Full Idea
If the law is such that a whole people could not possibly agree to it …it is unjust.
Gist of Idea
A law is unjust if the whole people could not possibly agree to it
Source
Immanuel Kant (True in Theory, but not in Practice [1792], 2-Concl)
Book Reference
Kant,Immanuel: 'Political Writings', ed/tr. Reiss,Hans [CUP 1996], p.79
A Reaction
Kant is explicitly trying to approximate Rousseau's general will. The categorical imperative was greatly influenced by Rousseau. The key point is not whether they accept it, but that unanimous acceptance is unthinkable. Unfair laws will fail.