Can an observer say: "Timmy believes that he believes that 666 is a prime number".... As tempting as it seems to say that, this is not true. Read what the aforementioned blog entry has to say:
What of Timmy's meta-beliefs? He might not have any, if he's very young, but let's suppose that he's aware of himself as a believing agent. What does he think he believes? Jack suggests to me the following: "Timmy believes that he believes that 666 is a prime number". But this attribution seems mistaken for exactly the same reasons. Timmy lacks the concept prime number, so he can't have any (even meta-) beliefs involving it. And nor can he have any de re beliefs about primeness (under whatever guise), because he lacks any alternative grasp of the property in question. He's not capable of having primeness feature in his mental content at all. (source)
Timmy does NOT believe that 666 is a prime number. He says what would conventionally suggest that he does, but he doesn't mean what he is saying. He believes that 666 is a cool number. His mistake is that he thinks that the utterance "666 is a prime number" expresses the idea that 666 is a cool number, oblivious to that the utterance "666 is a prime number" conventionally means that 666 is divisible by only itself and 1.... He may and may not believe that 666 is divisible only by itself and 1, but his utterance is simply unmeant to this concept.
In this series:
Expressions And Meanings - Part 1: Introduction
Expressions And Meanings - Part 2: Elaboration
Expressions And Meanings - Part 3: Linguistic Relativism
Expressions And Meanings - Part 4: Conclusion