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)
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.
Book Reference
Kant,Immanuel: 'The Metaphysics of Morals', ed/tr. Gregor,Mary [CUP 1991], p.187
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]