Why is "Colourless green ideas sleep furiously" considered meaningless?

Question

Forewarning: I'm a philosopher, and I'm broadly ignorant of linguistics, so forgive me in advance for any misconceptions or stupid questions.

I think that it's regarded as common knowledge in linguistics—since Chomsky—that there can be grammatical sentences that are nevertheless meaningless. The standard example trotted out for this is

Colourless green ideas sleep furiously.

Whether or not there can be meaningless but grammatical sentences, this doesn't seem to me like an example of one. To me, it simply seems meaningful, but false. It seems to me to wear its truth-conditions on its sleeve: for it to be true, there'd have to be ideas that are colourless, green, and sleep furiously. But these conditions aren't met: ideas aren't green, and since they don't sleep, they certainly can't sleep furiously. So the sentence just seems false—indeed, necessarily false, since it's impossible for something to be both green and colourless. But for a sentence to be false, it must be meaningful. So the sentence is meaningful.

Essentially the same question was raised here previously, although both the question and answer there seem a bit muddled, with the poster bringing in logic and Explosion when they seem beside the point.

I found a similar discussion in Quine's Theories and Things, where he outlines a position similar to mine, in opposition to Ryle, Russell, and Carnap:

Many... predicates will be useless in application to attributes; thus it would be false, at best, to affirm, and useless, at best, to deny, that an attribute is pink or divisible by four. Ryle branded such predications category mistakes; he declares them meaningless and so did Russell in his theory of types. So did Carnap.

Over the years I have represented a minority of philosophers who preferred the opposite line: we can simplify grammar and logic by minimizing the number of our grammatical categories and maximizing their size. Instead of agreeing with Carnap that it is meaningless to say 'This stone is thinking about Vienna', and with Russell that it is meaningless to say 'Quadruplicity drinks procrastination', we can accommodate these sentences as meaningful and trivially false. Stones simply never think, as it happens, and quadruplicity never drinks. (Theories and Things, 1981, p. 110)

Quine's position seems to me to be quite right, and, applied to "Colourless green ideas...", to lead to my above conclusion. However, Quine says that his position was a minority among philosophers during his time; is this also a minority position among linguists during ours?

Answer 1

It seems to me to wear its truth-conditions on its sleeve: for it to be true, there'd have to be ideas that are colourless, green, and sleep furiously. But these conditions aren't met: ideas aren't green, and since they don't sleep, they certainly can't sleep furiously.

Chomsky might well dispute that ideas aren't green, that ideas don't sleep, and that they can't sleep furiously. These claims are neither true nor false, since they are meaningless. In programmer-speak, expressions like these cannot be evaluated because they have type errors.

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Since you mention vacuous truths in the comments, allow me to give my own proof that ideas are green.

For something to be green, it means that if a person with good sight looks at it in good lighting, they experience the colour green. Since an idea cannot be looked at, the antecedent is false, and therefore the statement itself is true. Vacuously, everyone who looks at an idea will experience the colour green! QED.

But now allow me to prove that, in fact, ideas can be looked at. For something to be able to be looked at, it means if a person with good sight faces in its direction with their eyes open and no obstruction, they will see it. Since you cannot face in the direction of an idea, the antecedent is false, and therefore the statement itself is true. Vacuously, everyone who faces in the direction of an idea will see it! QED.

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These proofs exhibit a verbal fallacy by saying "X can't be Y" when what we really mean is, "it is meaningless to say X can be Y".

In the sentence "ideas can't be green", the word "can't" really means that there is no sense in which an idea can be green. That is, when you say "ideas can't be green", you are not saying that "ideas are green" is meaningful but false, you are saying "ideas are green" is not meaningful. But then, the negation of a meaningless sentence is also meaningless, not true.

Answer 2

Let's look at another famous example of a meaningless sentence: "the king of France is bald". Is this true or false?

Many semanticists/pragmaticists would say it's neither. It's not true, but "the king of France has hair" isn't true either, because there is no king of France. This is known as a truth value gap, where it's impossible to assign a truth value to a sentence because some precondition isn't met.

There are various different ways to address these in linguistics; personally, I'm partial to Austin's speech act theory, which says there are a lot of ways for a speech act to fail, and being untrue is only one of these. Having a missing reference is a different sort of failure, as is starting to cough halfway through so you never finish the sentence, or messing up one of your words.

(Another way is to say that the negation of "the king of France is bald" is not "the king of France has hair" but "either the king of France has hair or there is no king of France". The problem with this is that it doesn't line up with speakers' intuitions about how to negate propositions, so you have to start introducing multiple types of negation, and the conclusions about logical negation don't really say anything about linguistic negation.)

Answer 3

Quine clearly decodes this sentence as an entire bundle of statements:

• Ideas can have color
• AND ideas can have the green color and no color at the same time
• AND ideas sleep (which, as an aside, implies that ideas have a waking state, which usually is understood as observing and reacting to the environment by an animal)
• AND that it is possible to sleep in particular moods
• AND that (generally) the colorless, green ideas whose existence was postulated above sleep "furiously".

Declaring any of these sub-statements false makes the entire statement false, which is what Quine does.¹

The more traditional view by Russell and Chomsky emphasizes that natural language syntactically permits applying physical attributes to immaterial concepts, or formulate antinomies, but that semantics don't permit that: The sentence is compatible with the grammar of speech, but violates the "grammar of mind", if you want.

It is semantically ungrammatical, or nonsensical.

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_{Footnotes add so much to the credibility of a text, so here is one.}

_{¹ Interestingly, this is not as trivial as he (and Chomsky, if differently) makes it seem. Natural language is a beast, and if we decode "having color" as a statement about a subjective experience (and aren't they all?), we can imagine a person with synesthesia to experience green ideas, and if we are hard-pressed, ideas which are both green and colorless at the same time or in quick succession; if we must, we can ingest a mushroom or fast for a week to help the experience along. Metaphorically, ideas can certainly sleep, and if they birth monsters, may be their sleep was furious...}

Answer 4

I'm no philosopher. In fact, I landed on this page while searching for a solution to a "water accumulation" problem on my lawn. However, this intriguing discussion captured my attention, and I've followed it to the very last comment. Surprisingly, no one has addressed a critical perspective, so here is my attempt.

The statement "colourless green ideas sleep furiously" is not just meaningful—it is profoundly true.

Consider this scenario: A friend cries on your couch for the fourth time this month, heartbroken yet again by their unreliable partner. You grit your teeth, offering comfort, despite knowing that the real solution is for them to leave the toxic relationship. You refrain from saying it, aware that your advice will likely be ignored, and they’ll return, repeating the cycle.

An idea is a thought. Words can have subjective meanings, especially when combined in a sentence. A "green idea" signifies an optimal solution—logical, positive, and without drawbacks. Green conveys go, freshness, positivity (think green lights, evergreen). However, a "colourless green idea" lacks excitement; it’s dull, like a "baked potato" of ideas.

Life's paradox often sees us neglecting the best decisions because they lack allure. These "colourless green ideas" remain dormant, asleep. Eating vegetables, meal prepping, getting organized—all are beneficial yet uninspiring actions we often defer.

Back to my friend: it frustrates me to see the clear solution ignored. I want to shake them into seeing reason. Yet, as a friend, I offer unwavering support, knowing the cycle will repeat.

Thus, "colourless green ideas" do indeed sleep, sometimes furiously. This sentence is not only meaningful but deeply so.

Other instances where this applies include:

• Stalling on a promising business idea.
• Quitting smoking.
• Tidying up your room.

In essence, any inaction towards a positive outcome fits this description. Though my response may differ in style from others in this thread, I hope it conveys my thoughts effectively.