Immanuel Kant was a famous philosopher of the enlightenment, most famous for his system of ethics and morality dubbed 'Kantian Ethics'. However, he also looked at the ontological argument as set forward by Descartes, although it is not clear whether he knew of Anselm's original version. He said:
1) There is no contradiction in rejecting a concept together with all its predicates. (You can reject God as an whole, including his existence, and there is nothing contradictory about that)
2) Existence is not a predicate. (Predicates tend to add knowledge to something, like the sides of a triangle defines a triangle, but existence doesn't add any knowledge - it just tells us there is an instance of something)
When reviewing Kant's response, I felt I disagreed in half.
I do agree that you can reject a concept in all its entirety, including any predicates such as existence, for reasons including that of Kant's. However, I disagree with him on one point: I believe existence is a predicate, in some circumstances.
In order to prove this, I will first prove that existence can be a predicate.
Now, imagine someone has a friend who they talk to and play with. It would completely change the meaning and your understanding of the friend if they don't actually exist and are only an imaginary friend.
In the same way, God is defined as existent by some and non-existent by others; this changes how you view their opinion of God. Therefore, in opposition to Kant's idea, existence does add knowledge to something so can be a predicate.
Okay, so now I've established existence can be a predicate. But, what about it being subjective to context?
Imagine there is a being who was invisible. It would not matter whether this being is blue or green. In fact, any visual details other than 'it is invisible' are entirely pointless. These details would not change the meaning, as Kant says, of the being. Therefore in this case, colour would not be a predicate.
However, with a visual object, such as a football, colour could very well be a predicate. If the ball is pink, that will change the meaning completely than if the ball was camouflaged.
I say the same applies to existence, or in fact to any predicate. For example, whether a triangle exists or not has no relation to the meaning of the triangle, so existence in this case would not be a predicate.
Therefore, all predicates are subjective to the context; such as red and green to an invisible being.
