  Others have taken a different course on the issue of the consistency of change. Herakleitos (C6th BCE) wrote in a suggestive fashion, with his doctrine of the unity of opposites. However, his few surviving sentences are too obscure and fragmentary to give much confidence in interpretation. He spoke of the same river having different waters at different times, but there is no development of the observation. Similarly he spoke of the sea as being at one time both life-preserving (to fish) and death-dealing (to humans), and “the path up and the path down are one and the same.” These examples hardly force one to believe in true contradictions, however.