Full Idea
Through intuition we can only know objects as they appear to us (to our senses), not as they may be in themselves; and this presupposition is absolutely necessary if synthetic propositions a priori are to be granted as possible.
Gist of Idea
A priori synthetic knowledge is only of appearances, not of things in themselves
Source
Immanuel Kant (Prolegomena to Any Future Metaphysic [1781], 283)
Book Reference
Kant,Immanuel: 'Prolegomena to Any Future Metaphysic', ed/tr. Lucas,Peter G. [Manchester UP 1971], p.38
A Reaction
This idea is basic to understanding Kant, and especially his claim that arithmetic is a priori synthetic.