Expostulate means "to reason earnestly," "to remonstrate," "to demand": "The policeman expostulated with the motorist about the dangers of fast driving." Postulate is derived from the same Latin word as expostulate but means "to claim, assume, ask, or request without any degree of urgency or force": "The lecturer postulated the idea that all of us are selfish."
Dictionary of problem words and expressions. Harry Shaw. 1975.