Common knowledge

Joyce: What, reduced to their simplest reciprocal form, were Tzara's thoughts about Ball's thoughts about Tzara, and, Tzara's thoughts about Tzara's thoughts about Ball?
Tzara: He thought that he thought that he knew what he was thinking, whereas he knew that he knew that he knew that he did not.
Joyce: And did he?
Tzara: He did and he didn't.

(Tom Stoppard, Travesties).

Thanks Jim Bugden