This might be a better question to ask on a linguistics subreddit, but I figured I’d ask here first since it’s focused specifically on the Japanese language.
Most sources I’ve seen agree that the copula だ is probably a contraction of である, and that in the latter, で is the particle broadly meaning “by means of.” So for example, a sentence like これは本である could very literally be rendered as “Regarding this: existence occurs in the form of a book.”
However, The textbook I’m using, *Japanese: The Spoken Language* by Jorden and Noda, presents a different interpretation of で. According to them, で is actually the gerund (“て-form”) of the copula だ. So for example, that same Japanese sentence would instead be rendered as something like “Regarding this: being a book, it exists.”
I feel like Jorden’s explanation doesn’t really make sense, because である predates だ as a copula; but is there any linguistic justification for her explanation?
For reference, Jorden and Noda both have Ph.Ds. in linguistics, and JSL is the culmination of Jorden’s 40-odd years as a researcher and pedagogue. As a language textbook, it’s unique in that it doubles as a conduit for Jorden to express her ideas about Japanese linguistics, many of which she never published anywhere else. As such, I doubt this is just a hand-wavey explanation meant as a simplification for students.
by ExquisiteKeiran