Colorless green ideas sleep furiously
"Colorless green ideas sleep furiously" is a sentence composed by Noam Chomsky in his 1957 Syntactic Structures as an example of a sentence that is grammatically correct, but semantically nonsensical. The term was originally used in his 1955 thesis "Logical Structures of Linguistic Theory". Although the sentence is grammatically correct, no obvious understandable meaning can be derived from it, and thus it demonstrates the distinction between syntax and semantics. As an example of a category mistake, it was used to show inadequacy of the then-popular probabilistic models of grammar, and the need for more structured models.
The full passage says:
It is fair to assume that neither sentence (1) nor (2) (nor indeed any part of these sentences) has ever occurred in an English discourse. Hence, in any statistical model for grammaticalness, these sentences will be ruled out on identical grounds as equally "remote" from English. Yet (1), though nonsensical, is grammatical, while (2) is not grammatical.
- Colorless green ideas sleep furiously.
- *Furiously sleep ideas green colorless.
While the meaninglessness of the sentence is often considered fundamental to Chomsky's point, Chomsky was only relying on the sentences' having never been spoken before. Thus, even if one were to prescribe a likely and reasonable meaning to the sentence, the grammaticality of the sentence is concrete despite being the first time a person had ever uttered the statement, or any part thereof in such a combination. This was used then as a counter-example to the idea that the human speech engine was based upon statistical models, such as a Markov chain, or simple statistics of words following others.
Attempts at meaningful interpretations 
The sentence can be given an interpretation through polysemy. Both green and colorless have figurative meanings, which allow colorless to be interpreted as "nondescript" and green as "immature". The sentence can therefore be construed as "nondescript immature ideas have violent nightmares", a phrase with less oblique semantics. In particular, the phrase can have legitimate meaning too, if green is understood to mean "newly-formed" and sleep can be used to figuratively express mental or verbal dormancy.
Writers have attempted to provide the sentence meaning through context, the first of which was written by Chinese linguist Yuen Ren Chao. A literary competition was held at Stanford University in 1985, in which the contestants were invited to make Chomsky's sentence meaningful using not more than 100 words of prose or 14 lines of verse. An example entry from the competition, from C.M. Street, is:
- It can only be the thought of verdure to come, which prompts us in the autumn to buy these dormant white lumps of vegetable matter covered by a brown papery skin, and lovingly to plant them and care for them. It is a marvel to me that under this cover they are labouring unseen at such a rate within to give us the sudden awesome beauty of spring flowering bulbs. While winter reigns the earth reposes but these colourless green ideas sleep furiously.
Statistical challenges 
Fernando Pereira of the University of Pennsylvania has fitted a simple statistical Markov model to a body of newspaper text, and shown that under this model, "Furiously sleep ideas green colorless" is about 200,000 times less probable than "Colorless green ideas sleep furiously".
This statistical model defines a similarity metric, whereby sentences which are more like those within a corpus in certain respects are assigned higher values than sentences less alike. Pereira's model assigns an ungrammatical version of the same sentence a lower probability than the syntactically correct form demonstrating that statistical models can learn grammaticality distinctions with minimal linguistic assumptions. However, it is not clear that the model assigns every ungrammatical sentence a lower probability than every grammatical sentence. That is, "colorless green ideas sleep furiously" may still be statistically more "remote" from English than some ungrammatical sentences. To this, it may be argued that no current theory of grammar is capable of distinguishing all grammatical English sentences from ungrammatical ones.
Related and similar examples 
There is at least one earlier example of such a sentence, and probably many more. The pioneering French syntactician Lucien Tesnière came up with the French sentence "Le silence vertébral indispose la voile licite" ("The vertebral silence indisposes the licit sail").
The game of cadavre exquis (1925) is a method for generating nonsense sentences. It was named after the first sentence generated, Le cadavre exquis boira le vin nouveau (the exquisite corpse will drink the new wine).
In the popular game of "Mad Libs", a chosen player asks each other player to provide parts of speech without providing any contextual information (e.g., "Give me a proper noun", or "Give me an adjective"), and these words are inserted into pre-composed sentences with a correct grammatical structure, but in which certain words have been omitted. The humor of the game is in the generation of sentences which are grammatical but which are meaningless or have absurd or ambiguous meanings (such as 'loud sharks'). The game also tends to generate humorous double entendres.
There are doubtlessly earlier examples of such sentences, possibly from the philosophy of language literature, but not necessarily uncontroversial ones, given that the focus has been mostly on borderline cases. For example, followers of logical positivism held that "metaphysical" (i.e. not empirically verifiable) statements are simply meaningless; e.g. Rudolf Carnap wrote an article where he argued that almost every sentence from Heidegger was grammatically correct, yet meaningless. Of course, some philosophers who were not logical positivists disagreed with this.[vague]
The philosopher Bertrand Russell used the sentence "Quadruplicity drinks procrastination" to make a similar point; W.V. Quine took issue with him on the grounds that for a sentence to be false is nothing more than for it not to be true; and since quadruplicity doesn't drink anything, the sentence is simply false, not meaningless.
Examples like Tesnière's and Chomsky's are the least controversially nonsensical, and Chomsky's example remains by far the most famous.
Clive James wrote a poem titled "A Line and a Theme from Noam Chomsky" in his book, Other Passports: Poems 1958-1985. It opens with Chomsky's second meaningless sentence and discusses the Vietnam War.
Another approach is to create a syntactically-correct, easily parsable sentence using nonsense words; a famous such example is "The gostak distims the doshes". Lewis Carroll's Jabberwocky is also famous for using this technique, although in this case for literary purposes. In Russian schools of linguistics, the glokaya kuzdra example has similar characteristics.
Other arguably "meaningless utterances" are ones that make sense, are grammatical, but have no reference to the present state of the world, such as "The King of France is bald," since there is no King of France today (see definite description).
See also 
- Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo
- Moore's paradox
- Poverty of the stimulus
- Universal grammar
- Invisible Pink Unicorn
- Chomsky, Noam (1957). Syntactic Structures. The Hague/Paris: Mouton. p. 15. ISBN 3-11-017279-8.
- Chao, Yuen Ren. "Making Sense Out of Nonsense". The Sesquipedalian, vol. VII, no. 32 (June 12, 1997). Archived from the original on 2006-08-30. Retrieved 2006-08-30.
- "LINGUIST List 2.457". 1991-09-03. Retrieved 2007-03-14.
- Pereira, Fernando (2000). "Formal grammar and information theory: together again?". Philosophical Transactions of the Royal Society 358 (1769): 1239–1253. See also this post at Language Log.