If you tell him what happened, I'm sure he'll understand - he's a reasonable man.

reasonable 由词根 reason（理性、道理）和后缀 -able（能够...的）组成，字面意思是"能够讲道理的"。在句中形容人时表示"通情达理的、明事理的"，指这个人能够基于事实和逻辑做出合理判断。