We must not confuse a person's opinion of reality with distortion of what they know.
Ultimate truth is what conforms to reality. Falsehood is what does not. But to pass off what one thinks is reality when it is not reality is not the same as distorting what one knows to be the case.
There are 4 scenarios
- Agreeing with reality and saying so
- Agreeing with reality and saying other
- Disagreeing with reality and saying so
- Disagreeing with reality and saying other
The problem is we have disagreements over what reality is. Differing opinions may be related to arguing at cross purposes or real disagreement. If the opinions are incompatible it may be that both persons are wrong, but if one of them is correct, logically the other must be incorrect. The person arguing for the incorrect position corresponds to scenario 3. To call that person a liar is, in fact, not correct. The difficulty is the argument is over which position is correct. Calling the other person a liar at least implies that one is certain they are correct (they may be, but this misses the point there is disagreement); it may also imply the other person is misrepresenting what they know to be true, whereas they may actually believe their incorrect position.
So to prove someone a liar one needs to demonstrate the person is aware of some fact that contradicts their position and they were hiding this knowledge to suit their purposes.
This is important. Calling someone a liar seems to be yet another common way of refusing to debate the issues. It is really a form of equivocation: someone claims that not being in agreement with the facts (scenario 3) is as an adequate definition of liar, but in tarring someone as a liar suggests they are in the position of misrepresenting what they know to be true (scenario 2), and it is this (the implied scenario not the actual one) which is seen as a moral failure. Whether lying is ever acceptable is another topic.
And what of scenario 4, being in the position of telling what you think is a lie but in actuality corresponds to reality. Well that still makes you a liar, however if people act on your lies it is likely to result in less damage to society than scenario 2 (and perhaps scenario 3).