But he lived at poor peace with himself, and now he despaired of being understood. He probably committed that irreversible act because scientists attacked his ideas about irreversibility. If time ran backward, the collision would reverse itself perfectly. Then suppose the motion of each molecule could somehow be reversed. That's as silly as it is logical.
Time looks directionless on the molecular level, where motion is perfectly reversible.
But nothing is so perfect in our larger scale of sensory awareness. Here in the visible world the past cannot be undone. Time's arrow flies from past to future. Air never flows back into the tank. Boltzmann turned superb mathematics on the question.