But we shall not have read far before we discover that the term _Physiology_, as used by Galen, stands not merely for what we understand by it nowadays, but also for a large part of _Physics_ as well. This is one of the chief sources of confusion in his writings. Having grasped, for example, the uniqueness of the process of _specific selection_ ([Greek: holkê tou oikeiou]), by which the tissues nourish themselves, he proceeds to apply this principle in explanation of entirely different classes of phenomena; thus he mixes it up with the physical phenomenon of the attraction of the lodestone for iron, of dry grain for moisture, etc. It is noteworthy, however, in these latter instances, that he does not venture to follow out his comparison to its logical conclusion; he certainly stops short of hinting that the lodestone (like a living organ or tissue) _assimilates_ the metal which it has attracted! Setting aside, however, these occasional half-hearted attempts to apply his principle of a [Greek: physis] in regions where it has no natural standing, we shall find that in the field of biology Galen moves with an assurance bred of first-hand experience. [Sidenote: The Mechanical Physicists.] Against his attempt to "biologize" physics may be set the converse attempt of the mechanical Atomist school. Thus in Asclepiades he found a doughty defender of the view that physiology was "merely" physics. Galen's ire being roused, he is not content with driving the enemy out of the biological camp, but must needs attempt also to dislodge him from that of physics, in which he has every right to be. [Sidenote: The Anatomists.] In defence of the universal validity of his principle, Galen also tends to excessive disparagement of morphological factors; witness his objection to the view of the anatomist Erasistratus that the calibre of vessels played a part in determining the secretion of fluids (p. 123), that digestion was caused by the mechanical action of the stomach walls (p. 243), and dropsy by induration of the liver (p. 171). [Sidenote: Characteristics of the Living Organism.] While combating the atomic explanation of physical processes, Galen of course realised that there were many of these which could only be explained according to what we should now call "mechanical laws." For example, non-living things could be subjected to [Greek: phora] (passive motion), they answered to the laws of gravity ([Greek: tais tôn hylôn oiakizomena rhopais], p. 126). Furthermore, Galen did not fail to see that living things also were not entirely exempted from the operation of these laws; they too may be at least partly subject to gravity (_loc. cit._); a hollow organ exerts, by virtue of its cavity, an attraction similar to that of dilating bellows, as well as, by virtue of the living tissue of its walls, a specifically "vital" or selective kind of attraction (p. 325).