A phrase invented by Noam Chomsky. Often used by linguists to indicate the fact that a grammatically correct phrase can be semantically nonsensical.

Chomsky's whole point is that language is not logical; it's biological. There's nothing logical to the kind of extractions that one can make. You may have a logical meaning that doesn't make sense grammatically - so it's a biological, not logical, property. There are many examples of things that you can't say in English that would actually be logical to say.

It's a sentence that demonstrates that linguistic syntax is completely divorced from the meaning of a collection of words. The sentence is gramatically correct, yet presents an incomprehensible meaning. The rules that govern syntax in a language have nothing to do with the meaning that is conveyed.

The thing is, it's not semantically nonsensical. If you examine it with a bit of leeway, it's a perfectly normal sentence.

Colorless: dull, uninteresting. Flat, without energy.

Green: New, fresh, untested, unproven in the field.

Sleep: Lie dormant, paused, hibernating.

Furiously: With great energy, restlessness.

So it follows that a colorless green idea is a new, untried idea that is without vividness, dull and unexciting...To sleep furiously may seem a puzzling turn of phrase but one reflects that the mind in sleep often indeed moves furiously with ideas and images flickering in and out.

So what's the meaning of all this? That new ideas, not yet sharply defined, circulate in the unconscious, rapidly altering at a furious rate.

Hooray for the English language.

Easily the most important sentence in Linguistic-Cognitive Science.

Comes from a series of debates between Chomsky (a linguist) and Skinner (yes, the Behaviorist with the rats we all know and love) over the nature of language.

Skinner wanted to say that language is a sort of chain, and that we are conditioned to expect certain words in sequence. Say, after 'stop', we expect words like 'watch', 'there', 'light', 'time', and others. He said that's how we learn language; it's conditioned just like everything else in his theory, by exposure and repetition.

To refute this, Chomsky composed the sentence 'Colorless green ideas sleep furiously.' It is a sentence that is semantically correct (in that it violates no 'rules' of language, ie, no dangling participles, etc) but makes no logical sense. In addition, each pair of words, 'colorless' and 'green', 'ideas' and 'sleep', 'sleep' and 'furiously', you had probably never heard them paired up before. And yet, in a certain way, the sentence makes sense.

So, even though you've never heard the sentence before, and probably never heard those connections of words, the sentence makes some sort of sense. This violates completely the Behaviorist-Conditioned Response view that Skinner was trying to put forward. He was stumped, and had no real response, nor did his colleagues.

On a side note, there is apparently a yearly contest to write something that makes the phrase make sense. Of course, the descriptions given would likely never happen, but that's the point of it. Even though you've never been exposed to it (given the appropriate stimulus) you can still make sense of it (response).

Although first uttered to define the distinction between making sense and being syntactically well-formed, this sentence found a referent in the mid-1990s - it was a perfect description of the British Conservative Party's anodyne and ineffectual environmental policy.

Although this is probably of little interest or amusement value to those who do not remember the joys of living through the dying years of John Major's increasingly incompetent government, it does raise the point that the word "green" has gained a new meaning since Chomsky coined the phrase, and that thus semantic drift has the power to make (some kind of) sense out of nonsense.

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