=196.= Having determined that the earth is the centre of the universe, and the sun above it, the way is clear to incorporate in the system the doctrine of the four elements (στοιχεῖα, _naturae_)[34], which probably had its origin in a cruder form of physical speculation than the doctrine of the heavenly bodies. As we have seen above[35], the elements are not first principles of the Stoic physics, but hold an intermediate position between the two principles of the active and the passive on the one hand, and the organic and inorganic world on the other. Earth is the lowest of the elements, and also the grossest; above it is placed water, then air, then fire; and these are in constant interchange, earth turning to water, this into air, and this into aether, and so again in return. By this interchange the unity of the universe is maintained[36]. The transition from one element to the next is not abrupt, but gradual; the lowest part of the aether is akin to air[37]; it is therefore of no great importance whether we speak with Heraclitus of three elements, or with Empedocles of four. The two grosser elements, earth and water, tend by nature downwards and are passive; air and fire tend upwards and are active[38]. Zeno did not think it necessary to postulate a fifth element as the substance of soul, for he held that fire was its substance[39]. [Sidenote: Fire and breath.] =197.= Fire, heat, and motion are ultimately identical, and are the source of all life[40]. Thus the elemental and primary fire stands in contrast with the fire of domestic use; the one creates and nourishes, the other destroys[41]. It follows that fire, though it is one of the four elements, has from its divine nature a primacy amongst the elements[42], which corresponds to its lofty position in the universe[43]; and the other elements in turn all contain some proportion of fire. Thus although air has cold and darkness as primary and essential qualities[44], nevertheless it cannot exist without some share of warmth[45]. Hence air also may be associated with life, and it is possible to retain the popular term ‘spirit’ (πνεῦμα, _spiritus_) for the principle of life. In the development of the Stoic philosophy we seldom hear again of air in connexion with coldness; and between the ‘warm breath’ (_anima inflammata_) and the primary fire there is hardly a distinction; we may even say that ‘spirit’ has the highest possible tension[46]. [Sidenote: God in the stone.]