A lie is a lie and I can tell that whoever asked this idiotic question is intentionally trying to get a rise out of people and is a cheap way to go about it. If a child believes in Santa Clause does that make him real? If I believe that dragons existed does make it so?
If you believe a lie it's still a lie. The fact of the matter can't change based on someones mistaken belief. If you are delusional and ignore the lie or ignorant and don't understand that it's not a lie, it is still not going to happen or come into being
Webster define's lie as "..A false statement made with deliberate intent to deceive; an intentional untruth; a falsehood." One needs deliberate intent to deceive or to tell an intentional untruth...Therefore to tell an untruth that you believe is not a lie. This can make you ignorant, uninformed, and possibly an idiot...But telling an untruth you believe does not make you a liar.
It's a subject from which I'm torn between. I was watching that very "Seinfeld" episode and it made me think. At what point is there a line between outright slander and believing something, though it may not be true. I wonder how it works in law. What are your ideas?