Full Idea
The fact that we are able to act against our strongest desires reveals to us that we are free, and so are members of the intelligible world.
Gist of Idea
We must be free, because we can act against our strongest desires
Source
report of Immanuel Kant (Critique of Pure Reason [1781]) by Christine M. Korsgaard - Intro to 'Creating the Kingdom of Ends' Ch.1
Book Reference
Korsgaard,Christine M.: 'Creating the Kingdom of Ends' [CUP 1996], p.27
A Reaction
Can he prove that we can act against our strongest desires? If you choose to drown yourself in the sink, you may just be in the grips of a very strong desire to do so, which defeats the normal desire to survive.