I have found it very useful on the Internet to know the seven degrees of giving the lie (telling someone they lie, that is), in decreasing order from most to least insulting.
- The Lie Direct is simply "You lie" or "You are a liar". Crude, but useful as a challenge to mortal combat.
- The Lie Circumstantial is: "If anyone says such-and-such, he lies." This gives your opponent the opportunity to evade your wrath.
- Next comes the Countercheck Quarrelsome: "How dare you say such a thing!"
- Then the Reproof Valiant: "You know that is not true." I usually use this as the outer limit myself, though I sometimes advance to the Countercheck Quarrelsome.
- The next lower degree is the Reply Churlish: "You are no judge; your opinion is worthless."
- The Quip Modest I find hard to put into words; it is something like "I prefer it that way".
- And finally The Retort Courteous: "My opinion is otherwise."
- (redmonk proposed an eighth degree: the Non-Reply Apathetic; an example would be superfluous.)
sbp pointed out the interesting sentence "If anyone says he is Tony Blair, he lies" in connection with the Lie Circumstantial. It can be explicated as "If you claim to be Tony Blair and you're not, then you're a liar; if you claim to be Tony Blair and are, you lied about WMD to Parliament." We can neglect the existence of other people truly named "Tony Blair".