Soul

Medieval and Renaissance scholars understood anima (soul) as the entity whose presence made a thing alive. Following Aristotle (384–322 B.C.E.), they believed that plants and animals as well as humans possessed souls but that only the human soul survived death. United with a properly prepared body, the human soul carried out vegetative and sensitive functions. In the view of most Aristotelians down to the Renaissance, the soul did not require a body for intellectual functions. The mechanical philosophers of the seventeenth century, while not denying the existence of the human soul, argued that organs alone were sufficient for vegetative and sensitive functions. A comparison of the mechanist theories of René Descartes (1596–1650) with the vitalist theories of William Harvey (1578–1657) in physiology and embryology illustrates how early attempts to banish soul from the science of life foundered upon the variety and complexity of vital functions.

In De motu cordis (On the Motion of the Heart, 1628), Harvey showed, contrary to the prevailing Galenic physiology, that blood returned to the heart through the veins and that systole was the active phase of heart motion. Although he likened the heart’s motion to that of a pump, Harvey was no mechanical philosopher. He believed that the blood was the seat of the soul and that the heart restored and perfected the blood upon its return from the extremities before pumping it out again. Descartes readily accepted the circulation of the blood but denied that the heart possessed any “unknown or strange faculties” for the restoration of the blood. He claimed that the heat of the heart was sufficient to explain not only the restoration of the blood but cardiac motion as well. Where the vitalist Harvey could readily accept an active systole, the mechanist Descartes found an active diastole easier to accommodate. Descartes dismissed Harvey’s assertion of an active systole and claimed, instead, that drops of blood entered the ventricles, were vaporized by cardiac heat, distended the ventricles, and so achieved enough pressure to force open the valves and enter the arteries. Unable to explain active systole in a heart deprived of the souls vital powers, Descartes returned to the theory of active diastole, which Harvey had already shown was false.

In De generatione animalium (1651), Harvey, relying chiefly on the examination of chick eggs at different stages of development, proposed that fetal development took place by epigenesis, by the sequential derivation of parts from a principal particle that, for vertebrates, was the blood. Harvey believed that the blood—the first material to emerge from the homogeneous mass of the egg—became the seat of the soul and, as the source of animal heat and vital spirits, guided all subsequent differentiation. Harvey’s willingness to attribute epigenesis to the soul rather than to mechanical processes allowed him to avoid the absurd consequences of preformation.