The eighteenth century term “moral science” (or “moral philosophy”—‘philosophy’ and ‘science’ are often treated as synonyms at the time) does not quite mean what we might think it means. And so when people urge on us to remember that economics was once a moral science and should be reformed back into some such science, they often reveal their ignorance about the past of economics (recall).
In writing about science morale, Condorcet, for example, understood "by this term all those sciences that have as their object either the human mind itself, or the relations of men to another.” Moral sciences were opposed to physical sciences, and distinguished by the kinds of causes to be discussed. Moral sciences dealt with moral causes; and ‘moral’ meant something like ‘social.’ For example, institutions, norms, education, language, emotions, and property-relations (etc.) were all often thought of as moral causes. (By contrast, geography, climate, mechanics, and matter-theory (etc.) were all physical causes). So, for example, in his notoriously racist essay, Hume writes,
By moral causes, I mean all circumstances, which are fitted to work on the mind as motives or reasons, and which render a peculiar set of manners habitual to us. Of this kind are, the nature of the government, the revolutions of public affairs, the plenty or penury in which the people live, the situation of the nation with regard to its neighbours, and such like circumstances. By physical causes I mean those qualities of the air and climate, which are supposed to work insensibly on the temper, by altering the tone and habit of the body, and giving a particular complexion, which, though reflection and reason may sometimes overcome it, will yet prevail among the generality of mankind, and have an influence on their manners. "Of National Character"
So, eighteenth century ‘moral’ science and twentieth century ‘social’ science are closer in outlook than is commonly thought. Even so, the two practices have different presuppositions: since Weber social science often presupposes a version of the fact-value distinction (see here; or here), whereas in moral philosophy, ‘the natural course’ or ‘nature’ is often itself normative. If acting according to nature, or properly cultivated nature, is a key criterion or means toward the practice of virtue – as it is in many traditions --, then moral science might, indeed, be a guide to the practice of virtue.
Now, Adam Smith has a traditional and demanding understanding of virtue. His most explicit definition is as follows: “virtue is excellence, something uncommonly great and beautiful, which rises far above what is vulgar and ordinary” (The Theory of Moral Sentiments 220.127.116.11,). By ‘vulgar’ Smith does not mean somebody rude in our sense, but rather something akin to our ‘run-of-the-mill’ (which can include the rude in our sense). Few people writing today would accept such a demanding standard of virtue. Smith’s political economy did not presuppose, expect or describe virtuous people in this demanding sense, although he did hope that commercial society would make it more likely that such (rare) virtue could be practiced. While Smith thought of prudence as a virtue, moral science, as practiced by, (again) say, Adam Smith, presupposed and made explicit further moral values (humanity, equity, flourishing) that became extrinsic to the practice of late nineteenth century social science. But that story is too extended for a single Impression.