Like his master Hippocrates, Galen attached fundamental importance to clinical observation--to the evidence of the senses as the indispensable groundwork of all medical knowledge. He had also, however, a forte for rapid generalisation from observations, and his logical proclivities disposed him particularly to deductive reasoning. Examples of an almost Euclidean method of argument may be found in the _Natural Faculties_ (_e.g._ Book III. chap. i.). While this method undoubtedly gave him much help in his search for truth, it also not unfrequently led him astray. This is evidenced by his attempt, already noted, to apply the biological principle of the [Greek: physis] in physics. Characteristic examples of attempts to force facts to fit premises will be found in Book II. chap. ix., where our author demonstrates that yellow bile is "virtually" dry, and also, by a process of exclusion, assigns to the spleen the function of clearing away black bile. Strangest of all is his attempt to prove that the same principle of specific attraction by which the ultimate tissues nourish themselves (and the lodestone attracts iron!) accounts for the reception of food into the stomach, of urine into the kidneys, of bile into the gall-bladder, and of semen into the uterus. These instances are given, however, without prejudice to the system of generalisation and deduction which, in Galen's hands, often proved exceedingly fruitful. He is said to have tried "to unite professional and scientific medicine with a philosophic link." He objected, however, to such extreme attempts at simplification of medical science as that of the Methodists, to whom diseases were isolated entities, without any relationships in time or space (_v._ p. xv. _supra_). He based much of his pathological reasoning upon the "humoral theory" of Hippocrates, according to which certain diseases were caused by one or more of the four humours (blood, phlegm, black and yellow bile) being in excess--that is, by various _dyscrasiae_. Our modern conception of "hormone" action shows certain resemblances with this theory. Besides observation and reasoning, Galen took his stand on _experiment_; he was one of the first of experimental physiologists, as is illustrated in the present book by his researches into the function of the kidneys (p. 59 _et seq._). He also conducted a long series of experiments into the physiology of the spinal cord, to determine what parts controlled movement and what sensibility. As a practitioner he modelled his work largely on the broad and simple lines laid down by Hippocrates. He had also at his disposal all the acquisitions of biological science dating from the time of Aristotle five hundred years earlier, and reinforced by the discoveries in anatomy made by the Alexandrian school. To these he added a large series of researches of his own.