Does the いは in 或いは come from any term, and if so, how is it/was it used?


I was curious about いは in 或いは because ある is in its plain form however I can't
find anything on it. I don't have any physical dictionary, either nor
knowledge of if this is archaic Japanese




Surprisingly, [this does seem to
contain](https://kotobank.jp/word/%E6%88%96%E3%81%AF-428581) the 連体形 of ある,
followed by an "adverbial particle" (副助詞) い which the etymology doesn't
define, and the regular topic particle は.

The etymology explains that the original sense is ある人は ("a certain
person"),ある場合は ("in a certain situation"), which is reflected in the first
definition:

> 同類の事柄を列挙していろいろな場合のあることを表す  
>  Demonstrates the fact that there exists a number of situations, by
> enumerating a sequence of related possibilities

For this first definition, the dictionary gives the example:

> 或いは歌をうたい、或いは笛を吹く  
>  Either singing a song, or playing a flute

So together with the explanation ある場合は "In a certain situation", you can see
that the original definition would have meant something like "In one case,
singing a song; in another case, playing the flute".

This first definition was generalised to the more abstract notion of "X or Y".




Surprisingly, [this does seem to
contain](https://kotobank.jp/word/%E6%88%96%E3%81%AF-428581) the 連体形 of ある,
followed by an "adverbial particle" (副助詞) い which the etymology doesn't
define, and the regular topic particle は.

The etymology explains that the original sense is ある人は ("a certain
person"),ある場合は ("in a certain situation"), which is reflected in the first
definition:

> 同類の事柄を列挙していろいろな場合のあることを表す  
>  Demonstrates the fact that there exists a number of situations, by
> enumerating a sequence of related possibilities

For this first definition, the dictionary gives the example:

> 或いは歌をうたい、或いは笛を吹く  
>  Either singing a song, or playing a flute

So together with the explanation ある場合は "In a certain situation", you can see
that the original definition would have meant something like "In one case,
singing a song; in another case, playing the flute".

This first definition was generalised to the more abstract notion of "X or Y".

