Can someone try to make that argument more precise? It seems to me that the claim "Sorry. It can't be done" sounds plausible but fails in the most obvious limit case: a proof of a mathematical theorem doesn't become less correct if I found it by deliberately trying to prove the theorem. Since Bayesian reasoning approaches classical logic in the limit, the claim might be wrong for Bayesian reasoning too.
It is possible to gain evidence in favor of hypothesis X from Bob who you know has X as his bottom line. However, Bob can't force this outcome, because it's also possible that his attempt to convince you of X will backfire. For any fixed strategy on Bob's part, the effect on your beliefs tends to be towards the true value of X, not towards the value that Bob wants; with mixed strategies (or just silence) he can prevent you from gaining but can't reduce your net accuracy.
Applied to the finite likelihood case: Initially you assign some probability to X, and ...
In the spirit of contrarianism, I'd like to argue against The Bottom Line.
As I understand the post, its idea is that a rationalist should never "start with a bottom line and then fill out the arguments".
It sounds neat, but I think it is not psychologically feasible. I find that whenever I actually argue, I always have the conclusion already written. Without it, it is impossible to have any direction, and an argument without any direction does not go anywhere.
What actually happens is:
It is at the point 3 that the biases really struck. Motivated Stopping makes me stop checking too early, and Motivated Continuation makes me look for better arguments when defective ones are found for the conclusion I seek, but not for alternatives, resulting in Straw Men.