The expression "ignorance is bliss" means that it is easier to be happy if you don't know what is going on in the world. How does this expression relate to the conversation Montag has with Faber in Ray Bradbury's Fahrenheit 451?
Guy Montag's encounter with the former professor of English, Faber, has exposed the fireman to a mentality with which he had no previous experience. Faber has been systematically marginalized by the totalitarian society in which he lives. His knowledge of the contents of so many books marks him as a potential enemy of the state. Ray Bradbury's futuristic dystopian society in Fahrenheit 451 eschews any source of information not expressly approved by the autocratic government Montag has loyally served. As he begins to ponder the importance of books to so many people, however, he increasingly questions his role in enforcing the government's edicts against the possession of books. It is Faber who articulates for Montag the significance of information and the threat knowledge poses for the regime. As Montag's relationship with the former professor grows closer, they become unlikely allies in their efforts at subverting the system that has enslaved the population. Recognizing that Montag is under suspicion by his superior, Captain Beatty, Faber provides Montag with a small communications device that will enable the fireman to survive the interrogations to which he is certain to be subjected.
It is through these communications that Faber emphasizes the notion that "ignorance is bliss." Specifically, in coaching Montag through his meetings with Beatty, Faber states that "If you hide your ignorance, no one will hit you and you'll never learn." What Faber means by this comment is that, if one remains remote from the world of knowledge, then one is safe from reprisals. If, on the other hand, one projects knowledge, then one opens oneself to reprisals. It is safer to remain ignorant than to risk one's well-being by absorbing the information contained in books. Faber, in continuing to coach Montag, also states:
". . .remember that the Captain belongs to the most dangerous enemy of truth and freedom, the solid unmoving cattle of the majority. Oh, God, the terrible tyranny of the majority."
Again, the government's ability to maintain absolute, unquestioned control over the population is tied to its control of the information disseminated to the public. The less one knows, the happier he or she will be.