The power of this blog cannot be underestimated

On my endless commute I’ve been listening to a course on modern French history.  The professor knows his stuff, but he is not the most articulate lecturer I’ve ever heard.  Here’s an approximation of a sentence he uttered: “The importance of Charles de Gaulle, um, in post-war France, um, cannot, er, um, be underestimated.”  I swear that I knew this sentence was going to go awry even before he finished it.

Presumably the point he was trying to make was that de Gaulle was so important it would be impossible to overestimate that importance. No matter how high you made your estimate, you

This post isn’t about Charles de Gaulle

would always fall short of his real importance.  You could, I suppose, make the case that what he was trying to say that de Gaulle’s importance must not be underestimated–that is, his point was that you might be inclined to give a low estimate of his importance, but that would be a foolish mistake on your part.  In either case, the professor wasn’t saying what he wanted to say.

Here are some (of many) recent Google hits with the same problem:

“The complexity of bank reform cannot be underestimated”
“Iranian cyberthreat cannot be underestimated”
“The power of the English language cannot be underestimated” (hmm)

The great blog Language Log has more than one post about this construction (and similar ones). In this post, the author comes up with four potential explanations:

  1. Our poor monkey brains just can’t deal with complex combinations of certain logical operators;
  2. The connection between English and modal logic may involve some unexpected ambiguities;
  3. Negative concord is alive and well in English (or in UG);
  4. Odd things become idioms or at least verbal habits (“could care less”; “fail to miss”; “still unpacked”).

The author prefers the “poor monkey brains” explanation–as do I–but he feels obliged to work through the logical issues involved in explanation 2.  Here’s a taste:

Now, it’s a theorem of deontic logic that if it’s not permissible that A, then it’s obligatory that not A; or in symbols

¬PA  →  O¬A

This follows straightforwardly from the fact that PA (“A is permissible”) is defined as ¬O¬A (“not obligatory that not A”), and ¬¬O¬A becomes O¬A by cancellation of the double negative.

And since “cannot” can mean “not be permitted to”, while “must not” or “should not” can mean “be obliged not to”, it somewhat confusingly follows that “cannot” sometimes means the same thing as “must not” or “should not”.

If you say so.  Again, I think the likely explanation is that people can’t quite get the logic right when there are negatives involved, so they end up saying something that, if you work it through, means the opposite of what they intended. But as time goes by, the poor monkey brains explanation tends to give way to explanation 4–the phrase simply becomes an idiom that people don’t even try to understand.  Here’s the phrase’s Google Ngram, which shows that its use is very much a modern phenomenon:

Are we getting stupider?  Or is this just one of those things that happens in language?  I dunno, and I suppose I could care less.