He'd had the sense, moments earlier, that Caroline was on the verge of accusing him of being "depressed", and he was afraid that if the idea that he was depressed gained currency, he would forfeit his right to his opinions. He would forfeit his moral certainties; every word he spoke would become a symptom of disease; he would never again win an argument.
It was therefore all the more important now to resist depression - to fight it with the truth.