How do you reason with a lunatic?

You can't reason with a lunatic.

But how do you know when someone is truly nuts?  I think it is when you can't reason with such a person.

If a person states something is truth and will not brook any dissent on the matter, there's no reasoning with that person.  Is that person insane?  How do you answer such a question?

Let's say that two people see the same event.  There's a discussion about what happened, and one of the two persons declares that his version of events is the only one that is correct.  Someone who was not a party to the event and did not see it, would have to make a judgment about who is right and who is wrong.  How do you do that when one of the people will not listen to any other version of events?

Perhaps that is not a good example.  There has to be some standard of evidence and verification of accuracy.  For the example above, if one person truly has a problem perceiving what actually happens in front of him, his words are not reliable.  Or perhaps both of them are unreliable.  How do you judge that?  The only way to judge that is to submit to some standard of evidence.  But if you declare that your standard is the only one that matters or counts, you may be a little off your rocker.

No?

If you are going to make an assertion about what is truth, you need to be able to back it up.  If you are going to assert that what you say is unvarnished truth, you have to be a little bit off your rocker if you are going to expect to be believed provided that you don't have the evidence that is clearly and reliably in your favor.

So, what I'm getting at is the possibility of something that I didn't want to believe.  But the evidence seems to be there that someone may be a little bit nuts.

I have in mind someone, but I won't mention who that person is.