In two of my previous posts on the subject of speech, I have briefly discussed deception and gossip. Rash promises fall into neither category. Rash promises are when we promise what we cannot guarantee to we are able to deliver. Rash promises are wrong because they promise what we cannot guarantee.
There is only one thing that we can guarantee when we are speaking. We can guarantee our present intentions. Promises may appear to be about the future, but we do not have that kind of control over the future. We only have control over our present intentions. Consider an adult who promises that he will take his kids to the park later in the day. Suppose that his car breaks down. Now he cannot take anyone to the park because he has no car to drive there. Or consider another example: suppose that someone signed a contract to play piano in two weeks. Two days before the performance, she has a car accident and cannot leave the hospital for a week. She clearly cannot meet her prior promise.
In the prior examples, outside circumstances made the fulfillment of a promise impossible. But the promise itself can be impossible. Suppose that you scheduled an appointment at the doctor’s office but forgot that you had already scheduled an important meeting at work at the same time. Or suppose that you promised to steal money from the bank. In either case, you cannot fulfill the promise because you should have made the promise in the first place. One was immoral simply because of the act (stealing) while the other was immoral because of the circumstances.
There are two ways that a promise can be rash: it is either immoral or you have failed to account for circumstances. In the first case, you simply should not have made the promise in the first place. In the second, the promise is wrong because you are guaranteeing something that you have no power to guarantee. I could promise that it will rain tomorrow, but since I have no control over the weather, such a promise is rash.
What makes rash promises wrong is not that they are contrary to the true state of things. You are promising to do what you cannot do. Either because it is morally wrong or because you lack the power to do so. Such an act is not lying, nor is it a form of deception. However, it is acting contrary to truth. Since there is no reason to do so, it is wrong.
I have been speaking about promises, but this applies to any future directed statements that you are claiming to be under your power in some sense or other. The only difference between signing contracts, giving your word and promising is that you are giving some kind of sign that you are really sure about the matter.
There is some more to be said about this issue, but there is the remaining kind of speech that harms others.