On its philosophical side astrology was a system of astral theology developed, not by popular thought, but through the careful observations and speculations of learned priests and scholars. It was a purely Eastern science which came into being on the Chaldean plains and in the Nile valley. As far as we know, it was entirely unknown to any of the primitive Aryan races, from Hindostan to Scandinavia. Astrology as a system of divination never gained a foothold in Greece during the brightest period of her intellectual life. But the dogma of astral divinity was zealously maintained by the greatest of Greek philosophers. Plato, the great idealist, whose influence upon the theology of the ancient and even of the modern world was more profound than that of any other thinker, called the stars "visible gods" ranking them just below the supreme eternal Being; and to Plato these celestial gods were infinitely superior to the anthropomorphic gods of the popular religion, who resembled men in their passions and were superior to them only in beauty of form and in power. Aristotle defended with no less zeal the doctrine of the divinity of the stars, seeing in them eternal substances, principles of movement, and therefore divine beings. In the Hellenistic period, Zeno, the Stoic, and his followers proclaimed the supremacy of the sidereal divinities even more strongly than the schools of Plato and Aristotle had done. The Stoics conceived the world as a great organism whose "sympathetic" forces constantly interacted upon one another, governed by Reason which was of the essence of ethereal Fire, the primordial substance of the universe. To the stars, the purest manifestation of the power of this ethereal substance, were attributed the greatest influence and the loftiest divine qualities. The Stoics developed the doctrine of fatalism, which is the inevitable outcome of faith in stellar influence on human life, to its consequences; yet they proved by facts that fatalism is not incompatible with active and virtuous living. By the end of the Roman imperial period astrology had transformed paganism, replacing the old society of Immortals who were scarcely superior to mortals, except in being exempted from old age and death, by faith in the eternal beings who ran their course in perfect harmony throughout the ages, whose power, regulated by the unvarying celestial motions, extended over all the earth and determined the destiny of the whole human race.