With a tip of the hat to the estimable Simon Perry, this is a wonderful quotation on bullshit by Princeton philosopher Harry Frankfurt:
"It is impossible for someone to lie unless he thinks he knows the truth. Producing bullshit requires no such conviction. A person who lies is thereby responding to the truth, and he is to that extent respectful of it. When an honest man speaks, he says only what he believes to be true; and for the liar, it is correspondingly indispensable that he considers his statements to be false. For the bullshitter, however, all these bets are off: he is neither on the side of the true nor on the side of the false. His eye is not on the facts at all, as the eyes of the honest man and of the liar are, except insofar as they may be pertinent to his interest in getting away with what he says. He does not care whether the things he says describe reality correctly. He just picks them out, or makes them up, to suit his purpose."
Whenever I encounter quacks or bad psychics or similar, I very rarely believe they are liars.
After all, there must be easier and more lucrative ways of being dishonest.
So I prefer Professor Frankfurt's approach.
It does seem to explain a lot...
[This topic has also been covered with more erudition and insight by the wise old duck Le Canard Noir.]
[The full Frankfurt article is here.]