Edward Sapir (18841939). Language: An Introduction to the Study of Speech. 1921.
We may venture to surmise that while whom will ultimately disappear from English speech, locutions of the type Whom did you see? will be obsolete when phrases like The man whom I referred to are still in lingering use. It is impossible to be certain, however, for we can never tell if we have isolated all the determinants of a drift. In our particular case we have ignored what may well prove to be a controlling factor in the history of who and whom in the relative sense. This is the unconscious desire to leave these words to their interrogative function and to concentrate on that or mere word order as expressions of the relative (e. g., The man that I referred to or The man I referred to). This drift, which does not directly concern the use of whom as such (merely of whom as a form of who), may have made the relative who obsolete before the other factors affecting relative whom have run their course. A consideration like this is instructive because it indicates that knowledge of the general drift of a language is insufficient to enable us to see clearly what the drift is heading for. We need to know something of the relative potencies and speeds of the components of the drift.
It is hardly necessary to say that the particular drifts involved in the use of whom are of interest to us not for their own sake but as symptoms of larger tendencies at work in the language. At least three drifts of major importance are discernible. Each of these has operated for centuries, each is at work in other parts of our linguistic mechanism, each is almost certain to continue for centuries, possibly millennia. The first is the familiar tendency to level the distinction between the subjective and the objective, itself but a late chapter in the steady reduction of the old Indo-European system of syntactic cases. This system, which is at present best