A bet is the quickest way to see if someone is intellectually honest or dishonest. It instantly makes someone reevaluate their position, and if they refuse to bet, they show that their position is dubious.
Julian Simon seems to have understood this from an early age. From The Bet:
“I first learned to say ‘Do you want to bet?’ as part of an argument about facts when talking to my father,” Julian Simon wrote in his autobiography, A Life Against the Grain. “He would say outrageously wrong things in an authoritative fashion and refuse to hear any questions. There was nothing I could say except “Do you want to bet?”