Full Idea
How a law in itself can be the direct motive of the will (which is the essence of morality) is an insoluble problem for the human reason.
Gist of Idea
Reason cannot solve the problem of why a law should motivate the will
Source
Immanuel Kant (Critique of Practical Reason [1788], I.1.III)
Book Reference
Kant,Immanuel: 'Critique of Practical Reason (Third edition)', ed/tr. Beck,Lewis White [Library of Liberal Arts 1993], p.75
A Reaction
If that is the great man's final word, then it is tempting to switch to an empirical moral theory, such as that of Hobbes or Hume or E.O. Wilson, which starts from what motivations are available, and builds morality up from that.