 For four main reasons. First, Reid claims that “the greater part of mankind can never attain such extensive views of human life, and so correct a judgment of good and ill, as the right application of this principle requires” (EAP III.iii.iv: 164). Reid’s point here is that a principle of action should be action-guiding. It should be the sort of thing that, in a wide range of cases, an agent could consult when determining what to do and thereby come to understand what she morally ought to do. The principles of morality are action-guiding. “Every man of common understanding,” says Reid, is such that he is capable of knowing his duty (EAP V.i: 277). But gaining a conception of one’s good on the whole, let alone an accurate one, and an understanding of what genuinely contributes to it, is something that is very difficult to do. It requires that one “observe the connections of things, and the consequences of our actions,” thereby “taking an extended view of our existence, past, present, and future” (EAP III.iii.i: 153). Many ordinary persons will have neither the time nor the ability to do this, let alone actually gain an accurate notion of that in which their good on the whole consists. If this is right, however, then one’s good on the whole is not sufficiently action-guiding to be the most general and fundamental principle of action, as eudaimonists claim.