Consider some of the words that are listed in the OED (under synonym) as examples of as-near-as-it-comes synonymy. The first are serpent and snake. Are these words true synonyms? In terms of pure definition, yes—sort of. Serpent is defined in the OED as "any of the scaly limbless reptiles regarded as having the properties of hissing and 'stinging'; Zool. a reptile of the group Ophidia; a snake." Snake is defined as "one or other of the limbless vertebrates constituting the reptilian order Ophidia (characterized by a greatly elongated body, tapering tail, and smooth scaly integument), some species of which are noted for their venomous properties; an ophidian; a serpent." The difference at this level is minuscule: one definition is more comprehensive, mentioning the vertebrated skeleton and the scaly integument; the other is somewhat more colorful, accentuating the hissing noise that the beast is able to emit.
Were this all, one might agree that the two words are perfect synonyms. But the OED, complete as always, continues its definition of serpent, observing that nowadays, in ordinary use, the word is "applied chiefly to the larger and more venomous species; otherwise only rhetorical ... or with reference to serpent-worship."
Therein is the suggestion of reptilian synonymy blown suddenly asunder. For serpent is indeed the word we choose when we want to denote a snake that is bigger and nastier than most, and snake is the word we choose to describe any smooth and elongated creature that skitters from beneath the lawnmower blades. We say, on the one hand, "There is a snake in the basement" and, on the other, that missionaries were once thrown into "pits filled with serpents." But we don't proclaim there to be a serpent in the garden shed, and if we told a listener that a missionary was in a pit full of snakes, we would realize from the resulting questions—Were they big? Were they venomous?—that we had used the word wrongly, poorly, or incautiously. (This leads to another supposed synonymy: venom and poison. The words, however, are not exactly synonymous, because one can speak with venom yet perhaps not quite with poison. Venom is both a substance and a tone; poison is more a matter of chemistry.)
Examination of any words thought to be synonymous reveals a congruency of range but not an identical meaning. Take some other illustrative related examples from the OED: ship, vessel; compassion, fellow-feeling, sympathy; enormous, excessive, immense; glad, happy, joyful, joyous; kill, slay, slaughter; grieve, mourn, lament, sorrow. Some are very close indeed; there is little to distinguish a ship from a vessel, except that one doesn't say fishing ship or war vessel—suggesting that a vessel is likely to be engaged in peaceful activities, whereas a ship can have a more menacing role. One cannot quite imagine Nelson's having spoken of vessels on the horizon off Cape Trafalgar, or any dockside idler's speaking of the handsome lines of the ship that has just brought lobsters back from the Outer Banks. (In truth, he would probably say boat.) Others on the list are more obviously distinguishable. Sometimes the distinction is a matter of degree: one kills a man; one slays his child; one slaughters the villagers who sheltered the family. On other occasions the context suggests one choice rather than another: one feels compassion for the villagers in such circumstances, but fellow-feeling for the brother of the first who had to die.
The practices surrounding synonymy, which seem to have come into being at around the time Roget was beginning to codify the language (after Samuel Johnson had created his dictionary but before the members of the Philological Society of London had commenced work on the all-encompassing OED), are really quite simple. Currently, the high priest of this field is Ladislav Zgusta, a scholar of Czech extraction (reinforcing the notion that English is often more scrupulously regarded by those who come from less than pure English stock; James Murray, of the OED, was proudly a Scot, and Roget came from a Swiss Huguenot family). To be absolute synonyms, Zgusta says, words must be the same in three distinct ways: They must have the same designatum—that is, the same essential qualities. They must have the same connotation—the same associated features of meaning. And they must occupy the same range of usage and application—the contexts in which they are generally used must be identical.
Very few words satisfy this formidable set of criteria. Most of those that do are technical. In his standard work on lexicography, Dictionaries (1984), the near-legendary authority Sidney Landau offered as an example ten absolute synonyms, agreeing in designatum, connotation, and range, for the mad-cow-disease-related human ailment that is currently terrifying England and threatening France—Creutzfeldt-Jakob disease, widely known as CJD. The variants that Landau listed include Jakob-Creutzfeldt disease, spongiform encephalopathy, and Jones-Nevin syndrome. (The disease afflicting England, with ninety-one known cases and perhaps thousands more expected, is in fact now known, confusingly for my purposes here, as New-Variant CJD—and is different from Landau's plain-vanilla CJD. The new version varies more in terms of pathology than of lexical standing.)
The synonyms listed by the OED fall short of Zgusta's standards in a variety of ways. Serpent and snake have the same designatum—each is a long, slithery, cold-blooded, and scaly reptile. And they have more or less the same connotation. But they have very different ranges of use, as I have shown. So with only two of the three criteria satisfied, serpent and snake are not absolute synonyms: they are simply near synonyms, not to be substituted for each other without care and attention to the context.
Much the same can be said for beast and brute, or mirth and cheerfulness. The words in each pair share a designatum but not a connotation or range of possible use. A brute is invariably untamed; a beast often is, but he need not be. We can say "a beast of burden" or "a beast of the field"; we would never say "a brute of burden" and probably would not say "a brute of the field"—though we might allow "brute of the forest," since the word here reinforces an image in which all is mystery and secret danger.
An awareness of the nuances of synonymy, then, is fundamental to the speaking and writing of good English. Merely cataloguing synonyms, uncritically offering up lists of alternative words from which a speaker or writer may choose, makes for something less happy. Roget realized this back at the beginning of the nineteenth century, when the literate world was awash in dictionaries, thesauri, lexicons, and other guides for the betterment of verbal display. His goal was to make sure that what was written and spoken and read was impeccable. To this end he began a study of the language with the primary aim of classifying it and then distilling from that classification a guide to how it might best be made to work.
Peter Mark Roget, born in London on January 18, 1779, was in myriad ways a most extraordinary man. I said earlier that he was a polymath, and in the literal sense of the word—"a person of much or varied learning"—he truly was. Although the term is still widely used today, few men or women in Western societies are genuinely polymathic. In Britain the physician, philosopher, playwright, librettist, and writer Jonathan Miller springs to mind as a possibility; in America, sad to say, despite some astonishingly bright people, no one fits the bill.