This is only really true in a purely logical argument. In rational arguments, like the one we're having now, source does affect posterior expectation. That is,

This is only really true in a purely logical argument. In rational arguments, like the one we're having now, source does affect posterior expectation. That is, someone of unimpeachable character and a history of reliability saying "F" ought to increase your posterior expectation that F. Conversely, someone with deeply questionable character and reliability will affect your posterior expectation of F much less, or perhaps even decreasing the expectation if you sense a motive for skewing the truth. The source does matter. It is not irrelevant, nor is it an "attack" in the traditional sense to offer evidence that the source might be lying or misguided.