The lesson of the preceding discussion is as follows: There is such a thing as the moral law, but it doesn't rest on experience. Yet experience is the stuff of which knowledge is made. Prior to experience, there's nothing in us but forms, of which we're barely conscious. So Kant reasons that the moral law must be purely formal. The material of knowledge – derived as it is from experience alone – lacks moral value. So experience is not immoral, but amoral, foreign to morality.

What do we know of the moral law? One thing we know is that it's a form of the mind and that as such – forms being universal – it exists in all people. How can we know whether or not we should perform a certain action? Every time the maxim that guides our action can be elevated to a universal rule of conduct, Kant says, the action is good. In the opposite case, the action is bad. Kant formulated the moral law as follows: “Always act according to a maxim that you could will to be a universal law” (Groundwork of the Metaphysics of Morals). He went on to apply this formula to many specific cases. Should we steal? No, because we couldn't will theft to be a universal law, for to do so would spell the end of private property. The moral proposition being considered can't be elevated to a universal rule of conduct, so the action in question is bad.