We often speak of language as a monolithic entity that exists separately from its speakers. And while it is true that writing does give language an existence that is partly independent of people, language is fundamentally a mental process, existing in the minds of its speakers. And as individuals vary, so does their language. Languages vary at every level. Speakers of a language vary depending on their geographical origin, class, gender, and ethnicity. Even individuals do not speak a single form of language. Everyone has multiple styles and can shift the way they express themselves. For example, you probably don't speak the same way when delivering an oral report in class as you do chatting on the phone with your friends. All this variation gives rise, over time, to changes in the whole language. No matter what variety of English you speak, it is different from the varieties spoken in Shakespeare's time. It is even different from the language spoken in the early part of the twentieth century. Language change is natural, inevitable, and unstoppable. The only languages that do not change, that show no variation, are dead languages.

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."[1] 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.

Notes:

[1] 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.