In book 7, the monk Sanjaya preaches doubt. His main tenets are, "Do not believe people. Do not believe society... Do not believe the weather reports. Do not believe the Yankees will ever win the playoffs."
Tatta to Migaila: "What does [Buddha] see in [Sariputta and Moggallana]] that we don't got?" "Well, they do seem to be psychics."
Later on, when speaking with Prince Crystal, Buddha speaks of how a doctor needs to examine a patient to know how to treat him, and he can't do that if the patient is stubborn or simply cries without being cooperative. When Prince Crystal demands what that has to do with his plight (he is suffering from massive guilt and can't let go from it), Buddha turns into Black Jack for a panel and simply answers: "In other words, I'm a doctor." A moment as awesome as it is funny.