soul, seeds, and the like. Whereas the other part is claimed by the mathematicians also, e.g. how vision is to be explained, what causes the image on the mirror, what is the origin of clouds, thunder, rainbows, halos, comets, and the like. They hold that there are two principles in the universe, the active principle and the passive. The passive principle, then, is a substance without quality, i.e. matter, whereas the active is the reason inherent in this substance, that is God. For he is everlasting and is the artificer of each several thing throughout the whole extent of matter. This doctrine is laid down by Zeno of Citium in his treatise On Existence, Cleanthes in his work On Atoms. Chrysippus in the first book of his Physics towards the end, Archedemus in his treatise On Elements, and Posidonius in the second book of his Physical Exposition. There is a difference, according to them, between principles and elements ; the former being without generation or destruction, whereas the elements are destroyed when all things are resolved into fire. Moreover, the principles are incorporeal and destitute of form, while the elements have been endowed with form. Body is denned by Apollodorus in his Physics as that which is extended in three dimensions, length, breadth, and depth. This is also called solid body. But surface is the extremity of a solid body, or that which has length and breadth only without depth. That surface exists not only in our thought but also in reality is maintained by Posidonius in the third book of his Celestial Phenomena. A line is the extremity of a surface or length without breadth, or that which has length alone. A point is the extremity of a line, the smallest possible mark or dot.