The second objection Kant levels at the ontological argument that bears reference to that used by and contrasted by Descartes is that existence can be denied, even if it is a predicate, without contradiction, because we also, in denying existence, deny the thing itself; while it is impossible to say that “this supremely perfect being that exists does not have the perfection of existence”, because that is internally incoherent, it is possible to say that “this supremely perfect being which does not exist does not have the perfection of existence”. Kant expresses it more flowingly and fully, saying that ‘if… I annihilate the predicate in thought, and retain the subject, a contradiction is the result; and hence I say, the former belongs necessarily to the latter. But if I suppress both subject and predicate in thought, no contradiction arises; for there is nothing at all, and therefore no means of forming a contradiction.’ (Kant); this runs directly counter to Descartes’ belief that to deny God’s existence is internally inconsistent and illogical. While this second objection seems to accept existence as a predicate, it does so only in so far as it must in order to rule out the possibility of it being so; it criticises the logical reasoning of Descartes rather than his premises, saying that his conclusion does not follow from his premises, or rather it does not follow from his stated premises.
Source of quote: Kant, Immanuel,
Critique of Pure Reason, trans Meiklejohn, J. M. D, 1890; Project Gutenberg,
http://www.gutenberg.org/etext/4280