The ontological argument is the name that Kant himself assigned to arguments which attempt to demonstrate God’s existence from reason alone.  Starting with an a priori definition of God, such as that God is “that than which nothing greater can be conceived of” (St Anselm, Proslogion II) or that God is “supremely perfect” (Descartes, Meditation V), ontological arguments show that existence – or necessary existence – is part of that definition and thus that God’s existence is de dicto necessary, as the fact that a man is unmarried is de dicto necessary if he is a bachelor.  Having coined the term “ontological argument”, Kant went on to criticise these arguments in the opening to his Critique of Pure Reason (1781), arguing that the arguments fail because 1) all existential statements must be synthetic, 2) existence is not a perfection and 3) existence is not a predicate.  He concludes that the ontological argument is “so much labour and effort lost” because it cannot do what it sets out to do and prove God’s existence from reason alone.  Nevertheless, despite being enormously influential, Kant’s criticisms fail to establish the impossibility of an ontological argument for God’s existence because they depend on Kant’s worldview which he asserts dogmatically and fails to argue for. 

Kant’s claim that the ontological argument fails because all existential statements must be synthetic is nothing more than an assertion of his own critical worldview, developed in the years after Hume “awoke me from my dogmatic slumbers and gave a completely different direction to my enquiries…” in 1770.  Kant claims that “If, then, I try to conceive a being, as the highest reality (without any defect), the question still remains, whether it exists or not. For though in my concept there may be wanting nothing of the possible real content of a thing in general, something is wanting in its relation to my whole state of thinking, namely, that the knowledge of that object should be possible a posteriori also…” He points out that there is nothing self-contradictory about saying “God does not exist”, because existence cannot be part of the concept of God, but must always be established empirically, synthetically, so that existence is always contingent and includes the possibility of non-existence. Yet, what privileges a posteriori knowledge over a priori knowledge, beyond Kant’s assertion?  As GW Hegel noticed straightaway and as WV Quine late pointed out in his “Two Dogmas of Empiricism” (1951) Kant simply asserts that meaningful claims must be either synthetic or analytic and that all existential claims must be synthetic.  Who says that the empirical senses are the arbiters of existence or that nothing can be said to exist that is incapable of being experienced through the senses?  How could it be reasonable to suggest that the senses of a hairless ape inhabiting temperate regions of an insignificant planet orbiting a small star on an outer spiral of the milky way provide the only window on reality?  As Plato and Descartes would have agreed, the “reality” we experience through our senses is ever-changing, imperfect and seen through the filter of senses and concepts in the mind.  Concepts and ideas are permanent and our window on them through reason less misty, so it is more reasonable to see these as ultimate reality.  Further, for followers of Wittgenstein, even if Kant’s worldview is accepted, as Norman Malcolm pointed out, Kant’s worldview and thus “language game” is just one out of many.  To those who hold a different worldview – such as a Platonic worldview in which ultimate reality is metaphysical and the means of accessing it reason – the claim that all existential statements must be synthetic is untrue and unconvincing.  It follows that Kant’s criticism fails to destroy the possibility of an ontological proof for God’s existence.  

Kant’s claim that the ontological argument also fails because existence is not a perfection fails because, as Charles Hartshorne pointed out, while ordinary contingent existence is not a perfection, necessary existence might well be.  Kant’s example of 100 thalers being the same in concept whether it exists or not, and not 101 thalers or 150 by existing, seems to make his point convincingly… and yet neither Anselm nor Descartes conceives of God’s existence being like the existence of other, contingent things.  A chocolate cake, a unicorn, a man or an island might be said to exist or not to exist, but only God can necessarily exist, so while existence might not alter the concept of cake, unicorn, man or island, necessary existence is of the essence of God as the supremely perfect being.  Charles Hartshorne suggested that Anselm’s ontological argument in Prosologion 3 might well evade Kant’s criticism that existence is not a perfection for this reason, nevertheless, Kant would reject this criticism of his point, rejecting the whole concept of necessary existence.  As he had already said, all existential statements must be synthetic, so necessary existence is, in Kant’s worldview, impossible.  JN Findlay noted this, suggesting that “it was indeed an ill day for Anselm when he hit upon his famous proof. For on that day he not only laid bare something that is of the essence of an adequate religious object, but also something that entails its necessary non-existence.” (Findlay, 1948) And that Anselm’s ontological argument in Proslogion III, by demonstrating how God’s contingent existence is impossible, shows that God’s existence is impossible because nothing can necessarily exist.  Yet, as I have already argued, Findlay and Kant are doing nothing more than asserting their own worldview, which excludes necessary existence not because it is inconceivable but because it is not compatible with their stated position which privileges things which can be experienced through the empirical senses. Further, as Hartshorne also pointed out, if it makes sense to talk about God’s necessary non-existence then it makes just as much sense to talk about God’s necessary existence. And, as Norman Malcolm pointed out, what is possible and impossible depends more on our language game than on what it might or might not refer to objectively.  It seems that in arguing that existence is not a perfection Kant got no further than Gaunilo had in refuting the ontological argument; again he just asserted that it was incompatible with his own limited worldview.  

Finally, Kant wrongly argued that existence is not a predicate.  By this Kant meant that existence cannot be accidentally predicated of God, because – as Anselm had pointed out – God’s existence is not something that might or might not be true of God.  Also, existence is wrongly used as an accidental predicate in relation to other things, because it adds nothing to the concept of the object (is not a perfection).  Rather, existence is the basis on which anything else can meaningfully be predicated.  As Russell pointed out, if I say “the present King of France is bald” I imply that there is a present King of France, which makes my predication of baldness meaningful.  If there is no present King of France, predicating anything of him is meaningless.  Similarly, if I say that God exists, I imply that there is a God who exists, smuggling my conclusion into my premises and in this way begging the question.  It follows that existence is not an accidental predicate and could not be used as one of God even if it were.  Further, existence cannot be essentially predicated of anything because… all existential statements must be synthetic. Here again, Kant reverts to asserting his worldview.  While He is right to point out that existence is not an accidental predicate because it is not a perfection, this is beside the point when it comes to the Ontological Argument, as Anselm pointed out in his Responsio to Gaunilo.  In this context, the question concerns whether existence is of the essence and nature of God, whether God necessarily exists.  As Hartshorne reasoned, necessary existence might well be predicated essentially of God, even if contingent existence cannot be used as an accidental predicate of anything.  For Hartshorne there are three alternatives for us to consider: 

i.God is impossible 

ii.God is possible, but may or may not exist 

iii.God exists necessarily. 

because God cannot be just a possibility, since he is by definition preeminent, so God’s existence is necessary.  Hartshorne’s modal ontological argument has been refined in different ways, including by Plantinga and Craig, but in essence – for those who share his worldview at least – it proves God’s necessary existence.  It seems that Kant’s criticism that existence is not a predicate fails to undermine the ontological argument either, at least beyond those who share his worldview.   

In conclusion, Kant’s criticisms of the Ontological Argument are persuasive if and only if you share his limited and dogmatic worldview.  Of course, many Philosophers did and do share this worldview – including Lotze, Schopenhauer, Russell and Findlay – and they will not accept the existence of other worldviews because it is part of their worldview to see their own perspective as the only truthful perspective on reality.  Nevertheless, for anybody who holds a different worldview – such as for St Anselm, Descartes, Hartshorne or Malcolm – Kant’s criticisms are ineffective and there is still the possibility of proving God’s existence a priori, from reason alone.  If ultimate reality is metaphysical and accessed through reason not empirical experience, and if existence is not defined in terms of being contingent existence, then the concept of God can de dicto necessarily contain His necessary existence despite His being incapable of empirical observation.