Knowing what separates wakeful reality from dream states seems rather simple on its surface. After all, even if a dream feels quite real in the moment, it’s unbound from continuity and the natural laws of our (presumed) waking lives. Yet proving that you’re awake, rather than just intuiting it, has been a perilous task for philosophers across the centuries. Beginning with the ancient Chinese philosopher Zhuang Zhou’s famed butterfly dream, this TED-Ed animation tackles how thinkers from Al-Ghazali in medieval Persia, to René Descartes and Thomas Hobbes in 17th-century France and England, to neuroscientists today have approached the question of whether we can ever truly know we’re awake.