The topic which examines into causes they say is also divisible into two parts; and with reference to one of its considerations, the investigations of physicians partake of it; according to which it is that they investigate the dominant principle of the soul, and the things which exist in the soul, and seeds, and things of this kind. And its other division is claimed as belonging to them also by the mathematicians, as, for instance, how we see, what is the cause of our appearance being reflected in a mirror, how clouds are collected, how thunder is produced, and the rainbow, and the halo, and comets, and things of that kind. LXVIII. They think that there are two general principles in the universe, the active and the passive. That the passive is matter, an existence without any distinctive quality. That the active is the reason which exists in the passive, that is to say, God. For that he, being eternal, and existing throughout all matter, makes everything. And Zeno, the Cittiæan, lays down this doctrine in his treatise on Essence, and so does Cleanthes in his essay on Atoms, Chrysippus in the first book of his Investigations in Natural Philosophy, towards the end, Archedemus in his work on Elements, and Posidonius in the second book of his treatise on Natural Philosophy. But they say that principles and elements differ from one another. For that the one had no generation or beginning, and will have no end; but that the elements may be destroyed by the operation of fire. Also, that the elements are bodies, but principles have no bodies and no forms, and elements too have forms. Now a body, says Apollodorus in his Natural Philosophy, is extended in a threefold manner; in length, in breadth, in depth; and then it is called a solid body; and the superficies is the limit of the body having length and breadth alone, but not depth. But Posidonius, in the third book of his Heavenly Phænomena, will not allow a superficies either any substantial reality, or any intelligible existence. A line is the limit of a superficies, or length without breadth, or something which has nothing but length. A point is the boundary of a line, and is the smallest of all symbols. They also teach that God is unity, and that he is called Mind, and Fate, and Jupiter, and by many other names besides. And that, as he was in the beginning by himself, he turned into water the whole substance which pervaded the air; and as the seed is contained in the produce, so too, he being the seminal principle of the world, remained behind in moisture, making matter fit to be employed by himself in the production of those things which were to come after; and then, first of all, he made the four elements, fire, water, air, and earth. And Zeno speaks of these in his treatise on the Universe, and so does Chrysippus in the first book of his Physics, and so does Archedemus in some treatise on the Elements.