THE ENGLISH LANGUAGE is endowed with many more words than the average adult will ever need. Of necessity, therefore, the elements of this massive vocabulary must struggle, must prove themselves useful, to survive. They lead a hectic but unstable life; and like plants and animals, they must contend with the rigors of competitive survivorship.

Utilitarian words, used incessantly in daily discourse, are guaranteed survival because they fulfill a vital, continuing and indispensable role. Other words, not quite as commonly employed, may languish for a while; but sooner or later their usefulness in speaking or writing has waned, and they then become archaic relics, living only in unabridged dictionaries, and some may even become extinct. Sometimes, though, by assigning new meanings to old words, and thus answering a newly arisen need to fill an erstwhile empty lexical niche, they achieve a resurrected life. And, thus, like struggling creatures in an overcrowded, intensely competitive world, words evolve by competing, by being better suited than other words or to fit newer meanings sought by users of the language.

There are, sadly, no wise arbiter-linguists guiding us as to which words should be preserved and which discarded; and further, what meanings should be consigned to each living word. There is only the remorseless marketplace of everyday rhetoric. Words live or die solely by their relative utility.

Consider two simple, unassuming, every day words: quantity and quality. These two, similar in spelling and frequently linked in conversation, are vastly different in classical meaning. Quantity (from the Latin, quantus, meaning how much, how great) is currently defined as yielding a specified amount of whatever is being discussed; it is a measure of magnitude whether in size, volume, area, length or by whatever physical attribute is under discussion; in general, quantity specifies an amount of something, small or large.

Quality (from the Latin, qualis, meaning how constituted or of what kind), on the other hand, is a word specifying the innate attributes, character, nature or distinctive features of something. It is a neutral word not offering any gradient subtext. It is, or at least it was, an adjectival noun merely announcing the existence of a quality or aspect of something under consideration without any sense of comparative rating.

The Roman emperor Marcus Aurelius (42-118 CE), in his many avuncular admonitions to imperfect humans, once declared: “Look beneath the surface: let not the several quality of a thing nor its worth escape thee.” The quality of mercy, for example, is not strained, according to Shakespeare writing in the early 17th Century. In saying so, he doesn’t presume to append any intent or meaning to the word quality other than to employ it to point to an attribute called mercy.

But somehow, over the centuries, quality has shifted to signify elegance, superior temperament, exemplary faculty, distinction, superior grace or eminence. In a word, quality — without further qualification — has come to be synonymous with high quality. Quality has thus lost its impartiality and is now used as an independent character, meaning an enviable status found in relatively few. Quality in air travel, for example, can now mean going to Paris in a first-class seat.

Quantity has also evolved into a word that now tacitly suggests great abundance, multitude or magnitude; in a word, undistinguished vastness. Where formerly it was a rhetorical way of expressing quantity — whether great or small — somehow, the word has morphed into something that hints only at largeness. “I am confronted with a quantity of chores.” This can only mean, in the current vernacular, a lot of chores. And further, using the word, quantity without further amplification, suggests that whatever is being considered is probably of limited value, pedestrian in appearance and, well, of poor quality.

Quantity and quality are useful words. But in their rhetorical trajectory they have become polarized in meaning, cascading in intent to one extreme of interpretation or another so that one, quality, signifies exceptionally uncommon attributes while the other, quantity, has come to indicate great quantity and tacitly, inferior quality.

Back in Shakespeare’s day the accepted meanings of English words had longer life spans than in today’s lexical marketplace. Most 17th Century English citizens were marginally literate and therefore relied upon the educated — largely the clergy — to define and then protect the definitions assigned to living words. Permanency in word definitions was at least generational and slow to evolve. But even then there were literary adventurers attaching new and exciting meanings to mainstream words, particularly as workplace professions and vocations developed their own inner languages, beginning as jargon or cant and then secondarily spreading into the general public discourse. Many a new meaning was born to some restricted lingo until later joining the general vernacular stream of words.

The words may be the same from generation to generation; but their meanings, as accepted by the general public, have evolved as dramatically as styles in clothing. Consider, as an example, the word cool. Grandmother would define it as meaning comfortable, free of excessive heat. Mother might define it as meaning something dignified but lacking in enthusiasm, while pleasantly conventional; but a teenager would say that it is a word to signify something that is great, exceptional, out of this world, a word that should display a handful of exclamation points after it.

Stanley M. Aronson, M.D., a weekly contributor, is dean of medicine emeritus, Brown University ( smamd@cox.net).