なぜ動詞の過去形を用いると，反実仮想 (counterfactual) となるのか．英語の「仮定法過去」にまつわる意味論上の問題は，古くから議論されてきた．時制 (tense) としての過去は理解しやすい．現在からみて時間的に先行する方向に隔たっている事象を記述する場合に，過去（形）を使うということだ．一方，法 (mood) の観点からは，過去（形）を用いることによって，現実と乖離している事象，いわゆる反実仮想を記述することができる，といわれる．心的態度の距離が物理的時間の距離に喩えられているわけだ．
時制としての過去ではなく反実仮想を表わす過去形について，Taylor (177--78) の主張に耳を傾けよう．
The counterfactual use of the past tense is restricted to a small number of environments---if-conditionals (1), expressions of wishes and desires (2), and suppositions and suggestions (3):
(1) If I had enough time, . . .
(2) a. I wish I knew the answer.
b. It would be nice if I knew the answer.
(3) a. Suppose we went to see him.
b. It's time we went to see him.
The past tense in these sentences denotes counterfactuality at the moment of speaking, and not at some previous point in time. (1) conveys that, at the moment of speaking, the speaker does not have enough time, the sentences in (2) convey that the speaker does not know the answer, while in (3), the proposition encoded in the past tense, if it is to become true at all, will do so after the moment of speaking, that is to say, the past tense refers to a suggested future action. There are also a number of verbs whose past tense forms, under certain circumstances and with the appropriate intonation, can convey the present-time counterfactuality of a state of affairs represented in a past tense subordinate clause . . . :
(4) a. I thought John was married (. . . but he apparently isn't).
b. I had the impression Mary knew (. . . but it seems she doesn't).
These sentences might occur in a situation in which the speaker has just received information which causes him to doubt the (present-time) factuality of the propositions "John is married", "Mary knows". That it is a present, rather than a past state of affairs, that is at issue is shown by the choice of tense in a tag question. Imagine (5) uttered in a situation in which both speaker and addressee are preparing to go to a concert:
(5) But I thought the concert began at 8, does't it?/?didn't it?
The tag doesn't it is preferential in the present tense. In (5), the speaker is questioning the apparent counterfactuality, at the time of speaking, of the proposition "The concert begins at 8".
・ Taylor, John R. Linguistic Categorization. 3rd ed. Oxford: OUP, 2003.
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