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A lie is a lie and a liar is a liar, but those who lie and swallow their own lie as the truth are worse. What makes matters even worse is that they drag others along with them because the voice of their own self sounds truthful to them. They don't pretend to believe in their lie, they actually do believe. A mind that clings to a wrong notion with all its might breaks more than a mind that would merely pretend.
A willing liar still has that real story somewhere in the equation. Once the inner story is the sole story, there's no way to convince them, and that makes them more dangerous. From that mentality every move they make looks right in their own sight. Harm to others would resemble safety and progress, and destruction resembles salvation.
Such people cause the most destruction when they have power, whether financial power or political power or power that comes with a high ranking position in an institutions
I see something like this happen in public discourse. We tend to cling to our opinions as closely as we can even if it's wrong. We provide shreds of fact entangled with private bias and call the combination evidence of a subjective truth.
We make up rumors without any form of authentic verification. Regardless of believes and opinions, there's factual truth and it exists. However some people hold their own opinions as facts and never let go.
The remedy to this is plain humility because the main cause of such a tendency is pride. The unwillingness to accept that you can be wrong and subject to mistakes and should allow yourself to be corrected.
You need to adapt the practice of saying "I may be wrong." Those words alone keep your own mind open and your heart flexible enough to accept positive change. Truth never does taste good. The truth hurts they say, and some who don't want to endure the pain would rather hold tightly to a lie. Those people are in a bad position but for those that think their lies are true are even worse. It really is the worst case scenario.
Perhaps the real lesson is not just to beware of liars who think they tell the truth but to make sure we are not becoming one of them ourselves and to make sure we don't appoint people like that in positions of authority and power.