This explanation is based on Aristotle’s fundamental doctrine of the opposite _qualities_, heat, cold, wetness, and dryness, that are found combined in pairs in the four _elements_, earth, air, fire, and water. The theory was of the utmost importance for the whole subsequent development of science and was not displaced until quite modern times. It was not an original conception of Aristotle, for something resembling it had been set forth long before his time in figurative language by Empedocles (_c._ 500-_c._ 430 B. C.), as Aristotle himself tells us.[44] The same view had been foreshadowed by Pythagoras (_c._ 580-_c._ 490 B. C.) at an even earlier date and was perhaps of much greater antiquity. But Aristotle developed the doctrine and was the main channel for its conveyance to later ages, so that his name will always be associated with it. Matter in general and living matter in particular was held by him to be composed of these four essential so-called _elements_ (στοιχεῑ), each of which is in turn compounded from two of the primary _qualities_ (δυνάμεις) which Aristotle brought into relation with the elements. Thus earth was cold and dry, water cold and wet, air hot and wet, and fire hot and dry (Fig. 7b). [44] _Metaphysics_, i. 4. _De generatione et corruptione_, ii. 1. [Illustration: Fig. 7b. The Four Elements and the Four Qualities.] The theory of the elements and qualities is applicable to all matter and not specially to living things. The distinction between the living and not-living is to be sought not so much in its material constitution, but in the presence or absence of ‘soul’, and his teaching on that topic is to be found in his great work περὶ ψυχῆς, _On Soul_. He does not think of matter as organic or inorganic—that is a distinction of the seventeenth century physiologists—nor does he think of things as divided into animal, vegetable, and mineral—that is a distinction of the mediaeval alchemists,—but he thinks of things as either with soul or without soul (ἔμψυχα or ἄαψυχα).