Why can の and が both mark subjects in relative clauses?


> 夢 **の** ある人

and

> 夢 **が** ある人

I understand that both have obviously the same meaning, but why is it also
correct to use の, which, as I learned, has the function of either a possessive
particle or of a nominaliser. I heard this example in the first few seconds of
following video (so you get the context):

[ゴルゴ松本「少年院で漢字を使った魂の授業」](https://www.youtube.com/watch?v=M_pF4a_TxmU&list=LL1HAVqMMAdDKm0hSGsnLd-w&index=14
"ゴルゴまつもと「しょうねんいん　で　かんじ　を　つかった　たましい　の　じゅぎょう」")

EDIT: As I read in the linked post in the comment now, I saw following: "In
more precise terms: の can act like a subject (nominative) particle in
descriptive (attirbutive/relative) clauses. " – user54609

To further my question now, is there historic context to this rule? How come の
can take this role as well?




This is actually an interesting little story.

Old Japanese, as far as we can tell, didn't have a dedicated subject marker -
if you wanted a subject that wasn't the topic also, you just left it unmarked.
It had two genitive particles, though, *nə and *ŋga (modern の and が); which
varied according to a kind of animacy hierarchy - *ŋga with personal pronouns
and names IIRC (I've forgotten some of the details), *nə with the rest.

However, like several nearby languages that have this kind of relativisation-
through-adnominalisation strategy (where relative clauses look like main
clauses with different verbal morphology), the subjects of relative clauses
could be marked as genitive. (Korean and Turkic both do/did this IIRC,
Mongolic and/or Tungusic might.) This seems to be a reflection of the fact
that noun clauses and verb clauses share some interesting structural
similarities, especially crossover between subjects and possessors - compare
English 'I don't like his having done that'.

Over time (starting around the Old > Middle Japanese transition IIRC), people
reanalysed subject genitives inside relative clauses as just subject markers,
and started using them within main clauses as well. The Ryuukyuuan languages
have mostly stopped here, and maintain the dual use of reflexes of *nə and
*ŋga in both subject and possessor functions. Mainland Japanese further
reduced was now が to only marking subjects, and restricted what was now の to
only marking possessors.

This case is the exception to that split. It's not clear whether it's a simple
retention from earlier times or instead a further example of subjects and
possessors patterning alike, and it's probably both.

(Yaeyama IIRC is an example of a Ryuukyuuan language that doesn't have an
animacy hierarchy in subject markers, but it doesn't mark subject or object at
all and leaves role marking to word order.)




This is actually an interesting little story.

Old Japanese, as far as we can tell, didn't have a dedicated subject marker -
if you wanted a subject that wasn't the topic also, you just left it unmarked.
It had two genitive particles, though, *nə and *ŋga (modern の and が); which
varied according to a kind of animacy hierarchy - *ŋga with personal pronouns
and names IIRC (I've forgotten some of the details), *nə with the rest.

However, like several nearby languages that have this kind of relativisation-
through-adnominalisation strategy (where relative clauses look like main
clauses with different verbal morphology), the subjects of relative clauses
could be marked as genitive. (Korean and Turkic both do/did this IIRC,
Mongolic and/or Tungusic might.) This seems to be a reflection of the fact
that noun clauses and verb clauses share some interesting structural
similarities, especially crossover between subjects and possessors - compare
English 'I don't like his having done that'.

Over time (starting around the Old > Middle Japanese transition IIRC), people
reanalysed subject genitives inside relative clauses as just subject markers,
and started using them within main clauses as well. The Ryuukyuuan languages
have mostly stopped here, and maintain the dual use of reflexes of *nə and
*ŋga in both subject and possessor functions. Mainland Japanese further
reduced was now が to only marking subjects, and restricted what was now の to
only marking possessors.

This case is the exception to that split. It's not clear whether it's a simple
retention from earlier times or instead a further example of subjects and
possessors patterning alike, and it's probably both.

(Yaeyama IIRC is an example of a Ryuukyuuan language that doesn't have an
animacy hierarchy in subject markers, but it doesn't mark subject or object at
all and leaves role marking to word order.)

