This view of a "vital principle" as necessarily consisting of matter in a finely divided, fluid, or "etheric" state is not unknown even in our day. Belief in the fundamental importance of the Pneuma formed the basis of the teaching of another vitalist school in ancient Greece, that of the Pneumatists. [Sidenote: Galen and the Circulation of the Blood.] It is unnecessary to detail here the various ways in which Galen's physiological views differ from those of the Moderns, as most of these are noticed in footnotes to the text of the present translation. His ignorance of the circulation of the blood does not lessen the force of his general physiological conclusions to the extent that might be anticipated. In his opinion, the great bulk of the blood travelled with a to-and-fro motion in the veins, while a little of it, mixed with inspired air, moved in the same way along the arteries; whereas we now know that all the blood goes outward by the arteries and returns by the veins; in either case blood is carried to the tissues by blood-vessels, and Galen's ideas of tissue-nutrition were wonderfully sound. The ingenious method by which (in ignorance of the pulmonary circulation) he makes blood pass from the right to the left ventricle, may be read in the present work (p. 321). As will be seen, he was conversant with the "anastomoses" between the ultimate branches of arteries and veins, although he imagined that they were not used under "normal" conditions. [Sidenote: Galen's Character.] Galen was not only a man of great intellectual gifts, but one also of strong moral fibre. In his short treatise "That the best Physician is also a Philosopher" he outlines his professional ideals. It is necessary for the efficient healer to be versed in the three branches of "philosophy," viz.: (_a_) _logic_, the science of how to think; (_b_) _physics_, the science of what is--_i.e._ of "Nature" in the widest sense; (_c_) _ethics_, the science of what to do. The amount of toil which he who wishes to be a physician must undergo--firstly, in mastering the work of his predecessors and afterwards in studying disease at first hand--makes it absolutely necessary that he should possess perfect self-control, that he should scorn money and the weak pleasures of the senses, and should live laborious days. Readers of the following pages will notice that Galen uses what we should call distinctly immoderate language towards those who ventured to differ from the views of his master Hippocrates (which were also his own). The employment of such language was one of the few weaknesses of his age which he did not transcend. Possibly also his mother's choleric temper may have predisposed him to it.