Full Idea
The will is absolutely good if it cannot be evil - that is, if its maxim, when made into a universal law, can never be in conflict with itself.
Gist of Idea
The will is good if its universalised maxim is never in conflict with itself
Source
Immanuel Kant (Groundwork of the Metaphysic of Morals [1785], 437.81)
Book Reference
Kant,Immanuel: 'The Moral Law (Groundwork of Morals)', ed/tr. Paton,H.J. [Hutchinson 1948], p.98