ignorance. Persons of limited information, when they are at a loss to assign a cause for anything, very commonly reply that it is done by the spirits; and so they bring the spirits into play upon all occasions; even as indifferent poets are always thrusting the gods upon the stage as a means of unraveling the plot, and bringing about the catastrophe.Fernelius, and many others, suppose that there are aerial spirits and invisible substances. Fernelius proves that there are animal spirits, by saying that the cells in the brain are apparently unoccupied, and as nature abhors a vacuum, he concludes that in the living body they are filled with spirits, just as Erasistratus had held that, because the arteries were empty of blood, therefore they must be filled with spirits. But medical schools admit three kinds of spirits: the natural spirits flowing through the veins, the vital spirits through the arteries, and the animal spirits through the nerves; whence physicians say, out of Galen, that sometimes the parts of the brain are oppressed by sympathy, because the faculty with the essence, i. e., the spirit, is overwhelmed; and sometimes this happens independently of the essence. Further, besides the three orders of influxive spirits adverted to a like number of implanted or stationary spirits seem to be acknowledged; but we have found none of all these spirits by dissection, neither in the veins, nerves, arteries, nor other parts of living animals.
Here we have the point of view of the true investigator, the true scientific spirit. Abide by the facts and base your reasoning upon careful observation. Although Harvey lived at a period when physiological knowledge, as we understand it to-day, was almost wholly unknown, and when the influence of the "spirits" dominated all thought, yet he applied rational methods of scientific study and drew logical conclusions from his observations, with the result that to him belongs the honor of discovering the motion of the heart and the circulation of the blood. What he could not see he had no faith in, and so the theories concerning the spirits of the body he laid aside as having no foundation in fact. Would that to all of us might be given that same true appreciation of the importance of scientific observation upon which depends the advance of exact knowledge.
- Quoted from William Harvey: "An Anatomical Disquisition on the Motion of the Heart and Blood in Animals," translated from the Latin by Robert Willis. Everyman's Library, London and New York.