Although Galen is an eclectic in the best sense of influence of the term, there is one name to which he pays a very special tribute--that of his illustrious forerunner Hippocrates. Him on quite a number of occasions he actually calls "divine" (_cf._ p. 293). "Hippocrates," he says, "was the first known to us of all who have been both physicians and philosophers, in that _he was the first to recognise what nature does_." Here is struck the keynote of the teaching of both Hippocrates and Galen; this is shown in the volume before us, which deals with "the _natural_ faculties"--that is with the faculties of this same "Nature" or vital principle referred to in the quotation. [Sidenote: "The Natural Faculties."] If Galen be looked on as a crystallisation of Greek medicine, then this book may be looked on as a crystallisation of Galen. Within its comparatively short compass we meet with instances illustrating perhaps most of the sides of this many-sided writer. The "Natural Faculties" therefore forms an excellent prelude to the study of his larger and more specialised works. [Sidenote: Galen's "Physiology."] What, now, is this "Nature" or biological principle upon which Galen, like Hippocrates, bases the whole of his medical teaching, and which, we may add, is constantly overlooked--if indeed ever properly apprehended--by many physiologists of the present day? By using this term Galen meant simply that, when we deal with a living thing, we are dealing primarily with a unity, which, _quâ_ living, is not further divisible; all its parts can only be understood and dealt with as being _in relation to_ this principle of unity. Galen was thus led to criticise with considerable severity many of the medical and surgical specialists of his time, who acted on the assumption (implicit if not explicit) that the whole was merely the sum of its parts, and that if, in an ailing organism, these parts were treated each in and for itself, the health of the whole organism could in this way be eventually restored. Galen expressed this idea of the unity of the organism by saying that it was governed by a _Physis_ or Nature ([Greek: hê physis hêper dioikei to zôon]), with whose "faculties" or powers it was the province of [Greek: physiologia] (physiology, Nature-lore) to deal. It was because Hippocrates had a clear sense of this principle that Galen called him master. "Greatest," say the Moslems, "is Allah, and Mohammed is his prophet." "Greatest," said Galen, "is the Physis, and Hippocrates is its prophet." Never did Mohammed more zealously maintain the unity of the Godhead than Hippocrates and Galen the unity of the organism. [Sidenote: Galen's Physics.]