The deceiver first deceives himself.

The person who deceives you, accidentally or intentionally, is always, on some level, deceives him/herself. That's because it is just impossible to dare this dissonance that occurs in the body while lying without being deluded. The dissonance occurs when words do not coincide with thoughts, and thoughts do not coincide with feelings. When people in conflict are absolutely confident about something you don't agree with, or something that goes against common sense, agreements, or obvious facts, you can argue with them endlessly. Make a choice: to prove someone they are wrong, that is to explain that twice two makes four, or stop it and realize that they are deceiving themselves, not you.