- Michels, John (January 18, 1884). Science: Volume 3. Highwire Press, Jestor: American Association for the Advancement of Science. p. 74-75. 喺24 November 2021搵到.
[...] because of the improvement in philosophy [...] men began to break loose from the trammels of Greek and mediaeval metaphysics, and to realize that a process is not explained by the arbitrary assumption of some hypothetical cause invented to account for it. So long as the phenomena exhibited by living things were regarded, not as manifestations of the properties of the kind of matter of which they were composed, but as mere exhibitions of the activity of an extrinsic independent entity, a pneuma, anima, vital spirit, or vital principle which had temporarily taken up its residence in the body of an animal, but had no more essential connection with that body than a tenant with the house in which he lives, - there was no need for physiological laboratories. [...] Both Harvey and Descartes, however, still believed in a special locally placed vital spirit or vital force, which animated the whole bodily frame as the engine in a great factory moves all the machinery in it.