Colorless green ideas sleep furiously

Noam Chomsky (Kuva: Jean-Baptiste Labrune / cc 2.0)
Noam Chomsky (Kuva: Jean-Baptiste Labrune / cc 2.0)
November 16th, 2012

TL;DR - Syntax is easy. Semantics is hard. Computational semantics is double hard.

"Colorless green ideas sleep furiously." is a didactic, if somewhat flippant, sentence introduced by Chomsky in his seminal work Syntactic Structures. It's meant to illustrate the vast chasm between "syntax" and "semantics".

Syntax is the set of rules, the grammar, of a language. Syntax indicates how sentences can be constructed. Syntax may, for example, say a sentence can consist of a noun phrase followed by a verb phrase. Or it may state a noun phrase consists of zero or more adjetives followed by a noun. Or, for example, it might imply a verb phrase consists of a verb followed by zero or more adverbs. Indeed, it may place any number of seemingly bizarre restrictions and/or allowances on sentence structure.

English syntax, drilled in to our young impressionable minds but now only dimly recollected, indicates that the sentence "Colorless green ideas sleep furiously." is well-formed. In other words, it satisfies all the antiquated rules of English grammar. (Some of these grammar rules serve no more purpose than the human appendix. But, as English, and Humans, have evolved through thousands of years of aleatory action, neither really makes much sense.) A formal syntactic parse of our sentence is as follows

Parse of "Colorless green ideas sleep furiously".

Parse of "Colorless green ideas sleep furiously".


So, you see, syntax is easy. To determine if a sentence is syntactically well-formed you simply make sure it satisfies the grammar rules. That's it. It's so simple a computer could do it and, in fact, many a computer does.

Semantics,  on the other hand, is hard. Semantics is concerned with the "meaning" of a sentence. Ignoring the fact that we don't even know what we mean by "meaning", you can immediately see that semantics is a far more slippery subject than syntax.

For example, what does "Colorless green ideas sleep furiously." mean? (I certainly have no idea.) What does William Carlos Williams´ poem The Red Wheelbarrow

so much depends

a red wheel

glazed with rain

beside the white

mean? (I've my opinion, which likely is quite different from yours.) What do the the proceeding paragraphs of this blog post mean? What is the meaning of this sentence?

Even formulating an answer to such questions seems to lead to an infinite regress. You clarify the meaning of a sentence with another sentence; then the question becomes: What's the meaning of this clarifying sentence? It also needs an explanatory sentence, and on, and on... It's turtles all the way down.

Clarifying semantics for humans is hard. So, semantics for a computer, seems, well, almost impossible. However, that's the task we've yoked ourselves to, making a computer understand and answer your questions. Wish us luck!

Through this blog, we'll take this journey together. It'll be a fun ride.

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