He conceived, therefore, that soul is in no way fundamentally distinct from any other of the transformations of the ever-living Fire. And thus the problem which so grievously torments modern psychologists, that of the connection between soul and body, did not exist for him. And a notable corollary of his view is this. Since man has essential kinship with his environment, he can apprehend both the outer surface of things and their inner law; and it is in this recognition of their inner law that his true nature is to be found. Now if it be granted that this inner law can be apprehended by intuition as well as by conscious reasoning process, the corollary is one to which the nature-mystic can of his own master principle. The soul, as fire, depends on the cosmic Fire for sustenance, the breath being the physical medium; and in this regard, all that was said of Anaximenes and "Breath," or Air, will have its place. But Heracleitus has a further thought which is in full harmony with the nature-mystic's chief contention. He holds that _sense perception_ is also a medium, for the outer fire is thereby absorbed by the inner fire. The value of this thought remains in spite of the sage's doctrine of the body. For though the body is regarded by him as a clog on the activity of the inner fire, because it consists of water and earth (two forms in which the movement of the Fire is greatly reduced) it is nevertheless akin to the soul, and is itself destined, in the course of ceaseless change, to become Fire in its most living and active form. Such is the central doctrine of this noted thinker, round which all his other teaching turned. Let us now ask, as in the corresponding cases of Thales and Anaximander, why the particular element was chosen as the Ground of all things. The answer to this question will furnish, as in the previous cases, much matter for our special purpose, since the emphasis will lie rather on the physical properties and functions of fire, than on its more abstract ontology. It is obvious that Heracleitus would start with a knowledge of the speculations of his more immediate predecessors, and of the data on which they were based--the phenomena of circulation in nature, evaporation, mist, rain, melting, freezing, and the rest. And we find that in this direction he merely amplified the older systems, taking fire, instead of water or air, as his _Welt-stoff_. He also observed, with special care, certain suggestive cases of rarefaction by heat and condensation by cold; as also the facts of constant decomposition and renewal in the vegetable and animal worlds. But the phenomenon which stands out as the chiefest determinant of his thought is one which is always bound to act as a powerful stimulant on a thoughtful mind--that of combustion.