The defect of the earlier conceptions, their common illusion, rose from their seeking the cause outside the object, from their demanding an explanation of vital phenomena from a principle external to living, immaterial, and unsubstantial matter. Here this defect is less marked. The pluri-vitalists will in turn appeal to the vital properties as modes of activity, inherent in the living substance in which and by which they are manifested, and derived from the arrangement of the molecules of this substance—that is to say, from its organization. This is almost the conception of the present day. But this progress will only be realized at the end of the evolution of the pluri-vitalist theory. At the outset this theory seems an exaggeration of its predecessor, and a still more exaggerated form of the mythological paganism with which it was reproached. The archeus, the blas, the properties, the spirits—all have at first the effect of the genii or of the gods imagined by the ancients to preside over natural phenomena, of Neptune stirring up the waters of the sea, and of Eolus unchaining the winds. These divinities of the ancient world, the nymphs, the dryads, and the sylvan gods, seem to be transported to the Middle Ages, to that age of argument, that philosophical period of the history of humanity, and there metamorphosed into occult causes, immaterial powers, and personified forces. _Galen._—The first of the pluri-vitalists was Galen, the physician of Marcus Aurelius, the celebrated author of an Encyclopædia of which the greater part has been lost, and of which the one book preserved held its own as the anatomical oracle and breviary throughout the Middle Ages. According to Galen the human machine is guided by three kinds of spirits: _animal spirits_, presiding over the activity of the nervous system; _vital spirits_ governing most of the other functions; and finally, _natural spirits_ regulating the liver and susceptible of incorporation in the blood. In the sixteenth century, in the time of Paracelsus, Galen’s spirits became _Olympic spirits_. They still presided over the functional activity of the organs, the liver, heart, and brain, but they also existed in all the bodies of nature.