No man ever steps in the same river twice Definition
Heraclitus was famous for his insistence on ever-present change as being the fundamental essence of the universe, as stated in the famous saying, "No man ever steps in the same river twice" (see panta rhei below). This is commonly considered to be a key contribution in the development of the philosophical concept of becoming, as contrasted with "being", and has sometimes been seen in a dialectical relationship with Parmenides' statement that "whatever is, is, and what is not cannot be", the latter being understood as a key contribution in the development of the philosophical concept of being. For this reason, Parmenides and Heraclitus are commonly considered to be two of the founders of ontology. Scholars have generally believed that either Parmenides was responding to Heraclitus, or Heraclitus to Parmenides, though opinion on who was responding to whom has varied over the course of the 20th and 21st centuries. Heraclitus' position was complemented by his stark commitment to a unity of opposites in the world, stating that "the path up and down are one and the same". Through these doctrines Heraclitus characterized all existing entities by pairs of contrary properties, whereby no entity may ever occupy a single state at a single time. This, along with his cryptic utterance that "all entities come to be in accordance with this Logos" (literally, "word", "reason", or "account") has been the subject of numerous interpretations.