"Adherents of the no "ought" from "is" view could however easily meet part of the difficulty raised by Prior's example by reformulating their own position. What they intended to claim they might and would presumably say, is that no conclusion with substantial evaluative and moral content -- can be derived from factual premises. Yet the problem would remain for them as to why now anyone would accept their claim. For they have conceded that it cannot be derived from any unrestrictedly general logical principle. Yet their claim may still have substance, but a substance that derives from a particular, and in the eighteenth century new, conception of moral rules and judgments. It may, that is, assert a principle whose validity derives not from some general logical principle, but from the meaning of the key terms employed. Suppose that during the seventeenth and eighteenth centuries the meaning and implications of the key terms used in moral utterance had changed their character; it could then turn out to be the case that what had once been valid inferences from or to some particular moral premise or conclusion would no longer be valid inferences from or to what seemed to be the same factual premise or moral conclusion. For what in some sense were the same expressions, the same sentences would now bear a different meaning. But do we in fact have any evidence for such a change in meaning? To answer this question it is helpful to consider another type of counter-example to the 'No "ought" conclusions from "is" premises' thesis. From such factual premises as 'This watch is grossly inaccurate and irregular in time-keeping' and 'This watch is too heavy to carry about comfortably', the evaluative conclusion validly follows that 'This is a bad watch'. From such factual premises as 'He gets a better yield for his crop per acre than any farmer in the district', 'He has the most effective programme of soil renewal yet known' and 'His dairy herd wins all the first prizes at the agricultural shows', the evaluative conclusion validly follows that 'He is a good farmer.'
Both of these arguments are valid because of the special character of the concepts of a watch and of a farmer. Such concepts are functional concepts; that is to say, we define both 'watch' and 'farmer' in terms of the purpose or function which a watch or a farmer are characteristically expected to serve. It follows that the concept of a watch cannot be defined independently of the concept of a good watch nor the concept of a farmer independently of that of a good farmer; and that the criterion of something's being a watch and the criterion of something's being a good watch -- and so also for 'farmer' and for all other functional concepts -- are not independent of each other. Now clearly both sets of criteria -- as is evidenced by the examples given in the last paragraph -- are factual. Hence any argument which moves from premises which assert that the appropriate criteria are satisfied to a conclusion which asserts that 'This is a good such-and-such', where 'such-and-such' picks out an item specified as a functional concept, will be a valid argument which moves from factual premises to an evaluative conclusion. Thus we may safely assert that, if some amended version of the 'No "ought" conclusion from "is" premises' principle is to hold good, it must exclude arguments involving functional concepts from its scope. But this suggests strongly that those who have insisted that all moral arguments fall within the scope of such a principle may have been doing so, because they took it for granted that non moral arguments involve functional concepts. Yet moral argumetns from within the classical, Aristotelian tradition -- whether in its Greek or its medieval versions -- involve at least one central functional concept, the concept of man understood as having an essential nature and an essential purpose or function; and it is when and only when the classical tradition in its integrity has been substantially rejected that moral arguments change their character so that they fall within the scope of some version of the 'No "ought" conclusion from "is" premises' principle. That is to say, 'man' stands to 'good man' as 'watch' stands to 'good watch' or 'farmer' to 'good farmer' within the classical tradition. Aristotle takes it as a starting-point for ethical enquiry tha tthe relationship of 'man' to 'living well' is analagous to that of 'harpist' to 'playing the harp well' (Nicomachean Ethics, 1095a 16). But the use of 'man' as a functional concept is far older than Aristotle and does not initially derive from Aristotle's metaphysical biology. It is rooted in the forms of social life to which the theorists of the classical tradition give expression. For according to that tradition to be a man is to fulfill a set of roles each of which has its own point and purpose: member of a family, citizen, soldier, philosopher, servant of God. It is only when man is thought of as an individual prior to and apart from all roles that 'man' ceases to be a functional concept."
Alasdair MacIntyre, After Virtue: A Study in Moral Theory, 2nd ed. (Notre Dame: University of Notre Dame Press, 2003), pp.57-59