According to Aristotle, the Heracleitean flux was inconsistent with the highest law of thought, and made all predication impossible. It has been shown that the master himself recognised a fixed recurring order of change which could be affirmed if nothing else could. But the principle of change, once admitted, seemed to act like a corrosive solvent, too powerful for any vessel to contain. Disciples were soon found who pushed it to extreme consequences with the effect of abolishing all certainty whatever. In Plato’s time it was impossible to argue with a Heracleitean; he could never be tied down to a definite statement. Every proposition became false as soon as it was uttered, or rather before it was out of the speaker’s mouth. At last, a distinguished teacher of the school declined to commit himself by using words, and disputed exclusively in dumb show. A dangerous speculative crisis had set in. At either extremity of the Hellenic world the path of scientific inquiry was barred; on the one hand by a theory eliminating non-existence from thought, and on the other hand by a theory identifying it with existence. The luminous beam of reflection had been polarised into two divergent rays, each light where the other was dark and dark where the other was light, each denying what the other asserted and asserting what the other denied. For a century physical speculation had taught that the universe was formed by the modification of a single eternal substance, whatever that substance might be. By the end of that period, all becoming was absorbed into being at Elea, and all being into becoming at Ephesus. Each view contained a portion of the truth, and one which perhaps would never have been clearly perceived if it had not been brought into exclusive prominence. But further progress was impossible until the two half-truths had been recombined. We may compare Parmenides and Heracleitus to two lofty and precipitous peaks on either side of an Alpine pass. Each commands a wide prospect, interrupted only on the side of its opposite neighbour. And the fertilising stream of European thought originates with neither of them singly, but has its source midway between. IV.