The doctrine of the four elements was also adopted by Plato and amplified by Aristotle, with whose name indeed it is commonly associated. Aristotle, the greatest scientific thinker among the Greeks, exercised an authority almost supreme in Europe during nearly twenty centuries. His influence is to be traced throughout the literature of chemistry long after the time of Boyle. It may be detected even now. Probably few who write chemical memoirs to-day, and who follow the time-honoured practice of prefacing their own contributions to knowledge by a statement of what is already known on the subject, are aware that in so doing they are obeying the injunctions of Aristotle. His theory of the nature of matter is contained in his treatise on _Generation and Destruction_. It mainly differed from that of Empedokles in regarding the four “elements” as mutually convertible. Each “element” or principle was regarded as being possessed of two qualities, one of which was shared by another element or principle. Thus: Fire is hot and dry; air is hot and wet; water is cold and wet; earth is cold and dry. In each primal “element” one quality prevails. Fire is more hot than dry; air is more wet than hot; water is more cold than wet; earth is more dry than cold. The relative proportion and mutual working of these qualities determined the specific character of the “element.” Thus, if the dryness of fire is overcome by the moisture of water, air is produced; if the heat of air is overcome by the coldness of earth, water is formed; if the moisture of water is overcome by the dryness of fire, earth results. Ancient chemical literature contains many illustrations or diagrams symbolising the convertibility or mutual relations of the four “elements.”