1. Kant’s criticisms of the Ontological Argument
Immanuel Kant challenged Descartes’ version of the OA in his book ‘Critique of Pure Reason’.
His set out 4 criticisms against the OA.
The most famous of which was his claim that existence is not a predicate.

2. First criticism - contradiction
Both Anselm and Descartes claimed it was contradictory to state that “God does not exist” based on their belief that existence was part of God’s greatness or perfection.
Kant’s response was to claim that this statement has no solid grounding; if one rejects the subject (God) and the predicate (existence) then the contradiction is removed.
To consider a triangle but reject its three angles is impossible, but, Kant argues, ‘…there is no contradiction in rejecting the triangle together with its three angles.’ The existence of the triangle is not necessary.

3. Second criticism – nature of enquiry
Kant’s second argument is to say that it is wrong to claim that God exists when the OA is an enquiry into the possibility of God’s existence.
One cannot claim to be enquiring open-mindedly into God’s existence and then introduce the concept of existence as a necessary part of your description of God.

4. Third criticism – existential propositions
Kant’s second argument is to state that all existential (to do with existence) propositions are synthetic.
In other words, we have to use our senses to verify that something exists.

5. Fourth Criticism: God’s Necessary Existence
And so, existence itself cannot be considered one of its properties.
Kant argues that for something to exist, even in the mind, it must already have predetermined properties that make it what it is.
“We do not make the least addition to the thing when we further declare that this thing is.”
Therefore, as the object already has the properties that makes it what it is, existence cannot be considered as a property that adds anything, including greatness, to it.

6. God’s Necessary Existence
Kant’s example of necessary existence is that a triangle requires three angles for its existence otherwise it does not exist as a triangle.
but, Kant argues, ‘…there is no contradiction in rejecting the triangle together with its three angles.’ The existence of the triangle is not necessary.
To consider a triangle but reject its three
angles is impossible,
With this argument Kant draws into question points three and four of Anselm’s argument saying existence isn’t a necessary quality in making a thing what it is. It does not make the thing ‘greater’.

7. God’s Necessary Existence
Anselm states that this was the problem with Gaunilo’s parody.
An island, no matter how great, is dependent on other elements for its existence in order to maintain its ‘greatness’. Therefore, the island’s existence is not a ‘necessary existence’.
A necessary existence indicates ‘something that does not rely on anything else for its existence, rather other things rely on its existence as a necessary factor for their existence.’
A necessary existence is something that the non-existence of is impossible