Heraclitus from Ephesus, who lived around 500 BC, argues, in a fragment of his work On Nature, that reason should look beyond common sense and realize that the appearance of stability and permanence presented to our senses is false. All things are in a constant flux, even the 'unchanging' hills. Everything flows. His follower Cratylus popularized his teaching: 'You cannot step twice in the same river. Like the Taoists, Heraclitus saw change as a dynamic interplay of opposites: 'cold things warm themselves, warm cools'. 'He concluded that since all opposites are polar they are united: 'The up and the down is one and the same. Change takes place dialectically through the dynamic unity of opposites.