Full Idea
Since reason has been imparted to us as a practical power, which thus influences the will, its true function must be to produce a will which is good.
Gist of Idea
The function of reason is to produce a good will
Source
Immanuel Kant (Groundwork of the Metaphysic of Morals [1785], 396.7)
Book Reference
Kant,Immanuel: 'The Moral Law (Groundwork of Morals)', ed/tr. Paton,H.J. [Hutchinson 1948], p.62