At the dawn of recorded medical history Hippocrates stands for the fundamental and primary importance of _seeing clearly_--that is of _clinical observation_. And what he observed was that the human organism, when exposed to certain abnormal conditions--certain stresses--tends to behave in a certain way: that in other words, each "disease" tends to run a certain definite course. To him a disease was essentially a process, one and indivisible, and thus his practical problem was essentially one of _prognosis_--"what will be the natural course of this disease, if left to itself?" Here he found himself to no small extent in opposition with the teaching of the neighbouring medical school of Cnidus, where a more static view-point laid special emphasis upon the minutiae of _diagnosis_. Observation taught Hippocrates to place unbounded faith in the recuperative powers of the living organism--in what we sometimes call nowadays the _vis medicatrix Naturae_. His observation was that even with a very considerable "abnormality" of environmental stress the organism, in the large majority of cases, manages eventually by its own inherent powers to adjust itself to the new conditions. "Merely give Nature a chance," said the father of medicine in effect, "and most diseases will cure themselves." And accordingly his treatment was mainly directed towards "giving Nature a chance." His keen sense of the solidarity (or rather, of the constant interplay) between the organism and its environment (the "conditions" to which it is exposed) is instanced in his book, "Airs, Waters, and Places." As we recognise, in our popular everyday psychology, that "it takes two to make a quarrel," so Hippocrates recognised that in pathology, it takes two (organism and environment) to make a disease. As an outstanding example of his power of clinical observation we may recall the _facies Hippocratica_, an accurate study of the countenance of a dying man. His ideals for the profession are embodied in the "Hippocratic oath." [Sidenote: Anatomy.] Impressed by this view of the organism as a unity, the Hippocratic school tended in some degree to overlook the importance of its constituent _parts_. The balance was re-adjusted later on by the labours of the anatomical school of Alexandria, which, under the aegis of the enlightened Ptolemies, arose in the 3rd century B.C. Two prominent exponents of anatomy belonging to this school were Herophilus and Erasistratus, the latter of whom we shall frequently meet with in the following pages (_v._ p. 95 _et seq._). [Sidenote: The Empirics.]