Kluge: The Haphazard Construction of the Human Mind (Houghton Mifflin; 2008)

Take, for example, this seemingly benign sentence: Put the block in the box on the table. An ordinary sentence, but it can actually mean two things: a request to put a particular block that happens to be in a box onto the table, or a request to take some block and put it into a particular box that happens to be on the table. Add another clause, and we wind up with four possibilities:

Put the block [(in the box on the table) in the kitchen].

Put the block [in the box (on the table in the kitchen)].

Put [the block (in the box) on the table] in the kitchen.

Put (the block in the box) (on the table in the kitchen).

Most of the time, our brain shields us from the complexity, automatically doing its best to reason its way through the possibilities. If we hear Put the block in the box on the table, and there's just one block, we don't even notice the fact that the sentence could have meant something else. Language alone doesn't tell us that, but we are clever enough to connect what we hear with what it might mean. (Speakers also use a range of "paralinguistic" techniques, like point-

times happens when we plan to stop at the grocery store after work (and instead "autopilot" our way home, sans groceries). In a computer, both types of problems — tracking goals and tracking trees — are typically solved by using a "stack," in which recent elements temporarily take priority over stored ones; but when it comes to humans, our lack of postal-code memory leads to problems in both cases.

As it happens, there are actually two separate types of recursion, one that requires stacks and one that doesn't. It is precisely the ones that do require stacks that tie us in knots.

* According to legend, the first machine translation program was given the sentence "The flesh is weak, but the spirit is willing." The translation (into Russian) was then translated back into English, yielding, "The meat is spoiled, but the vodka is good."

ing and gesturing, to supplement language; they can also look to their listeners to see if they appear to understand.)

But such tricks can take us only so far. When we are stuck with inadequate clues, communication becomes harder, one reason that emails and phone calls are more prone to misunderstandings than face-to-face communication is. And even when we speak directly to an audience, if we use ambiguous sentences, people may just not notice; they may think they've understood even when they haven't really. One eye-opening study recently asked college students to read aloud a series of grammatically ambiguous* sentences like Angela shot the man with the gun (in which the gun might have been either Angela's murder weapon or a firearm the victim happened to be carrying). They were warned in advance that the sentences were ambiguous and permitted to use as much stress (emphasis) on individual words as they liked; the question was whether they could tell when they successfully put their meaning across. It turns out that most speakers were lousy at the task and had no clue about how bad they were. In almost half the cases in which subjects thought that they had successfully conveyed a given sentence's meaning, they were actually misunderstood by their listeners.! (The listeners weren't much better, frequently assuming they'd understood when they hadn't.)