And as there are three kinds of lives; the theoretical, the practical, and the logical; they say that the last is the one which ought to be chosen. For that a logical, that is a rational, animal was made by nature on purpose for speculation and action. And they say that a wise man will very rationally take himself out of life, either for the sake of his country or of his friends, or if he be in bitter pain, or under the affliction of mutilation, or incurable disease. And they also teach that women ought to be in common among the wise, so that whoever meets with any one may enjoy her, and this doctrine is maintained by Zeno in his Republic, and by Chrysippus in his treatise on Polity, and by Diogenes the Cynic, and by Plato; and then, say they, we shall love all boys equally after the manner of fathers, and all suspicion on the ground of undue familiarity will be removed. They affirm too, that the best of political constitutions is a mixed one, combined of democracy, and kingly power, and aristocracy. And they say many things of this sort, and more too, in their Ethical Dogmas, and they maintain them by suitable explanations and arguments. But this may be enough for us to say of their doctrines on this head by way of summary, and taking them in an elementary manner. LXVII. They divide natural philosophy into the topics of bodies, and of principles, and of elements, and of Gods, and of boundaries, and of place, and of the vacuum. And they make these divisions according to species; but according to genera they divide them into three topics, that of the world, that of the elements, and the third is that which reasons on causes. The topic about the world, they say, is subdivided into two parts. For that in one point of view, the mathematicians also have a share in it; and according to it it is that they prosecute their investigations into the nature of the fixed stars and the planets; as, for instance, whether the sun is of such a size as he appears to be, and similarly, whether the moon is; and in the same way they investigate the question of spherical motion, and others of the same character. The other point of view is that which is reserved exclusively for natural philosophers, according to which it is that the existence and substance of things are examined, [for instance, whether the sun and the stars consist of matter and form,] and whether the sun is born or not born, whether it is living or lifeless, corruptible or incorruptible, whether it is regulated by Providence, and other questions of this kind.