Full Idea
The will could be defined as the faculty of purposes, since they are always determining grounds of the faculty of desire according to principles.
Gist of Idea
The will is the faculty of purposes, which guide desires according to principles
Source
Immanuel Kant (Critique of Practical Reason [1788], I.1.II)
Book Reference
Kant,Immanuel: 'Critique of Practical Reason (Third edition)', ed/tr. Beck,Lewis White [Library of Liberal Arts 1993], p.61
A Reaction
Do animals have wills? Kant implies that you can only have a will if you have principles. Compare Hobbes' rather less elevated definition of the will (Idea 2362).
Related Idea
Idea 2362 The will is just the last appetite before action [Hobbes]