As ancient, medieval, and some more modern theories of medicine have traveled over the same diagnostico-theoretical roads as did the natural science of those periods, they were naturally subject to the same errors and aberrations. But the consequences of their errors differed materially. Whereas natural science, in the early and middle ages, with its faulty diagnostico-theoretical method, too frequently had recourse to supernatural factors to explain terrestrial phenomena, and thus created superstition instead of elucidation, the pathology of ancient as well as of medieval medicine avoided as much as possible any recourse to miraculous agencies in explaining the pathological phenomena of the body. This it was forced to do for the sake of self-preservation. For what would have become of the physicians with their art, which was of a purely material kind, working as it did with drug and knife, if they themselves had traced disease to supernatural causes? No one, under such conditions, would have had any dealings with mundane medical science. It is true, there have been times when such a state of things actually existed. The physician, with his earthly appliances, was always led astray as soon as metaphysical ideas had victoriously entered pathology. History affords numerous examples of this. The cult of relics, the belief in astrology during half of the middle ages, show plainly to what a degrading position the physician was reduced as soon as a pathology reckoning with earthly factors was replaced by a metaphysical theory of disease. Then the physician was either completely thrust aside—ἀλλ’ ὠθεῖται μὲν ἒξω νοσοῦντος ὁ ἰατρός, as says Plutarch (“Superstition,” Vol. I., page 412)—or he was forced to submit to a disgraceful interference. All schools of medicine, therefore, from the humoral pathology of the followers of Hippocrates to the so-called parasitism of the nineteenth century, have avoided as much as possible the acknowledgment that supernatural influences were active as pathological factors. Various as the principles of the countless medical schools may have been, they were all united in assuming as the starting-point of their speculations some material process of the body itself, in accordance with which they applied their therapeutic agencies.