You can use a person's honor against him only if he values honor. Only the honest man is threatened by the pointed finger. The liar is unaffected by that kind of accusation. I think it is because there is no such thing as "dishonesty". There is only honesty or it's lack. Not a thing and it's opposite, but a thing and it's absence. One or zero, not one or minus-one. One who is lacking a thing cannot be threatened at the prospect of its loss.