Full Idea
The only way for my intuition to precede the reality of the object and take place as knowledge a priori is if it contains nothing else than the form of sensibility which in me as subject precedes all real impressions through which I'm affected by objects.
Gist of Idea
A priori intuition of objects is only possible by containing the form of my sensibility
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 may be the single most famous idea in Kant. I'm not really a Kantian, but this is a powerful idea, the culmination of Descartes' proposal to start philosophy by looking at ourselves. No subsequent thinking can ignore the idea.