The first attempts to localize the soul go back to classical antiquity. The soul had originally been thought to reside in the liver, an organ to which no other function could, at that time, be attributed. Empedocles, Democritus, Aristotle, the Stoics, and the Epicureans had later held its abode to be the heart. Other Greeks (Pythagoras, Plato, and Galen) had opted for the brain. Herophilus (flourished c. 300 bc), a famous physician of the Greek medical school of Alexandria, had sought to circumscribe its habitat to the fourth ventricle of the brain; that is, to a small area immediately above the brain stem. Controversy persisted to the very end of the 16th century.

The departure of the soul from the body had always been central to the Christian concept of death. But the soul had come to mean different things to various classical and medieval thinkers. There was a “vegetative soul,” responsible for what we would now call autonomic function; a “sensitive soul,” responsible for what modern physiologists would describe as reflex responses to environmental stimuli; and, most importantly, a “reasoning soul,” responsible for making a rational entity (res cogitans) of human beings. The reasoning soul was an essentially human attribute and was the basis of thought, judgment, and responsibility for one’s actions. Its departure implied death. The Anatome Corporis Humani (1672) of Isbrand van Diemerbroeck, professor at Utrecht, appears to have been the last textbook of anatomy that discussed the soul within a routine description of human parts. Thereafter, the soul disappeared from the scope of anatomy.

The modern and entirely secular concept of brain-stem death can, perhaps rather surprisingly, find both a conceptual and a topographical foundation in the writings of René Descartes (1596–1650), the great French philosopher and mathematician who sought to bring analytical geometry, physics, physiology, cosmology, and religion into an integrated conceptual framework. Descartes considered the body and the soul to be ontologically separate but interacting entities, each with its own particular attributes. He then sought to specify both their mode and site of interaction; the latter he deduced to be the pineal gland. The pineal was to become, in the words of Geoffrey Jefferson, “the nodal point of Cartesian dualism.”

Before Descartes, the prevailing wisdom, largely derived from Greece, had regarded the soul both as the motive force of all human physiological functions and as the conscious agent of volition, cognition, and reason. Descartes succeeded in eliminating the soul’s general physiological role altogether and in circumscribing its cognitive role to the human species. Descartes’s writings about death show that his concept of the soul clearly implied both mind and the immaterial principle of immortality. It had to mean both things, for no one had ever conceived of survival after death without a mind to verify the fact of continued existence, to enjoy its pleasures, and to suffer its pains.

The relation between body and soul had been discussed in patristic literature, and, because of his Jesuit education, Descartes would have been familiar with these discussions. The church’s interest in these matters was strictly nonmedical, seeking only to reconcile earlier Greek theories with its own current doctrines. Descartes was the first to tackle these problems in a physiological way. With one foot still firmly on consecrated ground (and with Galileo’s difficulties with the Inquisition very much in mind), he sought to give a materialistic, even mechanistic, dimension to the discussion. In this sense, his De Homine (On Man; published posthumously in 1662) can be thought of as an updating of Plato’s Timaeus. His contemporaries viewed Descartes as having delivered the coup de grace to an earlier Greek tradition (dating back to several centuries before Christ) that had claimed that animals, as well as humans, had souls. This had been the subject of much discussion in the early Christian Church. During the 4th century, St. John Chrysostom (onetime archbishop of Constantinople) had denounced the idea, attributing it to the devil, who had allegedly managed by various maneuvers to deceive people as varied as Pythagoras, Plato, Pliny, and even Zoroaster.

Descartes probably was impressed by the central location of the unpaired pineal gland, situated where neural pathways from the retinas converge with those conveying feelings from the limbs. This “general reflector of all sorts of sensation” is, moreover, sited in the immediate proximity of the brain ventricles, from which (according to the wisdom of the day) “animal spirits” flowed into the hollow nerves, carrying instructions to the muscles. In his Excerpta Anatomica, Descartes had even likened the pineal to a penis obturating the passage between the third and fourth ventricles.

Descartes proved wrong in his beliefs that all sensory inputs focused on the pineal gland and that the pineal itself was a selective motor organ, suspended in a whirl of “animal spirits,” dancing and jigging “like a balloon captive above a fire,” yet capable in humans of scrutinizing inputs and producing actions “consistent with wisdom.” He was also wrong when he spoke of the “ideas formed on the surface” of the pineal gland, and in his attribution to the pineal of such functions as “volition, cognition, memory, imagination, and reason.” But he was uncannily correct in his insight that a very small part of this deep and central area of the brain was relevant to some of the functions he stressed. We now know that immediately below the pineal gland there lies the mesencephalic tegmentum (the uppermost part of the brain stem), which is crucial to generating alertness (the capacity for consciousness), without which, of course, there can be no volition, cognition, or reason.

It is a matter of vocabulary whether one considers the mesencephalic tegmentum either as being involved in generating a “capacity for consciousness” or as preparing the brain for the exercise of what Descartes would have considered the “functions of the soul” (volition, cognition, and reason). In either case, the total and irreversible loss of these functions dramatically alters the ontological status of the subject. Descartes specifically considered the example of death. In “La Description du corps humain” (1664) he wrote that “although movements cease in the body when it is dead and the soul departs, one cannot deduce from these facts that the soul produced the movements.” In a formulation of really modern tenor, he then added “one can only infer that the same single cause (a) renders the body incapable of movement and (b) causes the soul to absent itself.” He did not, of course, say that this “same single cause” was the death of the brain stem. Some 300 years later, in 1968, the Harvard Committee spoke of death in terms of “irreversible coma” (where Descartes had spoken of the “now absent soul”) and stressed, as had Descartes, the immobility of the comatose body. The religious and secular terms seem to describe the same reality.

There have been other neurological controversies concerning the locus of the soul. Early in the 18th century Stephen Hales, an English clergyman with a great interest in science, repeated an experiment originally reported by Leonardo da Vinci. Hales tied a ligature around the neck of a frog and cut off its head. The heart continued to beat for a while, as it usually does in the brain dead. Thirty hours later, the limbs of the animal still withdrew when stimulated. In fact, the elicited movements only ceased when the spinal cord itself had been destroyed. This observation gave rise to a great controversy. Reflex action at spinal cord level was not then fully understood, and it was argued that the irritability implied sentience, and that sentience suggested that the soul was still present. The “spinal cord soul” became the subject of much debate. It is now known that such purely spinal reflex movements may occur below a dead brain. It was shown during the 19th century that individuals executed on the guillotine might retain the knee jerk reflex for up to 20 minutes after decapitation.

The church is still concerned with the diagnosis of death, but the theological argument has, during the last half of the 20th century, moved to an entirely different plane. As mentioned earlier, in 1957 Pope Pius XII raised the question whether, in intensive care units, doctors might be “continuing the resuscitation process, despite the fact that the soul may already have left the body.” He even asked one of the central questions confronting modern medicine, namely whether “death had already occurred after grave trauma to the brain, which has provoked deep unconsciousness and central breathing paralysis, the fatal consequences of which have been retarded by artificial respiration.” The answer, he said, “did not fall within the competence of the Church.”