As … as

as ... as , so ... as . The STANDARD DICTIONARY says: A shade of difference in their meanings, as strictly used in comparisons, is often neglected. So ... as suggests that, in the comparison of the persons or things mentioned, there is present in the mind of the speaker a consciousness of a considerable degree of the quality considered; as ... as does not carry this impression. In “John is not as tall as James” there is no implication that the speaker regards either John or James as tall; there is merely a comparison of their heights. So, too, in “John is not as old as James” there is merely a comparison of ages. But if one says, “John is not so tall as James,” though the so is not emphasized, there is understood usually to be a reference more or less distinct to something uncommon in the height of James as compared with the stature of other men or of other boys of his age; the speaker regards James as being tall. “John is not so old as James” suggests that, in some relation or other, James is thought of as being old ; as in “James is taller than John.” “Yes, but my boy is not so old as yours.”

In affirmative sentences so ... as can not properly be used except in certain restricted constructions, and where the quality referred to is to be emphasized. It occurs oftenest in sentences that, though affirmative in form, carry a negative suggestion; as, “So good a cook as Polly is hard to find,” that is, “It is not easy to find so good a cook as Polly.”

    Few knights of the shire [in the 17th century] had libraries so      good as may now perpetually be found in a servants' hall.

    MACAULAY, History, ch. 3.

That is, “not many knights of the shire,” etc. In a simple affirmative comparison like “Jane is as good a cook as Polly,” so ... as is not used.

In interrogative sentences, as in negative sentences, a consciousness more or less distinct of a considerable degree of the quality referred to is conveyed by so ... as, but not by as ... as. “Is John as old as James?” and “Is your uncle so old as my father?” convey different impressions as to what the speaker means by old. In the question where as ... as is used there is no implication of considerable age in old.