Mind Matters
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Diners Club Party. Ambiguity Tests Matter

As we've seen before with things that we do unconsciously, the impression of simplicity that we get from the effortlessness we experience can be deceptive. The complexity of what actually goes on often becomes apparent when we attempt to make these inner workings explicit as precise instructions to a computer. Take something like: The man in the hat on the hill was happy. Are we talking about a man who's wearing a hat, being on a hill? Or a man being in a hat that a hill is wearing? If it just seems . . . "obvious," how do you explain why to a computer?  Simply telling it that hills don't wear hats doesn't work, because having dutifully noted that fact it would be none the wiser when confronted by a man (or hill) wearing a coat; yet surely we don't operate by having a separate rule to cover every possible eventuality. We're up against world knowledge again, and that's exactly where computers start to flounder.

The difficulty isn't so much that so many words have multiple definitions, as can be seen in any dictionary, and happily take on roles as noun, verb, adjective or whatever else suits the speaker's fancy. For example, "log" can mean a piece of tree, a device for measuring a ship's speed, a type of record, to cut down or harvest trees, to rock to and fro, an abbreviation for logarithm, or a Hebrew measure of liquids; "turn" in my dictionary has 32 senses listed. If it were just a question of keeping track of the temporary ambiguities of such phrases as the Chinese puzzle solver or a talk about books by Mr. Rider until later information  ruled out the incorrect possibilities, a computer would have no problem coping--although the programming would be a lot more complex. They perform perfectly well with convoluted but unambiguous constructions in artificial languages. However, when precision is relaxed to a degree typical of natural language, the results can be unresolvable by any rules. Very often, they're equally ambiguous to people:

Her son has grown another foot.
I shot the woman with the Polaroid

Humans, however, zoom in so quickly on the meaning that makes sense in the given context that they're rarely aware even that any alternative was possible. This ability to home in on the obvious sometimes blinds us to meanings perfectly legitimate within the grammar of the language, which the computer, meticulously applying its rules in the way it was told to, doesn't see anything "obviously" odd about at all. A classical example is from a parser written at Harvard in the 1960s, which was presented with the sentence Time flies like an arrow. Just five words, their one meaning pretty plain, yes? But to the surprise of the programmers, the computer found five!

Time proceeds speedily, in the way that an arrow proceeds.
Measure the speed of flies in the way you measure the speed of an arrow.
Measure the speed of flies in the way an arrow measures the speed of flies.
Measure the speed of flies that are like an arrow.
Time flies--a kind of fly--are fond of an arrow.

So how do humans jump straight to the intended meanings without bogging down over the impossibly implausible (interestingly reminiscent of the way our visual systems don't waste time trying to construct logically consistent but physically impossible objects)? One possibility is that our brains do in fact work in something like the way computers do, but unconsciously--i.e. storing large numbers of possible meanings all awaiting corroborating evidence and delivering only the final, confirmed one to consciousness. The other is that we make some kind of snap judgment based on experience, and just go with it until we're either vindicated or run into a wall. If these strategies have a vaguely familiar ring about them, they should. They're called breadth-first and depth-first searches.

For individual-word ambiguities, it seems we use something close to breadth-first. It is an experimentally established fact that immediately after we hear a word, words closely related to it are identified more quickly than neutral ones. If sky is flashed on a screen, for example, words like blue and cloud evoke faster responses than, say, peg. It's as if the mental dictionary were stored in the form of a network in which related meanings are clustered together, and activating a particular item leaves that area "primed" to expect further accesses. When a word has, say, two possible meanings, one of which makes no sense at all in the context being dealt with, both possibilities are nevertheless unconsciously recognized, and memory is primed ready to deliver close associations of both of them. Take the sentence He found several spiders, roaches, and other bugs in a corner of the room.  As would be expected from the above, a person listening to this retrieves words like "ant" and "insect" more rapidly immediately after hearing bug. It also turns out that words like "spy" are primed too, which are related to "bug" used in the sense of a surveillance device but which have no connection with the above context. This effect last only for milliseconds and leaves no awareness that it was ever there, but has been confirmed from psychological experiments. What this gives us is a word lookup is that is fast but not very selective, producing a lot of nonsense candidates that are then rapidly erased.

At the more complex level of phrases and sentences, however, we don't compute every possible branch of the pa sing tree. A clue to this is the quite sensible ambiguities that we simply don't see because we've already rejected them as irrelevant. Newspaper headlines provide a rich source for collectors:


And bureaucratese:

The report gives totals of the number of students broken down by sex, marital status, and age.

In fact, it sometimes happens that so insistent are we to see meaning that makes sense, we fail to find the only tree that is consistent with the grammatical structure.

The man who hunts ducks out on weekends.

Our first tendency is to blink, back up and look again, and then dismiss it as an error. But in fact it is perfectly grammatical, as:

The man who fishes is available all week. The man who hunts ducks out at weekends.

What happened the first time was that following a depth-first approach our parser took duck to be (most plausibly) the object of hunts and plunged on with that assumption, only to come to a perplexed halt later when the pieces wouldn't fit. Another example:

Glass windows are made of has blemishes.

makes perfect sense when seen in:

The glass that crystal goblets are made of is perfect, but glass windows are made of has blemishes.

What we see and hear is determined by our expectations as shaped by a lifetime of living in the world and getting to know how the parts of it typically relate to each other. One wonders what kind of an edifice the Blocks World stacking program would attempt to build, given the statements:

The ball is on the table.
John is on the ball.
The boss is on John's back.
The detective is on the boss's tail.
The commissioner is on the detective's side.

All in all, comprehending language comes through as a daunting prospect. As with anything else, you have to start somewhere.

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