His teaching, as we have it now, is a series of epigrams. This means his teachings are sayings and remarks, rather than systematic essays. Heraclitus is famous for his doctrine of change being central to the universe. His famous sayings, "All is flux", and "You cannot step twice into the same river" is still remembered today. Another of his sayings appeals to some psychologists:
"You cannot discover the depths of the psyche, even if you travelled every road to do so, such is the depth of its meaning".