Full Idea
The understanding can never accomplish a priori anything more than to anticipate the form of a possible experience in general.
Gist of Idea
A priori the understanding can only anticipate possible experiences
Source
Immanuel Kant (Critique of Pure Reason [1781], B303/A246)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.345
A Reaction
This is why many people think that Kant brough metaphysical (ontological) speculation to an end. He asserts that synthetic a priori knowledge is possible, but then imposes a huge limitation on it.