To draw a line through Peter's slip in the very tricky use of English to convey meaning, please consider the following phrases.....

In English we can say we dis-believe someon in many different ways...

"He is being economical with the truth"
"You're telling fibs"
"you're telling porkies" (English English only)
"You're mistaken"

It is a disciplinary offence in the English parliament to call someone a liar.

"I think the honorable member opposite is mistaken....."

Steve