Learn English – the difference between a “singular noun” and a “plural noun treated as singular”

grammatical-numbernouns

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 the comments on this question about "news", each of these words is actually a "plural noun [usually treated as singular]":

(edited) screenshot from comments on question 4146 about news
(highlighting/ellipsis added by me)
(NOAD = New Oxford American Dictionary)

One, Is this categorization valid? That is, is it correct to say, as the NOAD does, that these words are indeed plural (but treated as singular), rather than to say that they are singular? The accepted answer on that question, by user RegDwigнt, has a different analysis that speaks in terms of “news” being "uncountable", "used with singular verbs", and "etymologically, it used to be a plural form", and thereby carefully avoids addressing this issue, of whether these nouns are indeed plural-treated-as-singular. So my question remains.

Two, If so, 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).

Edit 2: The image and part "One" of the question were added later; previously I took for granted that the categorization was valid.

Best Answer

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".