What I did was assume that the importance of language would trump our shared knowledge. That is, even if he knew me to be lying in principle, he could only react to lying in fact. However, people do not operate on this level, except, to some degree, in court. Language failed me, but only because I purposefully perceived it inadequately. Language is much more than fact or even communication.
Language serves mostly as a symbol for consciousness.
In order for truth, the abstract quality of the symbol must be assumed.