What is the difference between a “singular noun” and a “plural noun treated as singular”?

Question

I'd always thought that words like "physics" and "mathematics" were singular: after all, we say "physics is the study of…" etc. But apparently, according to the comments on this question about "news", each of these words is actually a "plural noun [usually treated as singular]".

What makes these words plural? Is it the fact that they end in s? (Surely "bus" is not plural?) Is it history? (Were they used as plural at some time? How far back in history does one go to decide whether a noun is singular or plural?) Is it the fact that they don't have any distinct forms treated as plural?

Most importantly, why do we even have a grammatical category of "plural nouns treated as singular"? What purpose does it serve, and how are such nouns functionally distinguishable from nouns that are actually singular? When/if the reason is history, is there a rationale for saying "plural nouns treated as singular" rather than "singular nouns that were formerly plural"?

Edit: My question isn't just about "-ics" words, but all words in the category "plural nouns treated as singular" (assuming that the category isn't just -ics words).

Answer 1

Etymonline has this to say:

-ics
in the names of sciences or disciplines (acoustics, aerobics, economics, etc.) it represents a 16c. revival of the classical custom of using the neuter plural of adjectives with -ikos (see -ic) to mean "matters relevant to" and also as the titles of treatises about them. Subject matters that acquired their names in English before c.1500, however, tend to remain in singular (e.g. arithmetic, logic).

So yes, at some point in history, there were such things as physic (meaning "natural science"), mathematic (meaning "mathematical science"), etc. that were later turned into plural forms but kept being treated as singular.

Edit: having looked in a few more places, it appears that in contemporary English, it still makes some sense to have both the suffix -ic and its plural form -ics. According to the Collins English Dictionary, the former has kind of specialized in forming adjectives, while the latter is happily forming nouns:

-ic
suffix forming adjectives

1. of, relating to, or resembling: allergic, Germanic, periodic. See also -ical.
   [...]

[from Latin -icus or Greek -ikos; -ic also occurs in nouns that represent a substantive use of adjectives (magic) and in nouns borrowed directly from Latin or Greek (critic, music)]

[...]

-ics
suffix forming nouns (functioning as singular)

1. indicating a science, art, or matters relating to a particular subject: aeronautics, politics
2. indicating certain activities or practices: acrobatics

[plural of -ic, representing Latin -ica, from Greek -ika, as in mathēmatika mathematics]

The key here is that they are not just two unrelated suffixes. Much rather, one is etymologically a plural form of the other. As the American Heritage Dictionary succinctly puts it, -ics is "-ic + -s".

Answer 2

I still haven't seen any examples that aren't -ics words (besides 'news'), but let me post a tentative answer for the -ics case: I remembered something I had quoted in the inning/innings singular/plural question, and looked up Fowler. Under -ics, the book says:

-ics. 1. Among the names of sciences, arts, or branches of study, are a few words in -ic that rank as real English; the chief are logic, magic, music, & rhetoric; but the normal form is -ics, as in acoustics, classics, dynamics, ethics, mathematics, physics, politics, tactics. The substitution of -ic for -ics …

2. Grammatical number of -ics. This is not so simple a matter as it is sometimes thought. The natural tendency is to start with a fallacy: We say Mathematics is (& not are) a science; therefore mathematics is singular. But the number of is there is at least influenced, if not (whether legitimately or otherwise) determined, by that of a science. The testing should be done with sentences in which there is not a noun complement to confuse the issue:—Classics are, or is, now taking a back seat ; Conics is, or are, easier than I expected ; What are, or is, his mathematics like? ; Politics are, or is, most fascinating ; Your heroics are, or is, wasted on me ; Athletics are, or is, rampant in the big schools ; Tactics are, or is, subordinate to strategy.   The rules that emerge are:  (1) Singular for the name of a science strictly so used; Metaphysics, or Acoustics, deals with abstractions, or sound.  (2) Plural for those same names more loosely used, e.g. for a manifestation of qualities; often recognizable by the presence of his, the, &c.:  His mathematics are weak ; Such ethics are abominable ; The acoustics of the hall are faulty.  (3) Plural for names denoting courses of action or the like : Heroics are out of place ; Hysterics leave me cold.  (4) The presence of a singular noun complement often makes the verb singular: Mathematics, or even Athletics, is his strong point.

So that long discussion very well illustrates the circumstances in which -ics words are sometimes used as singular and sometimes plural (though I wouldn't say "His mathematics are weak", for some reason). Note that it seems to be discussing when a certain -ics word is singular and when it's plural, not saying that it's always plural and discussing when it's treated one way or the other.

Anyway, along with RegDwight's useful answer, my hypothesis for my original question is this: if a singular word X exists (e.g. mathematic), and the word Y is formed from X by the normal rules of English pluralisation, then word Y is called "plural", even if it has never been used in the plural.

Of course, all this still doesn't answer the latter question of what purpose this category serves etc.