If change is inevitable, that implies we must look to the way people use language now to establish our notions of correctness. The prescriptive tradition pays lip service to the inevitability of change. The standard most frequently offered is that of "present, national, and reputable use." That is, the usage of highly-regarded contemporary authors which is free of regional peculiarities. But often, the prescriptive tradition tends to treat change as bad, as evidence of corruption. It is conservative, clinging to older forms of the language well after they have died out in ordinary speech. For example, textbooks throughout the nineteenth century forced students to learn the old second person singular pronouns thou, thee, and thine, even though all but a handful of English speakers had abandoned their use over a century and a half earlier. Today, traditional grammar books continue to insist that students use whom in the appropriate place, although whom would seem to be defunct if we examine how people actually speak when they aren't consciously thinking about schoolbook rules.
 First formulated by George Cambell in his Philosophy of Rhetoric (1776). See Albert C. Baugh and Thomas Cable, A History of the English Language, 4th ed. (Englewood Cliffs, NJ, 1993), pp. 278-9.