Full Idea
For Kant, existence derives from a true affirmative subject-copula-predicate judgement; existence is not a real predicate, but is merely derivatively implied by the copula ('is').
Gist of Idea
Existence is merely derived from the word 'is' (rather than being a predicate)
Source
report of Immanuel Kant (Critique of Pure Reason [1781]) by Alex Orenstein - W.V. Quine Ch.2
Book Reference
Orenstein,Alex: 'W.V. Quine' [Princeton 2002], p.13
A Reaction
This is Kant's understanding of 'existence is not a predicate', prior to the later move of Brentano and Frege, which places existence claims in the quantifier, which is outside the proposition.