While every man Grantaire knew was off dreaming of carnal pleasures, of course Enjolras was dreaming about Jean Jacques Rousseau-- Grantaire's internal mocking of Enjolras' reverence for the man was interrupted by the coughing fit. His expression was instantly concerned, and he glanced wildly around for anything he could do to help. There was nothing Grantaite could do but wait until it subsided, and the story continued.
Enjolras was correct about one thing. Grantaire thought he was insane. Did he have a fever? because it must have gone to his brain. Clearly the sickness was loosening Enjolras' grip on reality.
"So all this," Grantaire wiggled the book in his hand, "is to settle an argument that began in a dream?" He glanced between Rousseau and Enjolras with a concerned expression.