“I promise to mow your lawn, but I don’t know whether I will.” Call promises of this form “Moorean,” based on their resemblance to Moore’s paradox. Moorean promises sound absurd. But why? In the literature on assertion, many have used Moore’s paradox to motivate a knowledge norm of assertion. I put forward an analogous norm on promising, according to which one should only make a promise if one knows that one will fulfill it. A knowledge norm explains why Moorean promises (...) are absurd, accounts for a variety of linguistic data, and sheds light on how promises generate obligations. (shrink)