Some nouns have no plural. 1st. Those which denote things measured or weighed, unless when they express varieties, as, sugar, sugars; wheat, wheats; oil, oils; wine, wines. Here, not numbers of individuals, but different species or classes, are signified. In this sense the nouns are used plurally.
2d. Names of abstract, and also of moral qualities, as, hardness, softness, prudence, envy, benevolence, have no plural. It is to be observed, however, that several nouns of this class ending in y, when they do not express the quality or property in the abstract, but either its varieties or its manifestations, are used plurally. Thus we say, levities, affinities, gravities, &c. There may be different degrees and different exhibitions of the quality, but not a plurality.
Where displays of the mental quality are to be expressed, it is better in all cases to employ a periphrasis. Thus, instead of using with Hume (vol. vii. p. 411) the plural insolences, the expression acts of insolence, would be preferable.
Some of those words which have no singular termination are names of sciences, as, mathematics, metaphysics, politics, ethics, pneumatics, &c.
Of these, the term ethics is, I believe, considered as either singular or plural.
Mathematics is generally construed as plural; sometimes, however, we find it as singular. “It is a great pity,” says Locke, (vol. iii. p. 427, 8vo. 1794,) “Aristotle had not understood mathematics, as well as Mr. Newton, and made use of it in natural philosophy.”
“But when mathematics,” says Mr. Harris, “instead of being applied to this excellent purpose, are used not to exemplify logic, but to supply its place, no wonder if logic pass into contempt.”
Bacon improperly uses the word as singular and plural in the same sentence. “If a child,” says he, “be bird-witted, that is, hath not the faculty of attention, the mathematics giveth a remedy thereunto; for in them, if the wit be caught away but a moment, one is new to begin.” He likewise frequently gives to some names of sciences a singular termination; and Beattie, with a few others, have, in some instances, followed his example.
“Thus far we have argued for the sake of argument, and opposed metaphysic to metaphysic.”—Essay on Truth.