Full Idea
Virtue is the moral strength of a man's will in fulfilling his duty, a moral constraint through his own lawgiving reason.
Gist of Idea
Virtue is the moral strength to fulfil duty, and arises from reason
Source
Immanuel Kant (Metaphysics of Morals I: Doctrine of Right [1797], 405 Intro XIII)
A Reaction
Since reason is the source of this determination, it seems that virtue is just a strong power of reason. Does that make clever people morally superior? What if he shows great strength is pursuing a warped sense of duty?
Book Reference
Kant,Immanuel: 'The Metaphysics of Morals', ed/tr. Gregor,Mary [CUP 1991], p.206