Does "ought" imply "can"? Philosophers have argued about this for centuries, because while people usually do what they have to do, sometimes they cannot.
For example, when a person makes a promise to do something, she has an obligation to do it. If someone has an obligation to do something, then she can do it. Yet sometimes people make promises and can't keep them.
This results in an inconsistency. The first two sentences entail that whenever a person makes a promise to do something, then she can do it. Yet if she can do it, then that people make promises, and can't always keep them, must be wrong.
This problem is hard to solve because it is part of the meaning of a promise that it creates an obligation to do what is promised. "If someone has an obligation to do something, then she can do it," is the thesis that "ought" implies "can." Yet you cannot require a person to do what she cannot do. For example, pretend Betty borrows 10 dollars from Carol on Monday for lunch. Her parents being rich, they promise to give her 50 dollars on Tuesday for living expenses. However, on Monday night, the parents are robbed of all their money and cannot send the money to her on Tuesday. Ergo, Betty has an obligation to pay Carol 10 dollars, but she cannot pay.
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