Thomas Scanlon criticizes Hume’s theory that the obligation to do what you promise depends on a social convention. Instead, Scanlon argues, the obligation to do what you promise is derived from a series of moral principles. These principles put moral limits on what we can do to deceive others in general; promising is just a specific kind of deception.
As for where these principles come from, that falls outside of the scope of our reading. Still, I can give you a brief answer. Scanlon’s book is devoted to showing that the core of morality that is concerned with what we owe to one another can be explained like this.
An act is wrong if its performance under the circumstances would be disallowed by any set of principles for the general regulation of behaviour that no one could reasonably reject as a basis for informed, unforced, general agreement. (Scanlon 1998, 153)
That is why he keeps referring to what principles people could or could not reasonably reject in his chapter on promises. The theory, in other words, is that if we explore the idea of what people could reasonably reject carefully enough, we can discover the fundamental principles of morality.
Obviously, this is a very different approach than Hume’s. Hume’s position is that you can think as hard and as long as you like, but you won’t find the answers about what we owe to one another. Those depend either on human conventions (in the case of artificial virtues) or facts about human nature (for the natural virtues).
Really, the whole piece is an implicit criticism of Hume. Hume says that you cannot figure out why promises are obligatory without invoking conventions and Scanlon says he can, indeed, figure out why promises are obligatory without invoking conventions. So there, Hume!
But there is a more pointed shot at Hume at the outset of the chapter. Scanlon describes several cases in which one person, A, gets another person, B, to do something by misleading B into thinking that A will do something in response that B wants. Scanlon thinks A’s behavior is wrong in all of these cases, despite the fact that (a) there is no convention between A and B or (b) A does not promise B (and so A’s behavior is not contrary to the rules of a convention of promising). This leads him to conclude that the reason why breaking a promise is connected to the reasons why it is wrong to mislead others in general.
These are the cases of the spear and the boomerang (p. 297) and the farmers (p. 298). We will want to talk about them and how Hume might respond.
Scanlon proposes a series of principles: M, D, L, and F. Only F is specific to promises.
There are two things that are distinctive about it.
First, it does not depend on whether the second party has done anything in response to being led to believe that the first party will do something. That is supposed to be the lesson of the Guilty Secret story. They guy would be embarrassed if you told the story, so he asks you to promise not to do so. There is nothing he would have done differently if you had not promised.
Scanlon draws a lesson from this example about what makes promises different from other ways of getting people to have expectations about what you are going to do. The lesson is that what is unique about promises is that they provide assurance. I am not completely clear on how being assured that someone will do something differs from expecting that someone will do something, but that’s what the man says.
Second, Principle F requires the first party to do what the second party expects. The other principles allow the first party to either warn the second party that she is mistaken about what he intends to do or to compensate the second party for her losses if he does not do what he led her to expect he would do.
One point about all of these principles is that they depend on the first party creating an expectation in the second party. It is because the other guy expects I will do something that I am either obliged to do it or to give warnings or whatever. That will be important.
Finally, we get another circularity objection (307). What is it with these things?
A says “I promise to mow your lawn” to B.
12 o’clock: why is A obliged to mow B’s lawn?
3 o’clock: because B expects A to mow the lawn. (I told you that would be important)
6 o’clock: why does B expect A to mow the lawn?
9 o’clock: because A is obliged to mow the lawn.
I found Scanlon’s answer on pp. 308-309 difficult to follow. So let’s try to talk it through.