Much discussion has been set up by the common attribution to Herakleitos in antiquity of the doctrine of the ultimate conflagration of all things. But for this there is no ground in any actual passage preserved from his works; and it appears to have been a mere misconception of his doctrine in regard to Fire. His monistic doctrine was, in brief, that all the opposing and contrasted things in the universe, heat and cold, day and night, evil and good, imply each other, and exist only in the relation of contrast; and he conceived fire as something in which opposites were solved. [519] Upon this stroke of mysticism was concentrated the discussion which might usefully have been turned on his criticism of popular religion; his negative wisdom was substantially ignored, and his obscure speculation, treated as his main contribution to thought, was misunderstood and perverted. A limit was doubtless soon set to free speech even in Elea; and the Eleatic school after Xenophanes, in the hands of his pupil Parmenides (fl. 500 B.C.), Zeno (fl. 464), Melissos of Samos (fl. 444), and their successors, is found turning first to deep metaphysic and then to verbal dialectic, to discussion on being and not being, the impossibility of motion, and the trick-problem of Achilles and the tortoise. It is conceivable that thought took these lines because others were socially closed. Parmenides, a notably philosophic spirit (whom Plato, meeting him in youth, felt to have "an exceptionally wonderful depth of mind," but regarded as a man to be feared as well as reverenced), [520] made short work of the counter-sense of not being, but does not seem to have dealt at close quarters with popular creeds. Melissos, a man of action, who led a successful sally to capture the Athenian fleet, [521] was apparently the most pronounced freethinker of the three named, [522] in that he said of the Gods "there was no need to define them, since there was no knowledge of them." [523] Such utterance could not be carried far in any Greek community; and there lacked the spirit of patient research which might have fruitfully developed the notable hypothesis of Parmenides that the earth is spherical in form. [524] But he too was a loose guesser, adding categories of fire and earth and heat and cold to the formative and material "principles" of his predecessors; and where he divagated weaker minds could not but lose themselves. From Melissos and Parmenides there is accordingly a rapid descent in philosophy to professional verbalism, popular life the while proceeding on the old levels.