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They sound alike. Are they cognate historically?

Morphologically, is よ in both cases a particle or part of the morpheme in -ましょう?

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I think ましょ might be a sound change from the other surface form of the 未然形 of ます, which is ませ. So, ませう -> ましょう. –  snailboat Dec 27 '12 at 22:36
@snailplane, oh, the long vowel. Sounds feasible, since you can trace a diphthong becoming a long vowel even in the spelling. –  katspaugh Dec 27 '12 at 22:49
Not merely feasible, but well-documented. The historical spelling of 〜ましょう was indeed 〜ませう. –  Zhen Lin Dec 28 '12 at 6:25
Zhen Lin, thank you! @snailplane, if you might post an answer, I would gladly accept it. –  katspaugh Dec 28 '12 at 10:22
(Though someone might have already said this) goo dictionary says the「う」is an auxiliary verb... ましょ+う[ませう]:[連語]《丁寧の助動詞「ます」の未然形+推量の助動詞「う」》dictionary.goo.ne.jp/leaf/jn2/208020/m0u –  Chocolate Dec 29 '12 at 21:24
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1 Answer

up vote 9 down vote accepted

The short answer to your initial question is no. The historical/etymological spelling of 〜ましょう was 〜ませう, which is the expected form of the volitional, since the irrealis (未然形) stem of 〜ます is 〜ませ. Regular sound change explains the rest: せう becomes しょう.

The same story applies to the consonant-stem (五段活用) verbs: the old spelling for 行こう was 行かう (as seen here) etc. However, something strange happened to the vowel-stem (一段活用) verbs; instead of 見む → 見う → **みゅう or せむ → せう → **しょう, these were replaced by the forms with 〜よう we know today.

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Thanks for the detailed and informed answer! –  katspaugh Dec 28 '12 at 17:25
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