方言 (dialect) をどうみるかという問題は，（社会）言語学の古典的なテーマである．この問題には，言語と方言の区別という側面もあれば，方言どうしの区分という側面もある（cf. 「#4031. 「言語か方言か」の記事セット」 ([2020-05-10-1])，「#1501. 方言連続体か方言地域か」 ([2013-06-06-1])）．そもそも方言とは言語学的にいかに定義できるのか，という本質的な問いもある．
英語史研究では中英語方言 (me_dialect) が広く深く調査されてきたが，現在この分野の権威の1人といってよい Williamson (481) が，（中英語）方言とは何かという核心的な問題に言及している．議論の出発点となりそうな洞察に富む指摘が多いので，いくつか引用しておこう．
The most significant and paradoxical finding of dialect geography has been the non-existence of dialects, the objects which it purports to study. A "dialect" is a construct: a reification of some assemblage of linguistic features, defined according to criteria established by the dialectologist. These criteria may be linguistic, extra-linguistic, or some combination of these two kinds.
A (Middle English) dialect can be considered to be some assemblage of diatopically coherent linguistic features which co-occur over all or part of their geographical distributions and so delineate an area within the dialect continuum. Adding or taking away a feature from the assemblage is likely to alter the shape of this area: addition might reduce the area's size, subtraction, increase it. Any text which contains that assemblage of features can be considered as having its provenance within that area.
Linguistic variation across space exists because languages undergo change as a natural consequence of use and transmission from generation to generation of speakers. A linguistic change takes place in a community and subsequently becomes disseminated through time and across space as the next generation of speakers and neighboring communities adopt the change. A "dialect continuum" emerges from the language contacts of speakers in neighboring communities as they share some features, but not others. A continuum is thus made up of a set of overlapping distributions of linguistic features with varied geographical extents --- some extensive, some local. A continuum is not static and is constantly shifting more or less rapidly through time.
特に2つめと3つめの引用は，方言連続体 (dialect_continuum) の精確な定義となっている．この方言観は「#1502. 波状理論ならぬ種子拡散理論」 ([2013-06-07-1]) とも通じるだろう．
・ Williamson, Keith. "Middle English: Dialects." Chapter 31 of English Historical Linguistics: An International Handbook. 2 vols. Ed. Alexander Bergs and Laurel J. Brinton. Berlin: Mouton de Gruyter, 2012. 480--505.
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