Full Idea
It would not be a duty to strive after a certain effect of our will if this effect were impossible in experience.
Gist of Idea
It can't be a duty to strive after the impossible
Source
Immanuel Kant (True in Theory, but not in Practice [1792], Intro)
Book Reference
Kant,Immanuel: 'Political Writings', ed/tr. Reiss,Hans [CUP 1996], p.62
A Reaction
'Ought implies can' has become a familiar slogan. The quickest way to get shot of a tiresome duty is to persuade yourself that it is impossible. The seemingly impossible is occasionally achieved.