Stylistic Variance Does Not Establish Multiple Authors

[The argument goes that because there are marked contrasts in style within the book of Isaiah, there must be multiple authors who each contributed different sections of the book.] ...such differences as there are may be easily accounted for by the change in situation which confronted Isaiah in his later years, and also by the maturing of his literary genius. Numerous parallels to this may be pointed out in the history of world literature. Thus in the case of John Milton, we find far more striking disssimilarities between <i>Paradise Lost</i>, which he composed in later years, and the style of <i>L'Allegro</i> or <i>Il Penseroso</i>, which appeared in his earlier period. A similar contrast is observable between his prose works such as <i>Christian Doctrine</i> and <i>Aeropagitica</i>. Or, to take an example from German literature, Goethe's <i>Faust Part II</i> presents striking contrasts in concept, style, and approach as over against <i>Faust Part I</i>. These contrasts are far more obvious than those between Isaiah I and Isaiah II. In his <i>Dictionary of the Bible</i> (p. 339a), Davis points out that in the twenty-five years of Shakespeare's activity, four distinct periods can be distinguished in his dramatic productions, each period being marked by clear differences in style.

source: Gleason Archer, A Survey of Old Testament Introduction: Revised and Expanded, p 381 tags: Apologetics, Bible, Isaiah