Jamieson, Fausset and Brown on Jeremiah 32:8

8. Then I knew--Not that Jeremiah previously doubted the reality of the divine communication, but, the effect following it, and the prophet's experimentally knowing it, confirmed his faith and was the seal to the vision. The Roman historian, Florus (2.6), records a similar instance: During the days that Rome was being besieged by Hannibal, the very ground on which he was encamped was put up for sale at Rome, and found a purchaser; implying the calm confidence of the ultimate issue entertained by the Roman people.