Why is "I'm lying" a paradox and "he's lying" a proposition?

"I'm lying." When I said this, was he lying to everyone or telling everyone frankly that he was lying? ! The meaning of this sentence can be explained to you by a multiplication formula, positive and negative get negative, negative get negative, so it is a paradox; "He is lying" is a statement. Of course, the next step is to clarify his point of view, so it is a proposition!