The ancient philosophers did not agree among themselves as to the nature and origin of matter; some of them considering it as eternal in its essence, and others as mutable and changeable in form. The theory of atoms, published by DEMOCRITUS, and subsequently carried out so elaborately by EPICURUS and his disciples, seems to have reached even to our own times, with an increasing reputation and acceptance. According to this theory, the kinds of matter, or elements, must he regarded as infinitely various. HERACLITUS, who taught philosophy about 550 years before Christ, considered all things as derived from an elemental heat or fire;[25] a philosophy which seems to us to have formed the basis of the Hippocratic doctrines of life. Like HERACLITUS, HIPPOCRATES tells us, that the calidum was the first principle of things, and that by an expansion or extension of itself, it constitutes all the objects of the material world. He expresses himself in the following manner. That which we call warmth, or heat, seems to me to be something immortal; something which comprehends all things, which sees and knows all things, as well present as future. Thus assuming as a basis, that the calidum is an almighty, all-wise being, or in other words, a God, all in all, the cosmogony was developed as follows: Chaos he regarded as that condition of the calidum, which preceded any exertion of the Almighty faculties. In emerging from the chaotic state, the greatest part of the heat having assumed the uppermost place, formed the aether; another part having gained the lowermost place, constituted earth; a third portion, midway between earth and aether, became air; and a fourth part, establishing itself between the two latter, became water. So that by means of the extension of this all-wise, elemental calidum, we have the four elements, earth, air, fire, and water, out of which are ultimately composed all the aggregates of the material world.[26] Now, to apply this general principle to the formation of the living being man, who seems to be a sort of microcosm in himself, we are told, that that portion of heat which remained with earth, being expanded and spread abroad in divers places, in some more, in others less, the earth became dry, and something like membrane or pellicle was formed; the matters contained in which, being heated as by a sort of putrefaction, some parts became bone, some nerve, some veins and their contents, and some formed the cavities and their contents, as the urinary bladder, for example.[27]