Full Idea
Agreeable results could be brought about by other causes;…therefore nothing but the idea of the law in itself, which is present only in a rational being, can constitute that pre-eminent good which we call moral.
Gist of Idea
Other causes can produce nice results, so morality must consist in the law, found only in rational beings
Source
Immanuel Kant (Groundwork of the Metaphysic of Morals [1785], 401.15)
Book Reference
Kant,Immanuel: 'The Moral Law (Groundwork of Morals)', ed/tr. Paton,H.J. [Hutchinson 1948], p.66