(A disorganized, but I hope interesting, discussion on the faculty of promise. Something that I am immensely interested in.)
Nietzsche called promising “the memory of the will”; Hume equated the faculty of promise with transubstantiation. Promise is how consciousness gains a foothold in reality, how the uncertainty of the future may be collapsed into the certainty of the present.
If the future is completely uncertain, if we don’t know what is going to happen the next day, then we won’t be able to rest. We would be constantly panicking, afraid that this is going to happen, that something else won’t happen. As an organism directed towards our own survival, we need stability, and some measured, controlled chaos/uncertainty from time to time as we explore new territory—in fact, one can almost give a transcendental explanation for the existence of laws of nature (different from Kant’s derivation): that laws of nature must exist because thinking creatures like us have evolved.
This is similar to Schrödinger’s argument in What is Life, that genes must be governed by quantum processes or else mutations would go out of control due to thermodynamic disruptions and we won’t be here. But he expands on this further in his argument that we are negative entropy machines, i.e., we create islands of stability in an ultimately chaotic universe. We don’t consume energy when we eat, but we consume stability. When we excrete and when we lose heat we, along this logic, loses entropy (and thus increases stability). Isn’t the faculty of promise similar to this? Promise is just a negative entropy mechanism that we use to keep ourselves, and society, sane!
Psychologically speaking, promise has the exact same function: it tames my constantly changing self to a single course, so I have a stable identity to rely on. (It is significant here that Rawls identifies promise as an entering into a social relation and binding oneself to certain rules of the game, since we only understand ourselves, we are able to be a self, an “I”, only in virtue of being with society. In Hegelian jargon, self-consciousness understands itself in being-for-other and being-for-self, where the self is just an other that we are contending against.)
This is why, I guess, it is a good thing to keep one’s promise. Just so one does not descend into the hell of entropy (whose endpoint is death).