Troubles With Negatives
From Handling Of Words part of The Complete Plain Words by Sir Gowers(1954)

(1) Double Negatives . It has long been settled doctrine among English grammarians that two negatives cancel each other and produce an affirmative. As in mathematics - (- x) equals + x, so in language "he did not say nothing" must be regarded as equivalent to "he said something".

It is going too far to say, as is sometimes said, that this proposition is self-evident. The Greeks did not think that two negatives made an affirmative. On the contrary, the more negatives they put into a sentence the more emphatically negative the sentence became. Nor did Chaucer think so, for, in a much-quoted passage, he wrote:

He never yit no vileineye ne sayde
In al his lyf; unto no maner wight
He was a verray parfit gentil knyght.

Nor did Shakespeare, who made King Claudius say:

Nor what he said, though it lacked form a little,
Was not like madness.

Nor do the many thousands of people who find it natural today to deny knowledge by saying "I don't know nothing at all about it". And the comedian who sings "I ain't going to give nobody none of mine" is not misunderstood.

Such repeated negatives, says Jespersen, are usual in a great many languages in which the negative element is comparatively small in phonetic bulk, and is easily attracted to various words. If the negation were expressed once only, it might easily be overlooked; hence the speaker, who wants the negative sense to be fully appreciated, attaches it not only to the verb, but also to other parts of the sentence: he spreads, as it were, a thin layer of negative colouring over the whole of the sentence instead of confining it to one single place. This may be called pleonastic, but is certainly not really illogical.

Still, the grammarians' rule should be observed in English today. Breaches of it are commonest with verbs of surprise or speculation ("I shouldn't wonder if there wasn't a storm." "I shouldn't be surprised if he didn't come today"). Indeed this is so common that it is classed by Fowler among his "sturdy indefensibles". A recent speech in the House of Lords affords a typical instance of the confusion of thought bred by double negatives:

Let it not be supposed because we are building for the future rather than the present that the Bill's proposals are not devoid of significance.

What the speaker meant, of course, was

"Let it not be supposed that the bill's proposals are devoid of significance".

Another example is:

There is no reason to doubt that what he says in his statement... is not true.

Here the speaker meant, "There is no reason to doubt that his statement is true".

And another:

It must not be assumed that there are no circumstances in which a profit might not be made.

Avoid multiple negatives when you can. Even if you dodge the traps they set and succeed in saying what you mean, you give your reader a puzzle to solve in sorting the negatives out. Indeed it is wise never to make a statement negatively if it could be made positively. A correspondent sends me

The elementary ideas of the calculus are not beyond the capacity of more than 40 per cent of our certificate students,

and comments

"I am quite unable to say whether this assertion is that two-fifths or three-fifths of the class could make something of the ideas".

If the writer had said that the ideas were within the capacity of at least sixty per cent, all would have been clear. Here are two more examples of sentences that have to be unravelled before they yield any meaning:

Few would now contend that too many checks cannot be at least as harmful to democracy as too few.
The Opposition refused leave for the withdrawal of a motion to annul an Order revoking the embargo on the importation of cut glass.

(2) Neither . . . Nor. Some books tell you that neither . . . nor should not be used where the alternatives are more than two. But if you decide to ignore this advice as pedantry you will find on your side not only the translators of the Bible,

Neither death, nor life, nor angels, nor principalities, nor powers, nor things present, nor things to come, nor height, nor depth, nor any other creature, shall be able to separate us from the love of God,

but also, though not quite so profusely, Sir Harold Nicolson,

Neither Lord Davidson nor Sir Bernard Paget nor Mr. Arthur Bryant will suffer permanently from the spectacle which they have provided.

(3) Nor and Or. When should nor be used and when or? If a neither or an either comes first there is no difficulty; neither is always followed by nor and either by or. There can be no doubt that it is wrong to write

"The existing position satisfies neither the psychologist, the judge, or the public".

It should have been

"neither the psychologist, nor the judge, nor the public".

But when the initial negative is a simple not or no, it is often a puzzling question whether nor or or should follow. Logically it depends on whether the sentence is so framed that the initial negative runs on into the second part of it or is exhausted in the first; practically it may be of little importance which answer you give, for the meaning will be clear.

He did not think that the Bill would be introduced this month, nor indeed before the recess.

"He did not think" affects everything that follows that. Logically therefore nor produces a double negative, as though one were to say "he didn't think it wouldn't be introduced before the recess".

The blame for this disorder does not rest with Parliament, or with the bishops, or with the parish priests. Our real weakness is the failure of the ordinary man.

Here the negative phrase "does not rest" is carried right through the sentence, and applies to the bishops and the parish priests as much as to Parliament. There is no need to repeat the negative, and or is logically right. But nor is so often used in such a construction that it would be pedantic to condemn it: if logical defence is needed one might say that "did he think it would be introduced" in the first example, and "does it rest" in the second were understood as repeated after nor. But if the framework of the sentence is changed to:

The blame for this disorder rests not with Parliament nor with the bishops, nor with the parish priests, but with the ordinary man,

it is a positive verb (rests) that runs through the sentence; the original negative (not) is attached not to the verb but to Parliament, and exhausts itself in exonerating Parliament. The negative must be repeated, and nor is rightly used.

(4) Not.

(a) "Not all".
It is idiomatic English, to which no exception can be taken, to write "all officials are not good draftsmen" when you mean that only some of them are. Compare "All that glitters is not gold". But it is clearer, and therefore better, to write "Not all officials are good draftsmen".

(b)"Not . . . but."
It is also idiomatic English to write "I did not go to speak but to listen". It is pedantry to insist that, because logic demands it, this ought to be "I went not to speak but to listen". But if the latter way of arranging a "not...but" sentence runs as easily and makes your meaning clearer, as it often may, it should be preferred.

(c)"Not . . .because."
Not followed by because sometimes leads to ambiguity. "I did not write that letter because of what you told me" may mean either "I refrained from writing that letter because of what you told me" or "It was not because of what you told me that I wrote that letter." Avoid this ambiguity by rewriting the sentence.