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.
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