ascribe to a subject, as we might ascribe omnipotence to it. It is something prior to all such ascriptions: first something must exist, and only then, once we have a subject to contemplate, can we begin to consider what that thing is like.

So Kant would reject as illegitimate a definition that is, even implicitly, drawn up in terms of existence. But he also highlights the hypothetical character of definitional claims in general. He uses the example of a triangle. A triangle is defined as a plane figure with three angles. And this definition does entitle us to deduce that, if a triangle exists, then it will have three angles: but it does not entitle us to deduce any more than that. In particular, it does not tell us whether any triangles actually exist in reality. So suppose we accept that the correct definition of 'God' is indeed to be drawn up in terms of infinite perfection, and that attributes like omnipotence and omniscience are particular aspects of this. We will then be entitled to conclude that, if God exists, then He will be omnipotent and omniscient. Given the definition, it really would be a contradiction for Him to exist and yet lack these attributes. But, if He doesn't exist, there will simply be nothing there, either to possess them or to lack them, and hence no opportunity for contradiction. The terms of the definition don't come into play at all, until we first have a subject to which to apply them. And so God's existence cannot be proved from His perfection, because, unless we already have independent grounds for postulating His existence, the question of His perfection or imperfection simply will not arise. Indeed, even if we continue to maintain against Kant that existence is a genuine property, and hence is capable of featuring in definitions after all, and maintain that it does in fact feature implicitly in the definition of 'God', all that this will give is the following: if God exists, then He exists. And that is true, but it is spectacularly uninformative as to whether or not He actually does so.

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