During the century and a half between Thales and the beginning of the Peloponnesian war, we have passed in review twelve distinct schemes of philosophy--Thales, Anaximander, Anaximenes, Xenophanes, Pythagoras, Parmenides, Herakleitus, Empedokles, Anaxagoras, the Apolloniate Diogenes, Leukippus, and Demokritus. Of most of these philosophers it may fairly be said that each speculated upon nature in an original vein of his own. Anaximenes and Diogenes, Xenophanes and Parmenides, Leukippus and Demokritus, may indeed be coupled together as kindred pairs yet by no means in such manner that the second of the two is a mere disciple and copyist of the first. Such abundance and variety of speculative genius and invention is one of the most memorable facts in the history of the Hellenic mind. The prompting of intelligent curiosity, the thirst for some plausible hypothesis to explain the Kosmos and its generation, the belief that a basis or point of departure might be found in the Kosmos itself, apart from those mythical personifications which dwelt both in the popular mind and in the poetical Theogonies, the mental effort required to select some known agency and to connect it by a chain of reasoning with the result--all this is a new phenomenon in the history of the human mind. [Side-note: Difficulties which a Grecian philosopher had to overcome--prevalent view of Nature, established, impressive, and misleading.] An early Greek philosopher found nothing around him to stimulate or assist the effort, and much to obstruct it. He found Nature disguised under a diversified and omnipresent Polytheistic agency, eminently captivating and impressive to the emotions--at once mysterious and familiar--embodied in the ancient Theogonies, and penetrating deeply all the abundant epic and lyric poetry, the only literature of the time. It is perfectly true (as Aristotle remarks[2]) that Hesiod and the other theological poets, who referred everything to the generation and agency of the Gods, thought only of what was plausible to themselves, without enquiring whether it would appear equally plausible to their successors; a reproach which bears upon many subsequent philosophers also. The contemporary public, to whom they addressed themselves, knew no other way of conceiving Nature than under this religious and poetical view, as an aggregate of manifestations by divine personal agents, upon whose volition--sometimes signified beforehand by obscure warnings intelligible to the privileged interpreters, but often inscrutable--the turn of events depended. Thales and the other Ionic philosophers were the first who became dissatisfied with this point of view, and sought for some "causes and beginnings" more regular, knowable, and predictable. They fixed upon the common, familiar, widely-extended, material substances, water, air, fire, &c.; and they could hardly fix upon any others. Their attempt to find a scientific basis was unsuccessful; but the