Both these first principles, it must be understood, were conceived of as bodies, though without form, the one everywhere interpenetrating the other. To say that the passive principle, or matter, is a body comes easy to us, because of the familiar confusion adverted to above. But how could the active principle, or God, be conceived of as a body? The answer to this question may sound paradoxical. It is because God is a spirit. A spirit in its original sense meant air in motion. Now the active principle was not air, but it was something which bore an analogy to it--namely aether. Aether in motion might be called a 'spirit' as well as air in motion. It was in this sense that Chrysippus defined the thing that is, to be a spirit moving itself into and out of itself, or spirit moving itself to and fro. From the two first principles which are ungenerated and indestructible must be distinguished the four elements which, though ultimate for us, yet were produced in the beginning by God and are destined some day to be reabsorbed into the divine nature. These with the Stoics were the same which had been accepted since Empedocles--namely earth, air, fire and water. The elements, like the two first principles were bodies; unlike them, they were declared to have shape as well as extension. An element was defined as that out of which things at first come into being and into which they are at last resolved. In this relation did the four elements stand to all the compound bodies which the universe contained. The terms earth, air, fire and water had to be taken in a wide sense: earth meaning all that was of the nature of earth, air all that was of the nature of air and so on. Thus, in the human frame, the bones and sinews pertained to earth. The four qualities of matter--hot, cold, moist and dry--were indicative of the presence of the four elements. Fire was the source of heat, air of cold, water of moisture, and earth of dryness. Between them, the four elements made up the unqualified being called Matter. All animals and other compound natures on earth had in them representatives of the four great physical constituents of the universe, but the moon, according to Chrysippus, consisted only of fire and air, while the sun was pure fire. While all compound bodies were resolvable into the four elements, there were important differences among the elements, themselves. Two of them, fire and air, were light; the other two, water and earth, were heavy. By 'light' was meant that which tends away from its own centre, by 'heavy,' that which, tends towards it. The two light elements stood to the two heavy ones in much the same relation as the active to the passive principle generally. But further, fire had such a primary as entitles it, if the definition of element were pressed, to be considered alone worthy of the name. For the three other elements arose out of it and were to be again resolved into it.