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I came across the above sentence while looking through a website. I'm confused as I was under the impression that ce dont was always used as the indefinite relative pronoun and de quoi is used in an interrogative sense. This can only be a statement, surely, so why the use of de quoi? I googled the sentence and found 34,000 results, and only marginally more with ce dont so this construction seems fairly common. Can anyone tell me why you would use this over ce dont?
My googling expertise gives me 329,000 returns for "Je ne sais pas de quoi il parle" and only 68,400 for "Je ne sais pas ce dont il parle" (notice I include the quotation marks).
So the use of "quoi" seems a lot more common than that of "ce dont" in that particular phrase if I am right.
As to the difference in meaning might there be more of an implied questioning in the phrase with "quoi" and more of a sense of a "statement" with "ce dont"?
That there seems to be an implied question with "de quoi" would seem to be the only explanation, but it still confuses me. When I see "je ne sais pas de quoi il parle" I only see a statement. I guess grammar isn't my strong point. How can "je ne sais pas de quoi il parle" be interrogative?
Yes I am probably wrong about the implied question.
Perhaps it is just that "de quoi" is a more common usage than "ce dont". (again x5 times more common for "de quoi" than for "ce dont" if that means anything.
I think very strictly what is happening here is that two slightly different structures are being confused:
(a) so-called "free relatives", which are effectively relative clauses without a noun to 'latch on to';
(b) actual indirect questions.
Much of the time, there's superficially not much difference between the two, either in English or French. But consider cases such as:
I ate [the apple that he had been talking about].
*I ate [which apple he had been talking about].
I ate [what he had been talking about].
*I wondered [the apple he had been talking about].
I wondered [which apple he had been talking about].
I wondered [what he had been talking about].
In (a), you have a verb ("eat"), which usually takes a "normal" object. And so because of that, it sounds odd to use a clause starting with "which..." as the object of "eat". Conversely, in (b), the verb "wonder" is a verb whose object is usually a question. So it sounds normal to use a clause starting with "which...", but odd to use a clause starting with "the...".
But, notice that in the last example in both (a) and (b), despite the syntactic difference that we've just seen with the type of clause you can use with "eat" vs "wonder", you can actually use a clause starting with "what..." in both cases. In (a), "what...about" is a 'normal' relative pronoun if you like, whereas in (b), it is actually an indirect question marker.
This is basically the situation you have in French: "ce dont..." corresponds more or less to "what" as in case (a), whereas "de quoi..." corresponds more or less to "what" as in case (b). And, as in English, the two cases do more or less coincide. So for example:
J'ai demandé ce dont elle avait besoin.
"I asked for/requested what she needed" (i.e. I didn't ask a question, but rather asked for the thing that she said she needed.)
J'ai demandé de quoi elle avait besoin.
"I asked what she needed" (i.e. I asked the question: "what is the thing that she needs")
In the case of "savoir", I think you effectively have a similar situation:
Je ne sais pas ce dont il a parlé.
"I am ignorant of the factual information about which he has been speaking"
Je ne sais pas de quoi il a parlé.
"I ask myself the question: what has he been speaking about"
So to summarise, what you basically have is that a handful of verbs, including "savoir" (and indeed "know" in English) can function either as in case (a) above or as in case (b). In English, when the subordinate clause starts with indefinite "what...", there's superficially no difference between case (a) and case (b). But in French, there is.
That makes sense for me.So it seems I was right after all about there being an implied question when "de quoi" was used.
Thank you for that very thorough answer. That clears everything up for me. It didn't occur to me that it could have been an indirect question. I thought the only way a clause could be interrogative was if it was followed by a question mark.
If you want to speak a very posh french you can say : Je ne sais de quoi il parle.