Among their limitations, possibly the most
manifest failing of AI systems is with regard to language: quite apart from the
already formidable problem of segmenting continuous speech and turning it into
words and phrases to be analyzed by computer, the most challenging problem is
to make the machine understand what is being
said. There is an often quoted example where a machine translates "The
spirit is willing but the flesh is weak" into the Russian equivalent of
"The vodka is strong but the meat is rotten". This example, dating
back to the 1960's or 70's, is probably a myth invented by journalists[1],
but the criticism unfortunately still applies to today's natural language
processing technology, as you can confirm easily by trying any web-based
translation tool.
But still, the very fact that such tools
are nowadays readily available on the web is indication that some kind of progress is gradually being made.
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The problem is much harder than you think:
You are generally not aware of the complex processing your brain does to
understand language. In particular, you are not aware that virtually every word
you hear in a sentence has multiple meanings, and that the words can often be
grouped together in different ways. The reason you don't generally notice this
is because when you hear a sentence your brain uses implicit knowledge about
the world in order to make sense of the sentence, and so you generally only
become conscious of one reasonable interpretation out of many possible ones.
Take for example the phrase[2]
"Time flies like an arrow". You understand this to mean that time
moves quickly just like an arrow does. But you may never have realized that it
can in fact also be interpreted to mean:
- you should
measure the speed of flying insects like you would measure the speed of an
arrow;
- you should
measure the speed of flying insects in the way an arrow would time them;
- you should
measure the speed of flying insects that are like arrows - i.e. You should time
those flies that are like arrows;
- Certain flying
insects, namely "time-flies," enjoy arrows (as in "Fruit flies
like a banana").
Certain jokes are funny because the normal
process of contextual disambiguation fails and gives rise to comical
interpretations. In the following newspaper headings[3]
there are at least two parsings, one of which is humoristic:
- Kids Make
Nutritious Snacks
- Enraged Cow
Injures Farmer with Ax
- Queen Mary
Gets Bottom Scraped
In addition to lexical and syntactical
ambiguity, another problem arises because language is essentially a
communicative act, based on the assumption of common knowledge between the
speakers, and on the speakers' presuppositions concerning what the other
speaker knows. Reference to previously mentioned material through the use of
pronouns is something which rarely poses problems to humans, but is exceedingly
difficult for computers.
- We gave the
monkeys the bananas because they were hungry
- We gave the
monkeys the bananas because they were over-ripe
have the same grammatical structure on the
surface. However, in one of them the word "they" refers to the
monkeys, in the other it refers to the bananas. A computer can only figure this
out if it knows something about monkeys and bananas.
A similar thing happens in understanding
the sentences:
- John saw the
bird flying over the mountain
- John saw the
stewardess flying over the mountain
Here, in the first example, the bird was
flying, whereas in the second, John was doing the flying (in an airplane).
Metaphor and analogy are also difficult
things for computers, which have problems dealing with anything more than the
literal meaning of phrases like:
- The early bird
gets the worm
where for true understanding, reference to
shared knowledge about birds and worms and the natural human condition of
laziness is necessary. How could a robot ever hope to understand what's funny
about the following sentences unless it knows about the human condition?
- There are 3
kinds of people: those who can count and those who can't.
- All I ask is a
chance to prove money can't make me happy.
And what about:
- The early bird
may get the worm, but the second mouse gets the cheese.
Here reference to shared knowledge is not
just about the world, but about things that are often said.