Sheldon: So, you're saying that friendship contains within it an inherent obligation to maintain confidences?
Penny: Well, yeah.
Sheldon: Interesting. One more question—and perhaps I should have led with this—when did we become friends?

Leonard: What do you mean, you're moving out? Why?
Sheldon: There doesn't have to be a reason.
Leonard: Yeah, there kinda does.
Sheldon: Not necessarily. This is a classic example of Münchhausen's Trilemma. Either the reason is predicated on a series of sub-reasons leading to an infinite regression, or it tracks back to arbitrary axiomatic statements, or it's ultimately circular, i.e. I'm moving out because I'm moving out.
Leonard: I'm still confused.
Sheldon: Leonard, I don't see how I could have made it any simpler.

Leonard: How could you just sit there and let them spy on me?
Sheldon: They were clever, Leonard. They exploited my complete lack of interest in what you were doing.