言語変化は言語内的・外的を含めた諸要因の組み合わせによって生じるという "multiple causation" の原則について，以下の記事を含めた各所で主張してきた．

　・ 「#443. 言語内的な要因と言語外的な要因はどちらが重要か？」 ([2010-07-14-1])
　・ 「#1232. 言語変化は雨漏りである」 ([2012-09-10-1])
　・ 「#1233. 言語変化は風に倒される木である」 ([2012-09-11-1])
　・ 「#1582. 言語内的な要因と言語外的な要因はどちらが重要か？ (2)」 ([2013-08-26-1])
　・ 「#1584. 言語内的な要因と言語外的な要因はどちらが重要か？ (3)」 ([2013-08-28-1])
　・ 「#1977. 言語変化における言語接触の重要性 (1)」 ([2014-09-25-1])
　・ 「#1978. 言語変化における言語接触の重要性 (2)」 ([2014-09-26-1])
　・ 「#1986. 言語変化の multiple causation あるいは "synergy"」 ([2014-10-04-1])
　・ 「#3152. 言語変化の "multiple causation"」 ([2017-12-13-1])
　・ 「#3271. 言語変化の multiple causation 再考」 ([2018-04-11-1])

　言語変化論を著わした Aitchison も同じ立場であり，上の記事でも関連箇所を引用・参照してきたが，もう1つ Aitchison (202) よりエッセンスともいうべき文章を引きたい．

   Change is likely to be triggered by social factors, such as fashion, foreign influence and social need. However, these factors cannot take effect unless the language is 'ready' for a particular change. They simply make use of inherent tendencies which reside in the physical and mental make-up of human beings. Causality needs therefore to be explored on a number of different levels. The immediate trigger must be looked at alongside the underlying propensities of the language concerned, and of human languages in general.
   A language never allows disruptive changes to destroy the system. In response to disruptions, therapeutic changes are likely to intervene and restore the broken patterns --- though in certain circumstances therapeutic changes can themselves cause further disruptions by setting off a change of changes which may last for centuries.
   Above all, anyone who attempts to study the causes of language change must be aware of the multiplicity of factors involved. It is essential to realize that language is both a social and a mental phenomenon in which sociolinguistic and psycholinguistic factors are likely to be inextricably entwined. 'Nothing is simple' might be a useful motto for historical linguists to hang in their studies . . . .

　引用最後にあるように，言語とは社会言語学的および心理言語学的（＝主流派の理論言語学と解してよい）な要因が密接に絡み合った社会的かつ心理的な現象である．言語学は，両方に目を配らなければならない．

　・ Aitchison, Jean. Language Change: Progress or Decay. 4th ed. Cambridge: CUP, 2013.

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