His poem on "Nature" was composed in the epic metre, and in it he expressed in beautiful forms the most abstract ideas. Zeno of Elea (fl. 500 B.C.) was a pupil of Parmenides, and the earliest prose writer among the Greek philosophers. He developed the doctrines of his master by showing the absurdities involved in the ideas of variety and of creation, as opposed to one and universal substance. Other philosophers belonging to Iona or Elea may be referred to these schools, as Heraclitus, Empedocles, Democritus, and Anaxagoras, whose doctrines, however, vary from those of the representatives of the philosophical systems above named. Heraclitus (fl. 505 B.C.) dealt rather in intimations of important truths than in popular exposition of them; his cardinal doctrine seems to have been that everything is in perpetual motion, that nothing has any permanent existence, and that everything is assuming a new form or perishing: the principle of this perpetual motion he supposed to be _fixe_, though probably he did not mean material fire, but some higher and more universal agent. Like nearly all the philosophers, he despised the popular religion. Empedocles (fl. 440 B.C.) wrote a doctrinal poem concerning nature, fragments of which have been preserved. He denied the possibility of creation, and held the doctrine of an eternal and imperishable existence; but he considered this existence as having different natures, and admitted that fire, earth, air, and water were the four elements of all things. These elements he supposed to be governed by two principles, one positive and one negative, that is to say, connecting love and dissolving discord. Democritus (fl. 460 B.C.) embodied his extensive knowledge in a series of writings, of which only a few fragments have been preserved. Cicero compared him with Plato for rhythm and elegance of language. He derived the manifold phenomena of the world from the different form, disposition, and arrangement of the innumerable elements or atoms as they become united. He is the founder of the atomic doctrine. Anaxagoras (fl. 456 B.C.) rejected all popular notions of religion, excluded the idea of creation and destruction, and taught that atoms were unchangeable and imperishable; that spirit, the purest and subtlest of all things, gave to these atoms the impulse by which they took the forms of individual things and beings; and that this impulse was given in circular motion, which kept the heavenly bodies in their courses. But none of his doctrines gave so much offence or was considered so clear a proof of his atheism as his opinion that the sun, the bountiful god Helios, who shines both upon mortals and immortals, was a mass of red-hot iron. His doctrines tended powerfully by their rapid diffusion to undermine the principles on which the worship of the ancient gods rested, and they therefore prepared the way for the subsequent triumph of Christianity.