Self-study
Ethicist: The general principle—if one ought to do something then one can do it—does not always hold true. This may be seen by considering an example. Suppose someone promises to meet a friend at a certain time, but—because of an unforeseen traffic jam—it is impossible to do so.
14.
Which one of the following is an assumption required by the ethicist's argument?
a
If a person failed to do something she or he ought to have done, then that person failed to do something that she or he promised to do.
This says: if you failed to do something you ought to have done, then you failed to do something you promised to do. In other words, every obligation comes from a promise. The ethicist doesn't need to believe that, because he could acknowledge obligations can come from other places besides promises. Maybe other obligations come from laws, or morals, or job duties. That wouldn't hurt the argument, as long as promises are one thing that create obligations.
If you liked this answer, you probably thought it meant “If NOT something you ought to do → you did NOT promise to do it” — but it doesn’t actually mean that. (A) essentially reverses the conditional link we want, which is to get from promise to ought.
b
Only an event like an unforeseen traffic jam could excuse a person from the obligation to keep a promise.
The traffic jam is just one example. The ethicist doesn't need it to be the only kind of event that could prevent someone from keeping a promise. Maybe a medical emergency or a natural disaster could also make it impossible. The argument would work just as well with any of those examples. So the assumption that only something like a traffic jam could excuse someone is far stronger than anything the argument requires.
c
If there is something that a person ought not do, then it is something that that person is capable of not doing.
This answer is about "ought not do" (obligations to avoid doing something), but the argument is entirely about "ought to do" (obligations to do something). The ethicist is trying to show that sometimes you ought to do something but can't. Whether or not the same issue arises for things you ought not to do is a separate question that doesn't affect this argument.
d
The obligation created by a promise is not relieved by the fact that the promise cannot be kept.
This must be assumed in order for the argument to preserve the possibility of a bridge from "promised" to "ought." The ethicist's example gives us someone who promised to meet a friend but can't. For this to work as a counterexample to the principle, it also has to be a case where the person ought to meet the friend but can't. (D) says the obligation from a promise isn't relieved just because the promise can't be kept. That's what connects "promised" to "ought" in this specific scenario: even though it's impossible to meet the friend, the person still ought to, because they promised.
Without this assumption, the example is irrelevant. If the obligation is relieved when the promise can't be kept, then the person stuck in traffic no longer "ought to" meet their friend. And if there's no "ought," there's no counterexample to the principle. The ethicist would have shown only that someone promised something impossible, not that someone ought to do something impossible.
e
If an event like an unforeseen traffic jam interferes with someone's keeping a promise, then that person should not have made the promise to begin with.
Whether the person should have made the promise in the first place is beside the point. The argument is about what happens after the promise has been made and can't be kept. Even if it were true that the person shouldn't have promised, that wouldn't affect whether the promise, once made, still creates an obligation.