標題の術語 mood と modality はよく混同される．『新英和大辞典』によると mood は「（動詞の）法，叙法《その表す動作状態に対する話者の心的態度を示す動詞の語形変化》」，modality は「法性，法範疇《願望･命令･謙遜など種々の心的態度を一定の統語構造によって表現すること」とある．両者の区別について，Bybee (165--66) が "Mood and modality" と題する節で丁寧に解説しているので，それを引用しよう．
The working definition of mood used in the survey is that mood is a marker on the verb that signals how the speaker chooses to put the proposition into the discourse context. The main function of this definition is to distinguish mood from tense and aspect, and to group together the well-known moods, indicative, imperative, subjunctive and so on. It was intentionally formulated to be general enough to cover both markers of illocutionary force, such as imperative, and markers of the degree of commitment of the speaker to the truth of the proposition, such as dubitative. What all these markers of the mood category have in common is that they signal what the speaker is doing with the proposition, and they have the whole proposition in their scope. Included under this definition are epistemic modalities, i.e. those that signal the degree of commitment the speaker has to the truth of the proposition. These are usually said to range from certainty to probability to possibility.
Excluded, however, are the other "modalities", such as the deontic modalities of permission and obligation, because they describe certain conditions on the agent with regard to the main predication. Some of the English modal auxiliaries have both an epistemic and a deontic reading. The following two examples illustrate the deontic functions of obligation and permission respectively:
Sally must be more polite to her mother.
The students may use the library at any time.
The epistemic functions of these same auxiliaries can be seen by putting them in a sentence without an agentive subject:
It must be raining.
It may be raining.
Now the auxiliaries signal the speaker's degree of commitment to the proposition "it is raining". Along with deontic modalities, markers of ability, desire and intention are excluded from the definition of mood since they express conditions pertaining to the agent that are in effect with respect to the main predication. I will refer to obligation, permission, ability, desire and intention as "agent-oriented" modalities.
The hypothesis implicit in the working definition of mood as an inflectional category is that markers of modalities that designate conditions on the agent of the sentence will not often occur as inflections on verbs, while markers that designate the role the speaker wants the proposition to play in the discourse will often occur as inflections. This hypothesis was overwhelmingly supported by the languages in the sample. Hundreds of inflectional markers that fit the definition of mood were found to occur in the languages of the sample. In fact, such markers are the most common type of inflection on verbs. However, inflectional markers of obligation, permission, ability or intention are extremely rare in the sample, and occur only under specific conditions.
mood は主として形態論（および付随して意味論）的カテゴリーで命題志向，modality は主として意味論的カテゴリーで命題志向のこともあれば行為者志向のこともある，ととらえてよさそうだ．意味論の観点からいえば，前者は後者に包摂されることになる．Bybee (169) による要約は次の通り．
The cross-linguistic data suggest, then, the following uses of the terms modality and mood. Modality designates a conceptual domain which may take various types of linguistic expression, while mood designates the inflectional expression of a subdivision of this semantic domain. Since there is much cross-linguistic consistency concerning which modalities are expressed inflectionally, mood can refer both to the form of expression, and to a conceptual domain.
・ Bybee, Joan. Morphology: A Study of the Relation between Meaning and Form. John Benjamins, Amsterdam/Philadelphia, 1985.
「#4564. 法助動詞の根源的意味と認識的意味」 ([2021-10-25-1]) に引き続き，法助動詞の2つの用法について．may を例に取ると，「?してもよい」という許可を表わす根源的意味と，「?かもしれない」という可能性を表わす認識的意味がある．この2つの意味はなぜ同居しているのだろうか．
この問題については英語学でも様々な分析や提案がなされてきたが，Sweetser の認知意味論に基づく解説が注目される．それによると，may の根源的意味は「社会物理世界において潜在的な障害がない」ということである．例えば John may go. 「ジョンはいってもよい」は，根源的に「ジョンが行くことを阻む潜在的な障害がない」を意味する．状況が異なれば障害が生じてくるかもしれないが，現状ではそのような障害がない，ということだ．
一方，may の認識的意味は「認識世界において潜在的な障害がない」ということである．例えば，John may be there. 「ジョンはそこにいるかもしれない」は，認識的な観点から「ジョンがそこにいるという推論を阻む潜在的な障害がない」を意味する．現在の手持ちの前提知識に基づけば，そのように推論することができる，ということだ．
つまり，社会物理世界における「潜在的な障害がない」が，認識世界の推論における「潜在的な障害がない」にマッピングされているというのだ．両者は，以下のような同一のイメージ・スキーマに基づいて解釈することができる．今のところ潜在的な障害がなく左から右に通り抜けられるというイメージだ．
┌───┐
│■■■│
│■■■│
├───┤
│／／／│
│／／／│
●─→………………………→
│／／／│
└───┘
1. In both the sociophysical and the epistemic worlds, nothing prevents the occurrence of whatever is modally marked with may; the chain of events is not obstructed.
2. In both the sociophysical and the epistemic worlds, there is some background understanding that if things were different, something could obstruct the chain of events. For example, permission or other sociophysical conditions could change; and added premises might make the reasoner reach a different conclusion.
この分析は must や can など，根源的意味と認識的意味をもつ他の法助動詞にも適用できる．must についての Sweetser (64) の解説により，理解を補完されたい．
. . . I propose that the root-modal meanings can be extended metaphorically from the "real" (sociophysical) world to the epistemic world. In the real world, the must in a sentence such as "John must go to all the department parties" is taken as indicating a real-world force imposed by the speaker (and/or by some other agent) which compels the subject of the sentence (or someone else) to do the action (or bring about its doing) expressed in the sentence. In the epistemic world the same sentence could be read as meaning "I must conclude that it is John's habit to go to the department parties (because I see his name on the sign-up sheet every time, and he's always out on those nights)." Here must is taken as indicating an epistemic force applied by some body of premises (the only thing that can apply epistemic force), which compels the speaker (or people in general) to reach the conclusion embodied in the sentence. This epistemic force is the counterpart, in the epistemic domain, of a forceful obligation in the sociophysical domain. The polysemy between root and epistemic senses is thus seen (as suggested above) as the conventionalization, for this group of lexical items, of a a metaphorical mapping between domains.
・ Sweetser, E. From Etymology to Pragmatics. Cambridge: CUP, 1990.
must, may, can などの法助動詞 (auxiliary_verb) には，根源的意味 (root sense) と認識的意味 (epistemic sense) があるといわれる．通時的にも認知的にも前者から後者が派生されるのが常であることから，前者が根源的 (root) と称されるが，機能に注目すれば義務的意味 (deontic sense) を担っているといってよい．
must を例に取れば，例文 (1) の「?しなければならない」が根源的（あるいは義務的）意味であり，例文 (2) の「?にちがいない」が認識的意味ということになる (Sweetser 49) ．
(1) John must be home by ten; Mother won't let him stay out any later.
(2) John must be home already; I see his coat.
一般的にいえば，根源的意味は現実世界の義務，許可，能力を表わし，認識的意味は推論における必然性，蓋然性，可能性を表わす．Sweetser (51) の別の用語でいえば，それぞれ "sociophysical domain" と "epistemic domain" に関係する．一見して関係しているとは思えない2つの領域・世界への言及が，同一の法助動詞によってなされているのは興味深い．
実際，英語に限らず多くの言語において，根源的意味と認識的意味の両方をもつ語が観察される．印欧語族はもちろん，セム，フィリピン，ドラヴィダ，マヤ，フィン・ウゴールの諸語でも似た現象が確認される．しかも，歴史的に前者から後者が派生されるという一方向性 (unidirectionality) の強い傾向がみられる（cf. 「#1980. 主観化」 ([2014-09-28-1])) ．
とすると，これは偶然ではなく，何らかの動機づけがあるということになろう．認知言語学では，"sociophysical domain" と "epistemic domain" の間のメタファーとして説明しようとする．現実世界の因果関係を認識世界の因果関係にマッピングした結果，根源的意味から認識的意味が派生するのだ，という説明である．
・ Sweetser, E. From Etymology to Pragmatics. Cambridge: CUP, 1990.
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