We react negatively to liars and their lies. But what is it to lie? The standard view goes all the way back to Augustine in the 4th century, when he wrote, “He may say a true thing and yet lie, if he thinks it to be false and utters it for true.” On this view, lying is a purely psychological act, and saying something objectively false is inessential to it. My lab discovered powerful evidence against this view. If someone makes a dishonest assertion that turns out to be true, then people reliably judge that he did not actually lie. Instead, he only thinks he lied. This supports the conclusion that lying is not a purely psychological act: objective falsity, or untruth, is essential to lying.