This sentence is interesting to me:

I don't know what you know.

It seems ambiguous. It has two possible meanings. It is so ambiguous that I'm actually struggling a bit to explain the differences without using ambiguous wording, as even context does not always bring clarity to this sentence:

  1. It could mean that I am acknowledging that you have knowledge that I don't have. For example, "You are an expert. I am a novice. I don't know what you know." In that case "what" seems to be referring to the knowledge itself.

  2. It could also mean that I am unaware of the collection of things that you are knowledgeable about. For example:

    • I give an explanation that is unnecessary, as you already know the reasons.
    • You: "I already know that, you don't need to explain."
    • Me: "I don't know what you know. I explained it just in case."

It also doesn't necessarily seem specific to this form. "I don't know the things that you know" has a similar ambiguity.

The above two differences are similar to the differences between (respectively):

  1. I don't have what you have.
  2. I don't know what you have.

Or, I guess, in general: "I don't X what you X" vs "I don't know what you X". The ambiguity arises when X is "know": The two constructions end up identical.

So, I have two questions:

  1. What is the source of this ambiguity? In particular, is the sentence ambiguous because I have learned to take grammatical shortcuts when using it (that is, is "I don't know what you know" grammatically incomplete, thus leading to ambiguity)?

  2. What is the difference between the two readings of this sentence? Does the "what" serve as a subtly different part of speech, for example? Or, considering the "I don't ____ what you have" parallels, perhaps the differences are in the "know"s? It's almost like the first "know" in the second meaning of "I don't know what you know" is subtly different than the other three "know"s.

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    The distinction has to do with what Ross calls "conjunctive vs disjunctive Wh-clauses". These are governed mostly by the matrix predicate; typical examples are What she did is important (conjunctive; factive) vs What she did is unknown (disjunctive; indefinite). – John Lawler Mar 18 '17 at 16:20

I don't know what you know.

The sentence is ambiguous in terms of its meaning because it's ambiguous in terms of its syntax too.

It is possible that the string what you know is a fused relative here (a special kind of relative clause construction sometimes also known as a free relative). In this case the string what you know is a noun phrase. It represents an entity. These kinds of fused relatives with what can be paraphrased using the words the thing(s) that. We can paraphrase the fused relative reading of the sentence (and make it slightly clearer by adding the word same) like this:

  • I don't know the same things that you know.

In this reading of the sentence we can consider the simple object of the sentence (as opposed to the full grammatical object of the sentence) as the pronoun what. This word represents the actual thing which is unfamiliar, unknown, to the speaker.

Alternatively, the string what you know could be read as an interrogative clause. In this case, if the you concerned was called Bob, for example, the sentence would mean something like:

  • I don't know the answer to the question: What does Bob know?

[I used Bob in the sentence above because the deixis of you could cause further problems here]

Here the whole interrogative clause what you know represents a question, the answer to which is unknown to the speaker.

Grammars like The Cambridge Grammar of the English Language give detailed analyses of the different structures of fused relatives and interrogative clauses. The most pertinent difference is that—according to their analysis—in the fused relative reading what you know is a noun phrase (a phrase headed by a noun or pronoun, in this case the word what) , whereas what you know in the interrogative reading is an interrogative clause (and therefore ultimately headed by a verb, in this case the verb know).

There is a nice test you can do, which will tell you whether an item is a fused relative or an interrogative clause. In the interrogative clause reading you can add the word else after the what and the sentence will still make sense and still be grammatical (although it will have changed its meaning somewhat). So the following sentence can only have the interrogative reading:

  • I don't know what else she knows.

The sentence above can only mean:

  • I don't know the answer to the question: What else does she know?
  • I have a doubt, as people here are won't to say, but its explication won't fit in a comment box. So I bloviated on for while in an answer. Tell me where I've gone off the rails so I can delete my "answer" and upvote yours. – deadrat Mar 18 '17 at 19:01
  • I agree: It’s not a fused relative - it’s an interrogative complement clause. "I don’t know what you know" doesn't mean "I don’t know the thing(s) that you know" --- that's a silly mistake (notice how the first "know" would be "sais" in French and the second would be "connais"!). It means "I don’t know the answer to the question 'What do you know?'". It would be a misguided diagnosis of it as the "connaitre" sense of "know" with a fused relative as object. – BillJ Mar 18 '17 at 19:39
  • @BillJ Do you consider "I don't know the things that you know" an incorrect/mistaken way of phrasing "I don't know which things you know" (where these phrases take meaning #2 in my post)? If not, then wouldn't "the things that you know", "which things you know", and "what you know" be equivalent in that reading, with the latter two (according to my understanding) being fused relative clauses? – Jason C Mar 18 '17 at 19:59
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    @JasonC Yes, I do. As a subordinate interrogative it's expressing a question rather than directly asking one. The main clause equivalent would be "what do you know?" and I think the two clauses (main and subordinate) would elicit the same set of answers such as "I know xyz". In other words, "I don't know what you know" is saying "I don't know what it is that you know". – BillJ Mar 18 '17 at 20:05
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    @JasonC To be clear, I take "I don't know what you know" to mean "I don't know what it is that you know". – BillJ Mar 18 '17 at 20:09

Araucaria has explained the syntactic ambiguity, but I don't think that fully explains the semantic ambiguity. I think the latter rests on two meaning of the verb to know -- to discover a fact and to understand a topic.

To illustrate, let's suppose that your friend Bob is a physicist and that you have only a layman's familiarity with modern physics. You've forgotten what it is exactly that Bob does for a living, but you remember that he either works with QED (quantum electrodynamics) or QCD (quantum chromodynamics). You know the former deals with electrons and light, while the latter deals with nucleons (particles in the nucleus, i.e, protons and neutrons). You have no working knowledge of quantum mechanics, group theory, or Hilbert spaces, but Bob does.

When you admit to Bob that you've forgotten his field of research, and you say

[1a] I don't know what you know

you mean

[1b] I need to discover the name of your field of expertise. Is is QED or QCD?

But if you remember that Bob works with QCD, and you say 1a, you mean something like

[1c] I don't understand all the mathematics that goes with quarks and gluons, but I'm impressed that you understand all that.

The fused relative implies 1b more strongly, especially if you use the singular:

[2] I don't know what it is that you know

but I think the interrogative reading

[3] What else does Bob know?

falls prey to the same ambiguity. Are you asking for a term in Bob's resume or are you asking for the state of Bob's mind?

The same syntactic analysis applies to

[4] I don't like what Bob likes

but I think this admits of only one meaning.

  • I'm still ponderficating. But I think the interrogative implies 1b and the relative 1c. I agree that know has two different meanings similar perhaps to connaitre and savoir (something like be familiar with and understand). But I think perhaps both readings can be available whether we have the FR or the Int (so we can have ambiguity unrelated to the phrasal category of the wh string). Still pondering. There are two readings to [4], it seems to me. "I don't like the same things which Bob likes" and "I'm unhappy with what Bob likes" – Araucaria Mar 18 '17 at 19:24
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    @Araucaria I'm having a hard time getting from I don't like what Bob likes to I don't like it that Bob likes it. Maybe it's just me. Refill my glass and pass me another cigar. – deadrat Mar 18 '17 at 19:36
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    Rounds and cigars are on me! – Jason C Mar 18 '17 at 19:47
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    One last thought to leave here before I head off to the pub. It may well be that one sense of the verb X forces the selection of a certain type of construction as a complement. However, turning that on it's head, it is clearly equally the case that having a construction as a complement will force us to give a verb, X, a particular interpretation. So which bit is forcing which bit is difficult to ascertain, it seems to me ... – Araucaria Mar 18 '17 at 20:39
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    @deadrat Let me fix that for you! +1 – Araucaria Mar 19 '17 at 4:14

I don't know what you know.

This sentence raise no flags with me. What the author of the sentence specifically means can only be known in context.

It is almost midnight .

Obviously this sentence is a statement of what time it is.
Or, maybe it is not a simple time statement.

In context, this might mean a time limit has almost been reached.

In another context, almost midnight could mean the end of time itself was near.
Attempting to determine what a sentence means out of context is not a very useful pursuit.


A single word statement like this has no particular meaning without context. If one were to hear something whistling in the air, and a sharp "down" yelled, then that person would have the context to understand it was time to grab some ground. And that is different from rehanging a painting that was too high.

I don't know what you know.

has to wait for context.

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