When we turn from Aristotle’s observations in the department of natural history to his discussion of the actual mechanism of the living body, the subject now contained under the heading _Experimental Physiology_, we are in the presence of much less satisfactory material. Aristotle here exhibits his weakness in physics and not being endowed with any experimental knowledge of that subject his physiological development is very greatly handicapped. He seems often to accept fancies of his own in place of generalizations from collated observations. This tendency of his was conveyed to his successors and delayed physiological advance for many centuries. It forms a striking contrast to the method of certain of the Hippocratic works such as the _Epidemics_ and the _Aphorisms_ which exhibit an investigator intent on recording actual observations and on deducing general laws therefrom. Had the Hippocratic method been extended by Aristotle beyond the field of natural history, where he freely follows it, to that of physiology, the succeeding generations might have established medicine far more firmly as a science. An important factor in Aristotle’s physical and physiological teaching is the doctrine that matter is continuous and not made up of indivisible parts. He thus rejected the atomic views of his predecessors Leucippus and Democritus which have been preserved for us by the poem of Lucretius. The different kinds of matter existing merely in their state of simple mixture formed various uniform or homogeneous substances, _homoeomeria_, of which the _tissues_ of living bodies provided one type. We now consider tissues as having structure made up of living cells or their products, but to Aristotle their structure was an essential fact following on their particular elemental constitution. The structure of muscle or flesh was perhaps comparable to that of a crystalline substance, for, as we have seen, Aristotle made no fundamental distinction between organic and inorganic _substances_, which are in his view alike subject to the processes of generation and corruption. The difference between them lies not in their structure but in their potential relation to the various degrees of soul, the vegetative, the animal, and the rational. ‘There are’, says Aristotle, ‘three degrees of composition, and of these the first in order is composition out of what some call the _elements_, earth, air, water, and fire.... ‘The second degree of composition is that by which the _homogeneous_ parts of animals (ὁμοιομερῆ), such as bone, flesh, and the like, are constituted out of [these] primary substances. ‘The third and last stage is the composition which forms the _heterogeneous_ parts (ἀνομοιομερῆ) such as face, hand, and the rest.’[62] [62] _De partibus animalium_, ii. 1; 646ᵅ 12.