Greek philosophy and the birth of neuroscience

Many leading concepts in modern neuroscience find their origin in the speculation of ancient Greek philosophers and physicians. Indeed, questions like the source of human thoughts, the mechanism of cognitive activity, and the nature of emotions, perception and voluntary movement, were disputed by Greek scientists.

Plato

From the sixth century B.C.E. to the second century A.D., an astounding assortment of conjectureswas proposed aimed to unravel crucial issues concerning the essence of soul, the location of intellect and the causes of neurological and psychiatric disorders. In a rational effort to describe and penetrate psychic phenomena, Greek philosophers elaborated theoretical solutions that still fascinate us for their inspired originality and the richness of implications.

The origin of psychic functions

In Plato’s Phaedo, Socrates (471–399 B.C.E.) squarely faces the issue of the origin of human thoughts and sketches out with synthetic efficacy the panorama of ideas of former philosophers about the source of men’s self-consciousness and cognitive faculties.

“And first of all I considered questions like this: . . . if the element by means of which we think is the blood or the air or the fire or nothing of that but rather it is the brain (enkephalos) that conveys sensations like hearing, seeing and smelling, so that memory and opinion are produced and, once they had firmly settled [in our mind], knowledge is generated in such way” (96 b)1.

This passage is of extreme interest for at least three reasons. Firstly, because it raises in a correct way the question as to “what it is in us that thinks”; secondly, because it provides the right explanation insofar as the brain is defined as the locus where human thoughts are elaborated; thirdly, because it establishes that the “substance” which receives sensations is just the same which elaborates them and produces memory, judgement and knowledge.

The Presocratic tradition

During the fifth century B.C.E. a novel, more unitary and abstract idea of soul prevailed in the Greek world. It made its appearance partly as a consequence of the linguistic achievements of the archaic lyrical poets, who developed a new conception of spiritual life. The contribution of natural philosophers to the process of theoretical clarification of the soul concept was relevant.

In Metaphysics, Aristotle informs us that natural philosophers were a group of innovative thinkers principally interested in explaining the constitution of all matter in terms of specific basic substances. These scientists explored different aspects of the physical and biological world and also tried to solve to the problem of the nature of soul. In addition, they faced the question of the relation between psychic activity and the body.

Some of the natural philosophers attributed to distinct physical principles the faculty to elaborate thoughts. Other natural thinkers were more interested on the bodily aspects of cognitive processes and argued that the seat of the intellect was localized to specific biological structures or parts of the human body, like blood, chest, diaphragm, heart’s cavity, membranes surrounding the heart, head, or brain.

Hippocrates

Alcmaeon, Hippocrates and the encephalocentric theory

Since the fifth century B.C.E., two main theories were being worked out by Greek philosophers and physicians to explain the origin of thinking activity: the encephalocentrism and the cardiocentrism.

The former considered the brain as the seat of human consciousness, sensation and knowledge; the latter attributed all these faculties to the heart. Both theories maintained a passionate and long-lasting controversy within the Greek scientific community, and the dichotomy between encephalocentrists and cardiocentrists continued even in the time of Galen and extended well into Renaissance.

The sensory and cognitive significance of the brain was probably first recognized by Alcmaeon of Croton no later than the early fifth century B.C.E. . He was a physician and, remarkably, made the first anatomical dissections on animal corpses. He stated that “all the senses are connected with the brain” through channel-like structures called “poroi”. In particular he described two of such poroi joining the eyes to the brain, no doubt the optic nerves. He claimed that the brain was the seat of consciousness and sensation because he recognized that all senses “are compromised if the brain is moved and changes its place” (DK 24 A 5), probably referring to concussions caused by head trauma. Alcmaeon distinguished between sensation and understanding. Man, says Alcmaeon, differs from the other animals in that he alone has understanding, whereas, they have sensation but do not understand.

Similar concepts are expressed by other philosophers and biologists of the fifth century. Hippon of Samos localized the principal part of the soul to the head and, in particular, to the brain. Anaxagoras of Clazomenae (ca. 500–428 B.C.E.) and Diogenes of Apollonia recognized that all sensations were connected to the brain.

For the great physician Hippocrates of Cos (about 400 B.C.E.) the human brain resembled that of all other animals, being cleft into two symmetrical halves by a vertical membrane. The Hippocratic Corpus contains outstanding concepts about the brain as the seat of human intellect and the cause of neurological disorders.

In one of the earliest treatises, De morbo sacro, we find the most important epistemological claim that epilepsy, called in antiquity “the sacred disease”, is not “any more divine or more sacred than other diseases, but has a natural cause, and its supposed divine origin is due to men’s inexperience and to their wonder at its peculiar character”. The cause of the sacred disease is to be found in the brain, which “has the most power in man. If it is in sound conditions, it is our interpreter of the things. . . The eyes and ears and tongue and hands and feet do whatsoever the brain determines; for there is an element of intelligence in the whole body . . ., but it is the brain that is the messenger to the understanding”.

In another remarkable passage of De morbo sacro, the brain was also regarded as the seat of judgment, emotions and aesthetic activity: “our pleasures, joys, laughter and jests arise from no other source than the brain; and so do our pains, grief, anxieties and tears. Through it . . . we also discern ugly and beautiful, bad and good, pleasant and unpleasant”. Thus thinking activity, moral consciousness, perceptive elaboration and control of body’s movement were functions all localized to the brain.

Hippocrates was also credited with the powerful image of human mind being “placed in the brain like a holy statue” (I, 4). In a psychiatric context, he explained mental insanity as a process of brain corruption induced by bile, one of the four humours (blood, yellowbile, black bile and phlegm) responsible for the states of health and illness.

Remarkably, the Hippocratic canon contains passages dealing with lateralization of the effects of brain injury. In one passage, it was stated that “an incised wound in one temple produces a spasm in the opposite side of the body”. In another relevant passage, it was recognized that loss of speech occurred with “paralysis of the tongue or of the arm and the right side of the body.

The doctrine of soul in Plato

Plato (427–347 B.C.E.) supported the concept of the primacy of the brain as the organ of the rational soul. The most complete formulation of Plato’s theory of human soul (psyche) is to be found in Timaeus. Besides the immortal soul, Plato recognized the existence of two other species of perishable souls. The first one was the source of the feelings, like boldness, fear, anger and hope; it was the thymos. This irascible part localized to the chest, above the diaphragm and near the heart and lungs. The heart had the task of keeping a watch over it, like a guardian. It was not completely separated by the immortal soul but was connected with this through the isthmus of the neck. The word diaphragm made here its first appearance as a technical term in the history of anatomy.

Wholly distinct and physically separated from the former two species was the soul of nourishment, the epithymetikon. This “craves for food and drink”. The concupiscible soul was situated in the region between the diaphragm and the umbilicus, near the liver, as distant as possible from the deliberating soul. It was the seat of passions, desires and unconscious life, like dreaming and foreboding. It “has neither opinion, nor reasoning, nor intelligence but pleasant and painful sensations”

Plato’s tripartite schema of human soul was not entirely original. As mentioned previously, Democritus, roughly contemporary with Plato, distinguished a rational soul related to the chest or the brain, and an irrational soul extended over the whole body. Some essential features of Plato’s tripartite view were derived indeed from Pythagorean speculation.

Philolaus of Croton (ca. 470–385 B.C.E.), almost contemporary with Socrates, developed a four-fold system of psychic and vital principles in man. He distinguished the following anatomical structures, each associated with distinct psycho-physical aptitudes. The head was conceived as the seat of the intellect; the heart was regarded as the foundation of life and sensation; the umbilicus was interpreted as the source of rooting and embryo’s development; the genital organs were considered the origin of fertilization as well as generation. A testimony from Diogenes Laertius identifies the origin of the concept of soul partition in the context of early Pythagoreans. According to this testimony, they conceived the human soul as a tripartite structure. The brain was the seat of the mind (nous) and the rational faculty (phrenes), whilst the heart was the place of courage, bravery and audacity (thymos) (DK 58 B 1a).

The heart and the cardiocentric theory

It was Aristotle (384–322 B.C.E.), the greatest of classical biologists and probably the first anatomist in the modern sense of this term, who gave a definite and authoritative character to the cardiocentric theory. According to Aristotle, the soul is defined as the substance or the form of a living body. It is indeed the primary cause of living, perceiving and thinking. He distinguished different soul faculties. The vegetative or nourishing soul belongs to both plants and animals. Sensitive and motor souls pertain only to animal. The intellectual soul (nous) is limited to man. All soul faculties reside in the heart. Only the nous is immaterial. Thus the heart occupies the uppermost place in Aristotle’s psychological and physiological hierarchy. It is regarded as the central organ of the body, the principle of life, the generator of body heat, the font of blood and the origin of vessels. Accordingly, it is the organ that develops first in the embryo.

Aristotle, however, provided also interesting contributions to brain anatomy. He observed that the brain in all animals was placed in the front portion of the head and was surrounded by two membranes, the meninges, which were patterned with blood vessels. The external envelop, situated next to the bone of the skull, was the thickest (no doubt, the dura mater); the internal, localized around the brain itself, was more delicate (probably, the pia mater or pia mater and arachnoid).

As to the brain, this was bloodless, cold and bipartite. Aristotle made for the first time the important anatomical distinction between cerebrum (enkephalos, brain) and cerebellum (parenkephalis, para-brain). The latter was positioned beyond the former, to the back, and its shape and texture were recognized to be different from those of the brain. In addition, Aristotle identified three possible nerves, he called “poroi” (“ducts”), two of which – the largest and the second largest – led to the cerebellum, the smallest to the brain itself.

He also recognized a cavity in the brain, a small hollow, probably the ventricular system, and made the interesting observation that Man has the largest brain in proportion of his size. Elsewhere, he referred to “liquidity about the brain”, a possible reference to the cerebrospinal fluid. In opposition to Plato’s opinion, he correctly described the spinal cord as an extension of the brain and recognized a similar constitution for both structures.

Aristotle implicitly assigned to the brain a somewhat ambiguous and indirect role in human consciousness and psychic activity insofar as this organ had the task to temper the excess of vital warmth produced by the heart. In this perspective, the brain was regarded as the inductor and generator of sleep.

Diocles of Carystus

Considered by Pliny the Elder one of the most reputable of the ancient physicians, Diocles of Carystus (flourished in the fourth century B.C.E.) reasserted the role of the heart, “the body’s leader from which the psychic pneuma moves to the body” (V, 1), as the physiological centre of sensation and thought. He made progresses in the description of the heart functional anatomy and recognized the two cardiac ears or auricles. This important discovery, however, was misinterpreted in the light of his theory of the cognitive function of the heart. Indeed, he attributed the role of sensory organs to these appendages, through which the heart would always be listening and understanding.

Most remarkably, Diocles later modified the doctrine of the cognitive primacy of the heart by suggesting that the right half of the brain provided sensation and the left intelligence, even though the heart remained in his opinion the centre for hearing and understanding. This heart–brain “double seat” theory, with the heart prevailing at last, is well exemplified by a series of fragments reporting Diocles’ view on some psychiatric and neurological diseases.

Praxagoras and followers

Praxagoras of Cos (born about 340 B.C.E.), the teacher of the great Alexandrian anatomist Herophilus, still ascribed all psychic faculties to the heart and blood vessels. He was credited to be the first who made a general distinction between arteries and veins, recognizing different functions to them. He conjectured that arteries would carry the pneuma and attributed to the only veins the role of transporting the blood. Being a physician, his view on the origin of thought, sensation and movement was strongly influenced by the problems he encountered in his clinical practice. According to him, the cause of madness was to be found in “a swelling of the heart, to which thoughts belong” (XVIII, 1) and delirium (phrenitis) “is inflammation of the heart, whose natural activity he holds to be mental sanity” (I, 2).

The cardiocentric thesis enjoyed a great success well beyond Aristotle, Diocles and Praxagoras. Among others, it was accepted and supported by the Peripatetic tradition, by Athenaeus of Attalia, the founder of the Pneumatist medical sect (first century B.C.E.), and by all the Stoics. In the Hippocratic treatise De corde, a text of controversial date but almost certainly post-Aristotelian, we find the statement that the mind (gnome) resides in the left ventricle, which rules the rest of the soul.

Human nervous system

The Alexandrian medical school

In Ptolemaic Alexandria, during the third century B.C.E., flourished a renowned medical school, whose most significant exponents were Herophilus of Chalcedon (335–280 B.C.E.) and Erasistratus of Ceos (310–250 B.C.E.).

Herophilus is considered the founder of human anatomy as a distinct branch of medicine. Some of his pioneering discoveries in almost every field of human dissection are regarded as the highest medical achievements until the 17th century.

Herophilus was credited with a series of accurate descriptions of neuroanatomical structures and his brain anatomy largely surpassed the standard of his time. He provided a clear distinction of the brain ventricles, recognizing that they allow passage of the psychic pneuma because they are in reciprocal communication, “and therefore there is an obligatory passage from the front ventricles into the ventricle of the cerebellum”. He gave a particularly precise account of this cerebellar ventricle, the fourth ventricle or the posterior ventricle, to which he attributed great functional significance as the seat of hegemonikon, the ruling principle of the body. In addition, he described and nominated some unique structures visible on the floor of this cavity, such as the “calamus scriptorius”, the posterior median sulcus and the “colliculus facialis”. He accepted Aristotle’s distinction between enkephalos and parenkephalis, recognizing that they are separated by a thick membrane (tentorium cerebelli). He described the tunic covering the ventricles of the brain as the “choroid meninx” and was considered, along with Erasistratus, to be the first anatomist who identified motor and sensory nerves, correctly locating their origin in the brain or in the spine. He gave careful description of at least seven pairs of cranial nerves, among which the optic, oculomotor, trigeminal, motor root of the trigeminal, facial, acoustic and hypoglossal nerves.

No less impressive was Herophilus’ physiological interpretation of his anatomical findings. He stated that the fourth ventricle and possibly the overlying cerebellum were the control centre of human motility. In his opinion, “the ventricle in the cerebellum exercises more control” in comparison to the anterior ventricles.

Remarkable was Herophilus’ view of nerve function, which was shared by Erasistratus as well: there are two kinds of nerves, the sensory (aisthetika neura) and the motor nerves (kinetika neura), and “the nerves that make voluntary motion have their origin in the cerebrum and the spinal marrow”. Herophilus’ rational approach to the dissection of the nervous system, his identification of the psycophysical command centre to the region of the hindbrain (the fourth ventricle and the cerebellum), the description of cranial and spinal nerves and the fundamental distinction between sensory and motor nerves were his extraordinary contribution to the development of what much later will become the neurosciences.

Younger than Herophilus, Erasistratus was himself a skilful neuroanatomist even if his main contributions were to be found in the field of physiology. He provided a remarkable description of human brain and cranial nerves, distinguishing nerves for sensation and nerves for movement. He assigned, however, the greatest functional significance to the thick membrane (pacheia meninx) enveloping the brain, the dura mater, which he long considered the command seat of sensitive, motor and cognitive functions. As a consequence, psychiatric and neurological disorders were interpreted as depending on pathological changes concerning this structure.

Nerve function

The first account of the nerves as hollowstructures regards the optic nerve and probably dates back to Alcmaeon of Croton. He described two “ducts” that proceeded down from the brain to the eyes (DK 24 A 10). The unique perforated structure of the optic nerve, with the central artery of the retina running inside its distal tract, had likely a great impact on the elaboration of a general theory of nerve function. Aristotle, as already mentioned, gave account of three possible nerve structures, he actually called poroi, which might have corresponded to the optic nerve and tract, and to trigeminal and oculomotor nerves. Herophilus too called the optic nerves poroi and so did Eudemus, a distinguished anatomist contemporary of Herophilus.

Thus nerves were conceptualized by Greek scientists as lumenal structures carrying the psychic pneuma. This permeated the hollow nerves and flew down from the cerebral ventricles to the sensory organs and to the muscles, performing sensitive functions and voluntary movement. The role attributed to the psychic pneuma can also be inferred by the pathogenic interpretation of several mental and neural diseases. As a result, the aetiological explanations of disorders of the motor and nervous system were conceived of in keeping with such a model of nerve physiological anatomy.

Galen

Galen of Pergamon (129 to about 216 C.E.) was one of the greatest physicians and biologists of all times. Galen devoted much experimental and theoretical work to the study of brain functions. He was a convinced assertor of the centrality of brain in elaborating thoughts, sensation and movements, and harshly fought against cardiocentric views sustained by Stoic philosophers. The brain – says Galen – is that part which “receives all sensations, produces images and understands thoughts”. Galen’s encephalocentrism was influenced by the work of a number of earlier scientists, in particular anatomists.

For him, the brain is the hegemonikon, the ruling principle and the regent part of the body. The brain alone is responsible for sensation and voluntary motion, which both are the main attributes of the rational soul.

Indeed, thanks to his rational investigative methodology and first-rate anatomical skill, Galen reached the conclusion that the nerves and the spinal cord were composed of the same substance of the brain. He also provided a clear demonstration that the spinal cord originated from the brain.

Galen did not attempt to localize the rational soul in a specific part of the brain. In his opinion, it was the psychic pneuma that functioned as the first instrument of the rational soul. Thus the psychic pneuma is the effector agent of the soul. Indeed, all Galen’s physiology of the brain is dependent upon pneuma.

Galen provided a very sophisticated description of the four ventricular structures deeply situated in the brain. These communicating cavities function, in Galen’s opinion, as an integrated whole. They are patent spaces, which harbour the psychic pneuma. This moves indeed within the interconnected spaces of the ventricular system, continues through the nerves to the rest of the body and conveys, in an unexplained way, sensation and movement. If nerves are cut, they are no longer able to conduct the psychic pneuma. As a consequence, sensation or movement disappear depending upon sensory or motor nerves are damaged.

Galen rejected Stoics’ cardiocentric assumptions on the ground that the heart produced neither sensation nor modification of consciousness when touched.

(Source: “Soul, mind, brain: Greek philosophy and the birth of neuroscience”, by Enrico Crivellato & Domenico Ribatti, 2007)

Aristotle