mute, or only believed mute
An unpopular après-garde filmmaker (Watt) either suffers a temporal lobe seizure and becomes mute or else is the victim of everyone else’s delusion that his (Watt’s) temporal lobe seizure has left him mute.
Context
Insubstantial Country keeps the muteness undecidable in exactly the theme's way: there is no fact of the matter about whether the filmmaker can't speak or whether everyone around him has agreed he can't — the failure of contact is located neither wholly in the sender nor the room but in the gap where the two can't be told apart.