Full Idea
If you concede that every existential proposition is synthetic, then how would you assert that the predicate of existence may not be cancelled without contradictions?
Gist of Idea
If an existential proposition is synthetic, you must be able to cancel its predicate without contradiction
Source
Immanuel Kant (Critique of Pure Reason [1781], B626/A598)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.566
A Reaction
The point is that the Ontological Argument claims that "God does not exist" is a contradiction. Kant is echoing Hume here. The proposition that 'nothing exists' hardly sounds like a logical impossibility