Cupit makes a brilliant point about why it is wrong to break a promise.
When you promise, my trust in you is morally required of me. If I fail to trust, if you are at all trustworthy, I do you an injustice. This is unlike a mere statement of intent you might make.
So trust is essentially coerced, extracted from me by your right, I have no choice in this matter. But in that case, I’m at your mercy, and if you renege on your promise, you are responsible for my false belief.
And what happens when you renege is you make me into a dupe, a fool, an idiot for trusting you. This degrades my status and hence constitutes wrongful treatment.
It’s interesting that Cupit argues that this wrongful treatment is not unjust. I think he’s right, and to get an injustice we need to upgrade a mere promise into a full-blown contract.
A contract is an explicit invitation to capitalistic social cooperation. It is a sacrament of a meeting of the minds. So for two people to make a contract is to acknowledge each other as human with the relevant libertarian natural rights.
And to break the contract later is to violate those rights, thereby treat the other party as less than human. And that, according to justice as fittingness, is unjust.