According to Kant, the moral worth of an action consists not in the consequences that flow from it, but in the intention from which the act is done. What matters is the motive, and the motive must be of a certain kind. What matters is doing the right thing because it’s right, not for some ulterior motive.
“A good will is not good because of what it effects or accomplishes,” Kant writes. It is good in itself, whether or not it prevails. “Even if… this will is entirely lacking in power to carry out its intentions; if by its utmost effort it still accomplishes nothing… even then it would still shine like a jewel for its own sake as something which has its full value in itself.”
For any action to be morally good, “it is not enough that it should conform to the moral law – it must also be done for the sake of the moral law.” And the motive that confers moral worth on an action is the motive of duty, by which Kant means doing the right thing for the right reason.
In saying that only the motive of duty confers moral worth on an action, Kant is not yet saying what particular duties we have. He is not yet telling us what the supreme principle of morality commands. He’s simply observing that, when we assess the moral worth of an action, we assess the motive from which it’s done, not the consequences it produces.
If we act out of some motive other than duty, such as self-interest, for example, our action lacks moral worth. This is true, Kant maintains, not only for self-interest but for any and all attempts to satisfy our wants, desires, preferences, and appetites. Kant contrasts motives such as these – he calls them “motives of iinclination” – with the motive of duty. And he insists that only actions done out of the motive of duty have moral worth.
Sandel, J. Michael (2009) Justice: What’s the right thing to do? New York: Farrar, Straus and Giroux.