Origin of ません (-masen)?


I understand that polite forms like yomimasu are actually (or originated as)
infinitive yomi + auxiliary verb masu in its plain non-past form. Then past
form yomimashita is just infinitive yomi + plain past mashita.

So I wonder what is the origin of the negative form, such as yomimasen. Given
the above, one might naively expect a plain non-past negative form of masu,
which would give yomimasanai. Is -masen "explainable" in terms of other forms?
Is it a contraction from an originally longer form?




According to Shogakukan's big 国{こく}語{ご}大{だい}辞{じ}典{てん}, the verb ending _-masu_
ultimately derived from a combination of humble polite auxiliary verb 参{まい}る
plus the verb する, as a shift from either _‑ mairasuru_ or possibly _‑
maisuru_. The final _‑ su_ in modern _‑ masu_ conjugates identically to
classical _su_ / _suru_. The 未然形{みぜんけい} ("incomplete form") that is used to
form negatives ends in _‑ a_ in most classical 四段{よだん} verbs (which became
modern 五段{ごだん} or "type 1" verbs), such as _kaka ‑_ as the _mizenkei_ for
書{か}く, or _ika ‑_ as the _mizenkei_ for 行{い}く, etc. However, classical _su_ /
_suru_ is irregular, and its _mizenkei_ is _se ‑_ instead. (This is still the
_mizenkei_ for _suru_ in some modern dialects, such as Kansai-ben.) So the
negative of _‑ masu_ was formed as the _mizenkei_ of _‑ mase‑_ plus the
negative ending _-nu_ , as _‑ masenu_. Final _‑ nu_ contracts to just _‑ n_,
giving us modern _‑ masen_.




According to Shogakukan's big 国{こく}語{ご}大{だい}辞{じ}典{てん}, the verb ending _-masu_
ultimately derived from a combination of humble polite auxiliary verb 参{まい}る
plus the verb する, as a shift from either _‑ mairasuru_ or possibly _‑
maisuru_. The final _‑ su_ in modern _‑ masu_ conjugates identically to
classical _su_ / _suru_. The 未然形{みぜんけい} ("incomplete form") that is used to
form negatives ends in _‑ a_ in most classical 四段{よだん} verbs (which became
modern 五段{ごだん} or "type 1" verbs), such as _kaka ‑_ as the _mizenkei_ for
書{か}く, or _ika ‑_ as the _mizenkei_ for 行{い}く, etc. However, classical _su_ /
_suru_ is irregular, and its _mizenkei_ is _se ‑_ instead. (This is still the
_mizenkei_ for _suru_ in some modern dialects, such as Kansai-ben.) So the
negative of _‑ masu_ was formed as the _mizenkei_ of _‑ mase‑_ plus the
negative ending _-nu_ , as _‑ masenu_. Final _‑ nu_ contracts to just _‑ n_,
giving us modern _‑ masen_.

