a famous example sentence in Natural Language Processing.

does it mean, time flies (funny critters, probably out of Star Trek or something) like an arrow? (why they would like an arrow is beyond me, but sure...)

or does it mean, time (you know, as in space-time) flies like an arrow? (recall back to English class - yes, I know you slept through most of it.. anyway, this is a simile)

or even more outrageously, does it mean, time (as a verb; you know, like "time how long it take me to type 'supercalafragilisticexpealidocios' on this Twiddler") flies like an arrow? (time the flies, as if they were an arrow. a totally ludicrous interpretation, but the computer doesn't know that..)

well anyway, you can't know, out of context, what the sentence is supposed to mean, hence the trouble for a Natural Language Processing program

The canonical test sentence for machine translation software. However, don't bother: even the crappiest MT program will almost certainly have a smart response hard-coded. It contains several varieties of structural and lexical ambiguity which make mechanical parsing difficult, but any English speaker, or at least any English speaker who has not spent too much time programming in Prolog, will instinctively understand it - in the absence of any particular contextual information - as a simplistic simile about the passage of time; a prime example, therefore of the NLP problems with real-world knowledge.

When a Harvard natural language parser was given this phrase in the 1960s it identified the following five parse trees in reponse.
  1. Time proceeds as quickly as an arrow proceeds.
  2. Measure the speed of flies in the same way that you measure the speed of an arrow.
  3. Measure the speed of flies in the same way that an arrow measures the speed of flies.
  4. Measure the speed of flies that resemble an arrow.
  5. Flies of a particular kind, i.e. time-flies, are fond of an arrow.
Note that the parser was smart enough to not assume the following. Either it was lexically coded to understand this as an impossibility, or it just wasn't thorough enough.

Time, an abstract concept can not perform concrete actions.

  • Time flies through the air in the same manner that arrows do.

Speaking of real-world knowledge, this might be a good place to note the source of the sentence. It didn't just spring into existence as a Computer Science tagline.

光陰矢の如 し

"Time flies like an arrow" is an old Japanese proverb. In Japanese it's "Kouin yanogotoshi" - which, literally translated, actually means "time is like an arrow".

I suppose the closest English equivalent would be "time and tide wait for no man". However, it seems that the Japanese version is (now, at least) much more commonly used. You can see it in a lot of people's letters and and online diaries. Of course, a simple "time flies" would do, but it isn't as descriptive. The arrow simile, furthermore, implies not only time's speed, but its irreversibility as well. The arrow cannot be pulled back to the bow, and time, once lost, can never be regained. Terminator movies notwithstanding.

-- Many thanks to sekicho for help with the Japanese.

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