Full Idea
In nature the understanding cognizes only what exists, or has been, or will be. It is impossible that something ought to be other that what it in fact is. ...We cannot ask what ought to happen in nature, any more than what properties a circle should have.
Gist of Idea
We only understand what exists, and can find no sign of what ought to be in nature
Source
Immanuel Kant (Critique of Pure Reason [1781], B575/A547)
A Reaction
This seems to be the first clear recognition of what we now call 'normativity', which seems like a misfit in naturalistic views. Davidson derives a sort of mental dualism from it. Note that powers and dispositions can also not be directly cognised.
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.540
Related Idea
Idea 21457 Reason has two separate objects, morality and freedom, and nature, which ultimately unite [Kant]