How Can a Promise Create an Obligation?

“I may come tomorrow” and “I promise to come tomorrow” both concern the future, but the second appears to change the relationship between two people. The other person now has reason to require my presence, and I cannot explain my absence merely by saying that I later changed my mind. The philosophical puzzle is how a few words can generate an obligation that did not previously exist.

Hume argued that an inward willingness to be bound cannot mysteriously manufacture a moral duty by itself; promising has to be understood through social practice and shared expectations. Later theories have proposed different foundations. Some emphasise the constitutive rules of the practice of promising. Others ground obligation in the reasonable expectation that a promise creates and the plans made in reliance on it. A third group treats promising as a normative power: a capacity to alter the rights and duties between people.

Each account faces difficulties. If obligation comes only from expectation, could a promisor weaken a promise by preventing reliance? If it comes only from a social rule, we still need to explain why that rule deserves acceptance. The normative-power view captures the special character of promising, but must still justify why we should possess such a power.

My judgement is that a promise gains force when three conditions work together: it is voluntary and clear; the recipient is entitled to arrange their affairs around it; and both parties participate in a practice in which promises can be understood and answered for. A promise is not a prediction of one's future mental state. It partly transfers authority over an action to another person. By voluntarily narrowing freedom, it makes cooperation stable across time.

https://plato.stanford.edu/entries/promises/
https://plato.stanford.edu/entries/hume-moral/