The poem on Nature is addressed to Pausanias the physician, who was a son of Anchitus of Agrigentum, and a special friend of Empedocles. To Pausanias the philosopher begins his instruction with these words: "First learn what are the four chief roots of everything that is: fiery Zeus, and Herè, source of vital breath, and Aidoneus, and Nestis with her tears, who is the fount of moisture in the world." Thus Empedocles, after the fashion of the Pythagoreans, allegorized his four elements. In other passages he calls them "fire, water, earth, and air's immeasurable height;" or, "earth, and ocean with his countless waves, and liquid air, the sun-god, and ether girdling the universe in its embrace;" or again, "Hephæstus, rain, and radiant ether;" or lastly, "light, earth, heaven, and ocean." It will be seen that he designated his elements sometimes by mythological titles, sometimes by abstract terms, and sometimes by selecting one or other natural object--such as the sun, the air, the ocean--in which they were most manifest. It is well known that Empedocles was the first philosopher to adopt the four elements, which, since his day, continued to rule supreme over natural science, until modern analysis revealed far simpler and broader bases. Other speculators of the Ionian sect had maintained each of these four elements--Thales the water, Anaximenes the air, Heraclitus the fire, and perhaps (but this rests on no sure evidence), Pherecydes the earth. Xenophanes had said, "Of earth and water are all things that come into existence." Parmenides had spoken of dark and light, thick and subtile, substances. Each of these fundamental principles is probably to be regarded not as pure fire, or pure water, or pure air, but as a universal element differing in rarity, and typified according to the analogical necessities of language, by means of some familiar object. The four elements of Empedocles appear to have been suggested to him, partly by his familiarity with contemporary speculation, and partly by his observation of Nature. They held their ground so long in scientific theory, because they answered so exactly to a superficial view of the world. Earth with everything of a solid quality, water including every kind of fluid, fire that burns or emits light, air that can be breathed, appear to constitute an exhaustive division of the universe. Of the eternity of these four primal substances, according to the Empedoclean theory, there is no doubt. The philosopher frequently reiterates his belief in the impossibility of an absolute beginning or ending, though he acquiesces in the popular use of these terms to express the scientific conceptions of dissolution and recombination.