Full Idea
If something is held to be absolutely good or evil in all respects and without qualification, it could not be a thing but only the manner of acting, i.e., it could only be the maxim of the will, and consequently the acting person himself is good or evil.
Gist of Idea
Good or evil cannot be a thing, but only a maxim of action, making the person good or evil
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.63
A Reaction
It goes on to deny that pain is intrinsically evil, but his reason for the claim is not clear. Nevetheless, I think he is right. This remark is an important bridge between Enlightenment concerns with law and Greek concerns with character.