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It’s often said, and with good reason, that we are not told by the way things are how they ought to be; there’s no deriving an “ought” from an “is.” However, it is just as often said, and with just as good, or better, reason, that sometimes the way things ought to be does indeed tell us how they are; “ought” implies “can.” That a person ought to do something, that the world ought to include his doing of it, tells us something about what he’s like; it tells us what he can do, namely, the thing he ought. For simplicity, call this “the Maxim”: A person ought to perform a certain action only if he can do so. Appealing to the Maxim is not the only way some feel we can learn about how the world is from examination of how it ought to be. Many think, also, that from examination of what a person ought to do we learn something else about what he’s capable of doing; we learn, on this view, that he can do something other than what he ought. Those who accept this view accept the “Principle of Alternate Possibilities (PAP)”: A person is morally responsible for what he did only if he could have done otherwise.
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