With liars, the issue isn’t whether the liar lies 70 percent of the time, 50 percent of the time or “only” 10 or 2 percent of the time. The issue is that the person lies. Because you know that liars lie, you have no way of knowing whether or not the words coming out of their mouths are true, or not. Ever.

When you’re associating with a person, doing business with a person or trying to enjoy a personal relationship with a person, you’re counting on them to have a good relationship with reality. You’re counting on the person to be sane and rational. Being sane doesn’t mean you never make errors. Honest errors are part of thinking, and part of the fallibility of human reason. But when someone deliberately and knowingly says “it is” when knowing it ISN’T, they have revealed a disturbing lack of relationship with truth, facts and reality. At that point, all bets are off.

It’s one thing not to know the truth about something, or to be honestly mistaken; it’s another thing not to care what the truth is, and pretend as if the opposite were true. Regardless of the subject, people with this habit don’t tend to make very good or reliable friends. How could they? They’re not very reliable or stable with themselves. So you could never count on them.

Michael J. Hurd, Daily Dose of Reason