せず - What does it come from?


> 気にするまいと思っても、気に **せず** にはいられない。

In above sentence, I guess `せず` means `しない`, but what is the original form,
and how does it become `せず`?




The `-ず` form of a verb means "without doing" and is roughly equivalent to
`ないで`. It's not a contraction of anything and is a verb form on its own.

The `-ず` form is formed by taking the `-ない` form of a verb and replacing the
`-ない` with `-ず`. For the irregular verbs, `する` becomes `せず` and `[来]{く}る`
becomes `[来]{こ}ず`.




> what is the original form

The verb する.

> and how does it become せず?

ず is an auxiliary verb ([助動詞]{じょどうし}) which attaches to the [未然形]{みぜんけい} of
the verb. [せ is one 未然形 of
する](https://ja.wikipedia.org/wiki/%E3%82%B5%E8%A1%8C%E5%A4%89%E6%A0%BC%E6%B4%BB%E7%94%A8#.E3.81.99.E3.82.8B.EF.BC.88.E5.8F.A3.E8.AA.9E.EF.BC.89)
and is the one which ず attaches to.

