Heracleitus was a profound observer and theorist. His physical theory foreshadowed the modern theories of natural law and of relativity; his practical theories reappear in the psychology of Protagoras and the ethics of the Stoics. =4. The Eleatic School.= The town of Elea to which Xenophanes came in the course of his wanderings had been recently settled by the Ionian refugees from Phocæa, a great maritime city in Asia Minor, which had been conquered by the Persians (543 B. C.). Elea is now Castellamare on the west coast of Italy. It is celebrated as the birthplace of Parmenides and Zeno, who founded the so-called Eleatic school. a. =Parmenides= (b. 515 B. C.). Parmenides wrote about 470 B. C. He is represented as a serious and influential man, with a high moral character. He exercised strong influence upon such philosophers as Plato and Democritus, and was a political power in the city of Elea, of which he was a native. He was not a stranger to the Pythagoreans. The large fragment of his poem is the most ancient monument extant of metaphysical speculation among the Greeks. Parmenides takes the doctrine of Xenophanes with great seriousness, and what Xenophanes says about the Godhead, Parmenides says about all things. Xenophanes’ religious weapon of an unchanging cosmic substance becomes in the hands of Parmenides an academic doctrine of science and the basis of logical controversy. Parmenides used the conception of Xenophanes in his great didactic poem, _The Way of Truth and the Way of Opinion_, with the evident purpose of refuting the theory of Heracleitus. The fragment of the poem reveals the driest abstractions dressed in rich poetry. As a thinker Parmenides is the most important in this period. Zeno was the friend and pupil of Parmenides. (1) =The Cosmic Substance is Being.= The first assumption in the Milesian doctrine――that there is a single matter that ever remains identical with itself――was so self-evident to Parmenides that he does not attempt to prove it. He assumes it, as if it were cogent to everybody. However, he explains what he means by Being in a negative statement: Not-Being, or what is not, cannot be thought. Being and thought are so correlated that they are the same. Thinking always has Being as its content, and there is no Being that is not thought. _Being_ ＝ _Thought_. This explanation of Parmenides’ identification of thought and Being may be put in this logical form:―― All thinking refers to something thought, and therefore has Being for its content; Thinking that refers to nothing, and is therefore contentless, cannot be; Therefore, not-Being cannot be thought, much less can it be.