In Frankfurt's terms, what is the difference between a "bull session" and bullshit?

In a bull session everyone agrees the talk isn't meant as sincere belief; bullshit pretends it is sincere while actually being indifferent to truth.

Frankfurt uses the bull session — friends "shooting the bull" about religion, politics or sex — to sharpen the concept. In a bull session, participants try out ideas to see how they feel and how others react, without committing to them. Everyone shares an understanding that "what is said is not necessarily what the speaker really believes."

That shared understanding is exactly what bullshit lacks:

• Bull session: the normal link between saying and believing is openly suspended — no pretence it's being honoured.
• Bullshit: the speaker pretends to be sincerely asserting truth, while in fact being unconstrained by it.

Both are "unconstrained by concern with truth," but only the bullshitter is being deceptive about the enterprise itself.

Tip: The honesty of a bull session is that nobody is fooled about its nature. Bullshit smuggles the same truth-indifference into a context where you are meant to take it as sincere.