 As for the eternity of duration, al-Razi noted that time (the relative temporal notion) is constantly perishing and coming to be inasmuch as all past time has ceased to be, while future time is constantly coming to be. In this case, time itself is changing, but every change requires a duration during which it changes. If this duration itself underwent change, there would need to be a further duration and so on infinitely. To stop the regress al-Razi posited an unchanging duration, which is itself the precondition for the measured time of those philosophers following Aristotle.