A liar is a person who lies. Everybody lies. Therefore everyone is a liar?

Yes. But also no.

When you call someone a liar, you’re saying not that the person has lied, but that there is something inherently dishonest about her. It’s not what she did — it’s who she is. So how can we tell if we’re dealing with a liar and not merely a person who lied? Easy:

Liars are people we don’t like.

If a friend cheats on her significant other, we say she’s a good person who made a mistake, an honest person who told a big lie. But if our friend’s significant other cheats on her, we say he is a dirty liar and we hope he gets chlamydia and dies alone in a gutter. If a politician from the party we support gets caught lying into a microphone, we say “Well, that’s politics” or “He misspoke.” If a politician from any other party does the same thing, we say he is a dangerous liar and threaten to move to Canada (or buy 500 assault rifles) if he is elected.

We never follow through, of course. We’re always telling lies.