Donald Trump got the world talking about double negatives as he tried to clarify controversial remarks he made in Helsinki. But using them correctly can be a perilous task, says Alan Connor.
The US president announced that he had not meant to say that "I don't see any reason why it would be" Russia who had interfered in the 2016 US election.
"The sentence should have been", he added, "'why it wouldn't be Russia'. Sort of a double negative."
A double-negative sentence has one word which flips the meaning of the rest, and another which flips it right back again.
But it usually causes more confusion when it is used than when it isn't - and negatives can sneak in through such expressions as "fail to" or "underestimate" as well as "uns" and "nots".
It's hard to understate how often we find ourselves using two negatives when we don't mean to - in fact, this sentence begins with a common example, "it's hard to understate...".
What I meant is that it's easy to understate, or that it's hard to overstate: but we all make many mistakes like this, often without our noticing, and often without the people we're talking to noticing.
You may well have taken away the thought I wanted to express from "hard to understate", even though I didn't express it.
What's going on?
One explanation is that we think up two ways of saying that something isn't the case - "I will no longer be able to support the prime minister", say, and "I will be unable to support the prime minister" - and mash them into something like "I will no longer be unable to support the prime minister".
Put more crudely, our primitive brains aren't very good at applying clinical logic to our words, which means we often say the exact opposite of what we wanted to.
In this case, the person speaking is so keen to withdraw their support for the prime minister that he or she withdraws the withdrawal. Even so, the prime minister knows there's trouble ahead.
And it's even harder to notice when mistakes are made because sometimes two negatives absolutely do not make a positive.
No-one hearing the words "I can't get no satisfaction" is left in any doubt as to how satisfied Mick Jagger is feeling; likewise "Ain't no mountain high enough".
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If you stubbornly apply the logic of two-negatives-cancel-each-other here, you're missing the point.
Thoughtful writers might misuse negatives to be funny (Raymond Chandler's "a wallet that was not quite as big as a bale of hay") or pile them up through exuberance (Shakespeare's "I never was nor never will be false.")
In politics, though, we tend to look more closely at the words used.
In 1975, American Secretary of State Henry Kissinger said about Saudi Arabia: "I am not saying that there's no circumstances where we would not use force".
Diplomats might have been tempted to count and untangle the negatives here, but a more practical approach is to listen to what Kissinger said before and after the confusing line - he explains why he might want to deploy troops.
If you're working out whether there's going to be a war, the "logical" meaning of that sentence is not the most important thing.
What, then, of Donald Trump's "I don't see any reason why it would(n't) be"?
As so often, things are not quite as they appear. Trump insists that here, he actually intended a double negative and - a different kind of beast to "easy to understate" and the Kissinger remark - a double negative that would have meant exactly what he wanted it to.
So a charitable explanation might be that Mr Trump thought up an answer with a double negative, remembered at the last moment that double negatives sometimes go wrong, and promptly removed one of the negatives, accidentally ending up with a statement that sounded anti-American.
Unpacking the logic in this way makes the brain ache.
Happily, it's often not the best way of working out what what you want to know - what someone was actually thinking.
Kissinger's thoughts on force are clear from the rest of the interview. Mick Jagger's on satisfaction are clear from the rest of the song.
And the same is probably true about Donald Trump, Russian interference, and the rest of the press conference.
Logic is beside the point.