The principle “ought implies can” asserts, roughly, that an agent ought to do something only if it is possible for that agent to do it. In other words, being possible for an agent to perform a certain action is a necessary condition of that agent’s having a moral obligation to perform that action. In this way, the principle “ought implies can” is fruitfully understood not as a moral principle itself, but as a principle to which ethical theories must adhere. That is, for ethical theory or moral principle to be justifiable for a candidate, it must not violate the principle that ought implies can; it must not require that agents perform actions they simply cannot perform. In this way, the principle itself recognizes that ethics ...