#3848. ランキングの理論 "Optimality Theory"[ot][generative_grammar][phonology][phonetics][history_of_linguistics][fetishism]


 生成文法 (generative_grammar) において,複数の規則が前提とされており,それら効果がバッティングする場合,「どちらの規則が優勢か」「どちらが先に適用されるのか」という問題が生じることは不可避である.順序のランキングという発想が出ると間もなく「最適解」を求める "Optimality Theory" (最適性理論)が生じたのは必然といってよいかもしれない.かりそめにも何らかの規則を設定する分野であれば,言語にかぎらずランキングという発想が持ち込まれるのは自然のように思われる.私自身,言語に関心はあるとはいえ,決して理論派言語学者ではないが,ランキングという発想には共感を覚える.
 Crystal (342--43) によると,最適性理論 (= Optimality Theory = ot) とは次の通り.

optimality theory (OT) In phonology a theory developed in the early 1990s concerning the relationship between proposed underlying and output representations. In this approach, an input representation is associated with a class of candidate output representations, and various kinds of filter are used to evaluate these outputs and select the one which is 'optimal' (i.e. most well-formed). The selection takes place through the use of a set of well-formedness constraints, ranked in a hierarchy of relevance on a language-particular basis, so that a lower-ranked constraint may be violated in order to satisfy a higher-ranked one. The candidate representation which best satisfies the ranked constraint hierarchy is the output form. For example, in English the negative prefix in- (e.g. insufficient) has two output forms, im- before bilabials (as in impossible, immodest), and in- elsewhere (inarticulate, involuntary, etc.). The coexistence of these forms means that there is conflict between the class of faithfulness constraints (which require identity between input and output) and the class of constraints which impose restrictions on possible sequences have identical place of articulation --- which needs to be resolved by an appropriate ranking of the relevant constraints. Optimality theory thus aims to account for a wide range of phenomena by specifying the interaction of a small number of universal constraints, which apply variously across languages in producing phonological representations. A particular constraint may achieve high ranking in one language (i.e. its output accounts for many surface forms) and low ranking in another (i.e. its output accounts for only a small class of forms). Although initially developed in relation to phonology, during the later 1990s optimality theory came to be extended to morphology and syntax.

 OT は主として音韻論に強みを発揮する理論ではあるが,規則の適用順序に関する一般理論であると認めるならば,規則というものが想定され得るその他のいかなる分野(形態論にせよ統語論せよ)においても適用される幅広い原理と考えられる.
 各言語がそれぞれ特有のフェチ (fetishism) をもっているという言語観とも親和性のある言語理論だといえる.

 ・ Crystal, David, ed. A Dictionary of Linguistics and Phonetics. 6th ed. Malden, MA: Blackwell, 2008. 295--96.

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