Full Idea
That the concept of a thing is possible (not self-contradictory) is not yet sufficient for assuming the possibility of the thing itself (the objective reality of the concept).
Gist of Idea
That a concept is not self-contradictory does not make what it represents possible
Source
Immanuel Kant (Metaphysics of Morals II:Doctrine of Virtue [1797], 382 Intro I)
Book Reference
Kant,Immanuel: 'The Metaphysics of Morals', ed/tr. Gregor,Mary [CUP 1991], p.187
A Reaction
I take this to be an inkling of Kripke's a posteriori scientific necessities, which place far greater restrictions on the possibilies of what we seem to have conceived, in addition to the mere need for consistency.
Related Ideas
Idea 19427 True ideas represent what is possible; false ideas represent contradictions [Leibniz]
Idea 5613 The analytic mark of possibility is that it does not generate a contradiction [Kant]