--- Leo Strauss, "The Three Waves of Modernity," Political Philosophy: Six Essays, ed. Hilail Gildin, Pegasus-Bobbs-Merrill, 1975, p. 81-82
Excerpt from https://contemporarythinkers.org/leo-strauss/essay/the-three-waves-of-modernity/:
The crisis of modernity reveals itself in the fact, or consists in the fact, that modern western man no longer knows what he wants–that he no longer believes that he can know what is good and bad, what is right and wrong. Until a few generations ago, it was generally taken for granted that man can know what is right and wrong, what is the just or the good or the best order of society–in a word that political philosophy is possible and necessary. In our time this faith has lost its power. According to the pre-dominant view, political philosophy is impossible: it was a dream, perhaps a noble dream, but at any rate a dream. While there is broad agreement on this point, opinions differ as to why political philosophy was based on a fundamental error. According to a very widespread view, all knowledge which deserves the name is scientific knowledge; but scientific knowledge cannot validate value judgments; it is limited to factual judgments; yet political philosophy presupposes that value judgments can be rationally validated. According to a less widespread but more sophisticated view, the predominant separation of facts from values is not tenable.: the categories of theoretical understanding imply, somehow, principles of evaluation; but those principles of evaluation together with the categories of understanding are historically variable; they change from epoch to epoch; hence it is impossible to answer the question of right and wrong or of the best social order in a universally valid manner, in a manner valid for all historical epochs, as political philosophy requires.
