Full Idea
Intuitions which are possible a priori can never concern any other things than objects of our senses.
Gist of Idea
A priori intuitions can only concern the objects of our senses
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
Given the Kantian idea that what is known a priori will also be necessary, we might have had great hopes for big-time metaphysics, but this idea cuts it down to size. Personally, I don't think we are totally imprisoned in the phenomena.
Related Idea
Idea 16914 A priori intuition of objects is only possible by containing the form of my sensibility [Kant]