particles - Etymology of もん・もの

How do I make sense of the 終助詞 もん as in

おいしいもん
そうなんだもん

Although I have only heard it in 時代劇 speak, I guess it comes from もの, which I think should be も + の.

But what も can follow the 終止形 and what の can follow that も. Is も the same も that roughly means "also"? Is の the nominalizer の here?

P.S. I am not asking what it means, when it is used, etc. I want to know how to make sense of it grammatically. At the moment, it has also been suggested that もん・もの derived from 物, but it seems to follow different rules than what one would expect of a particle derived from a noun.

Answer

If I ignore semantics and discuss syntax only, I think that it went like this:

1. もの was originally 物, a lexical noun (実質名詞), which could be modified by a relative clause ending in 用言の連体形.


2. It was then grammaticalized into a formal noun (形式名詞), losing its literal meaning but still appearing in the same syntactic position, after a relative clause ending in 用言の連体形.


3. After the morphophonological merger of 連体形 and 終止形, it was possible to reanalyze 連体形＋もの as 終止形＋もの because the two looked and sounded the same. This in turn made it possible to reanalyze もの as a 終助詞.


4. Once it was possible for もの to be treated as a 終助詞, it became possible for it to appear after だ.

So now もの (or its contracted form もん) can appear as a 形式名詞 or a 終助詞. And of course the original word 物 is still around.