“Lying is second nature to him,” Schwartz said. “More than anyone else I have ever met, Trump has the ability to convince himself that whatever he is saying at any given moment is true, or sort of true, or at least ought to be true.” Often, Schwartz said, the lies that Trump told him were about money—“how much he had paid for something, or what a building he owned was worth, or how much one of his casinos was earning when it was actually on its way to bankruptcy.”
(From my link)