言語変化論の入門書として定評のある Language Change: Progress or Decay を著わした Aitchison は，いったい言語変化は進歩なのか，あるいは堕落なのかという問題について様々な角度から議論している．その最終章で，著者は進歩でも堕落でもないと結論づける．その理由は，進歩なり堕落なりという概念には「目指すべき完璧なゴール」の存在が前提とされるが，言語においてそれが何なのかについて一致した見解がないからだ．つまり，「完璧な言語」というものが規定できさえすれば，現実の言語変化がそれに向かっているならば「進歩」であり，逆であれば「堕落」であると述べることもできるだろう．しかし「完璧な言語」がいかなるものかについての共通の了解がない限り，進歩とも堕落とも判断できないはずだ．
　それでも仮に「完璧な言語」を，最大限に単純で効率的にコミュニケーションをこなせる言語と考えることは可能かもしれない．だが，その場合にも大きな問題が立ちはだかる．まず，何をもって言語の単純さ（あるいは複雑さ）を計測するのかという問題がある（cf. 「#293. 言語の難易度は測れるか」 ([2010-02-14-1]) や「#1839. 言語の単純化とは何か」 ([2014-05-10-1])，「#2820. 言語の難しさについて」 ([2017-01-15-1])）．同じく，効率的なコミュニケーションといっても計測が難しい．
　直接 Aitchison (237--38) の議論に耳を傾けよう．この辺りの事情が上手に表現されている．

The term 'progress' implies a movement towards some desired endpoint. What could this be, in terms of linguistic excellence? A number of linguists are in no doubt. They endorse the view of Jespersen, who maintained that 'that language ranks highest which goes farthest in the art of accomplishing much with little means, or, in other words, which is able to express the greatest amount of meaning with the simplest mechanism'.
   If this criterion were taken seriously, we would be obliged to rank pidgins as the most advanced languages. As we have already noted, true simplicity seems to be counterbalanced by ambiguity and cumbersomeness. Darwin's confident belief in the 'inherent virtue' of shorter and easier forms must be set beside the realization that such forms often result in confusing homonyms, as in the Tok Pisin hat for 'hot', 'hard', 'hat' and 'heart'.
   A straightforward simplicity measure then will not necessarily pinpoint the 'best' language. A considerable number of other factors must be taken into account, and it is not yet clear which they are, and how they should be assessed. In brief, linguists have been unable to decide on any clear measure of excellence, even though the majority are of the opinion that a language with numerous irregularities should be less highly ranked than one which is economical and transparent. However, preliminary attempts to rank languages in this way have run into a further problem.
   A language which is simple and regular in one respect is likely to be complex and confusing in others. There seems to be a trading relationship between the different parts of the grammar which we do not fully understand. This has come out clearly in the work of one researcher who compared the progress of Turkish and Serbo-Croatian children as they acquired their respective languages. Turkish children find it exceptionally easy to learn the inflections of their language, which are remarkably straightforward, and they master the entire system by the age of two. But the youngsters struggle with relative clauses (the equivalent of English clauses beginning with who, which, that) until around the age of five. Serbo-Croatian children, on the other hand, have great problems with the inflectional system of their language, which is 'a classic Indo-European synthetic muddle', and they are not competent at manipulating it until around the age of five. Yet they have no problems with Serbo-Croatian relative clauses, which they can normally cope with by the age of two.
   Overall, we cannot yet specify satisfactorily just what we mean by a 'perfect' language, except in a very broad sense. The most we can do is to note that a certain part of one language may be simpler and therefore perhaps 'better' than that of another.

　言語は進歩もしていなければ，堕落もしていない．ただただ変化しているのだ．関連して「#1382. 「言語変化はただ変化である」」 ([2013-02-07-1])，「#2525. 「言語は変化する，ただそれだけ」」 ([2016-03-26-1])，「#2544. 言語変化に対する三つの考え方 (3)」 ([2016-04-14-1])，「#3354. 「言語変化はオフィスの整理である」」 ([2018-07-03-1]) の記事も参照．

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

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