Soul, Mind, Brain: Greek Philosophy and the Birth of Neuroscience

History of Neuroscience

Soul, mind, brain: Greek philosophy and the birth of neuroscience

Enrico Crivellato, Domenico Ribatti

Department of Medical and Morphological Researches, Anatomy Section, University of
Udine Medical School, P.le Kolbe, n. 3, 33100 Udine, Italy

Department of Human Anatomy and Histology, University of Bari Medical School, Bari, Italy

Abstract

The nature of “soul” and the source of “psychic life”, the anatomical seat of cognitive, motor and sensory functions, and the origin of neural diseases were broadly debated by ancient Greek scientists since the earliest times. Within the space of few centuries, speculation of philosophers and medical thinkers laid the foundations of modern experimental and clinical neuroscience. This review provides a brief history of the leading doctrines on the essence of soul and the properties of mind professed by Greek philosophers and physicians as well as the early attempts to localize brain faculties and to explain neural disorders.
© 2006 Elsevier Inc. All rights reserved.

Summary

This is a survey of the contribution of Greek philosophy and medicine to the development of original concepts about the nature of soul, the faculties of mind, and the structure and function of the brain. At the end of this study we wish to single out three issues, which in our opinion, represent the greatest achievements transmitted by Greek thinkers to later generations of neuroscientists.

Firstly, Hippocrates’ conception of human brain. As early as the ﬁfth century B.C.E., this pioneer physician had already worked out an astounding and absolutely modern view of brain functions. Indeed, the brain was not only considered the seat of intelligence, sensory perception and motor control but it was also regarded as the source of pleasure and pain, the origin of emotions, and the font of moral judgment and aesthetic experiences.
Secondly, Plato’s tripartite conception of human soul. According to this notion, each part of the soul localized to different body sites and was linked to distinct anatomical structures. Although partly derivative and not central in the Greek biological and medical tradition, this hierarchical, highly ideological subdivision, with the brain in a pre-eminent position, was an organic attempt to distinguish different psychic faculties and to link them to speciﬁc structures. We can establish a tentative relationship between the diverse soul species identiﬁed by Plato and the functional properties of distinct neural compartments as clariﬁed by modern investigation. In this context, the rational soul would correspond to neocortical functions whilst the emotional soul would be more related to limbic activity. The vegetative soul would partly correspond to the hypothalamus and the peripheral autonomic nervous system.

The third most admirable contribution is to be found in Herophilus’ and Erasistratus’ discoveries. The modern approach to the dissection of the nervous system, the clear description of many neuroanatomical structures, the identiﬁcation of cranial and spinal nerves and the fundamental distinction between sensory and motor nerves represented the legacy of these great researchers to the development of neuroscience.

Worth of particular mention was Erasistratus’ hypothesis of a possible relationship between the intelligence of man and the number and complexity of the convolutions of his brain. Regrettably, this remarkable observation was scornfully discarded by Galen, who claimed that even the ass’ brain presented numerous convolutions. In Galen’s pneumatic physiology it was the psychic pneuma to be endowed with the properties now attributed to neurons [37,38]. Had Erasistratus’ conjecture been further and properly investigated, the history of neuroscience could have run a different way.

The weight of Greek heritage was enormous. The controversy between encephalocentrists and cardiocentrists continued well into Renaissance and beyond. It often took the form of a contention between Galenism and Aristotelism. Still in 1628, Harvey (1578–1657) wrote in De motu cordis et sanguinis in animalibus that the heart was the centre and the sun of the body microcosm and the source of sensitive, motor and vegetative life [25]. He was in polemic with the encephalocentric view of his master Hieronymus Fabricius of Acquapendente (1533–1619) who, in De musculi actione, claimed that the brain was the primary motor because it caused movement but it was unmoved.

In 1553, 75 years before Harvey’s statement, the Spanish mystical physiologist Miguel Servetus (1511–1553) had reasserted the primacy of the heart and claimed that the blood was the seat of human soul. Aristotelian cardiocentric theories were harshly fought by the Galenist physician Jean Fernel (1497–1558) who deﬁned them an unbelievable delirium (“deli-ratio incredibilis”) [12]. These conﬂictual points of view testify how the dispute between encephalocentrism and cardiocentrism was still alive at the beginning of the modern scientiﬁc era.

Ancient Greece. Mysia, Pergamon. Philetairos. 282 – 263 BC.

Introduction

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 since the beginning of Greek civilization. From the sixth century B.C.E. to the second century A.D., an astounding assortment of conjectures was 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 present overview traces the course of the intellectual revolution initiated by some Presocratic philosophers in the sixth century B.C.E. and led to its zenith by Galen in the second century A.D. We shall examine and discuss some leading theories and relevant doctrines in a chronological fashion. What emerges from this survey is an extraordinary panorama of ideas, which represent the fundamental contribution of Greek philosophy and medicine to the origin and development of the neurosciences (Fig. 1).
An important problem we are faced with is the fragmentary nature of some evidence and the selectivity of the sources. For authors whose work has been largely preserved – like Plato, Aristotle and Galen – we can rely upon entire treatises rel-ative to their written production. For many others, like the Presocratic philosophers or the Hellenistic physicians, available information is not only incomplete but also second-hand. This, of course, raises the obvious problem of the questionable relia-bility of such information sources because the original opinion may have been misunderstood by the source-author or incom-pletely reported. The exact nature of the theory professed by a given scholar may be difﬁcult to recover and obscurity must probably remain. Most fragments of Presocratic thinkers, for instance, are preserved as quotations or testimonia from Plato, Aristotle, Theophrastus and Hellenistic sources. Theophrastus’ work, in particular, became the standard source for most sub-sequent collections of “opinions” (doxai or placita) on Preso-cratic philosophy, forming the so-called doxographic tradition [27]. The most important information concerning authors like Diocles, Praxagoras, Herophilus and Erasistratus derives from Galen.

This is a 14th century CE manuscript in Armenian on the discoveries made by the ancient Greek philosopher Aristotle. The text of this document was first translated from Greek into Armenia in the 6th century CE. (Mesrop Mashtots Institute of Ancient Manuscripts, Yerevan, Armenia)

The Problem of 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 fac-ulties. “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 set-tled [in our mind], knowledge is generated in such way” (96 b)1 [36].

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.

This view represented a great scientiﬁc achievement but was not shared by all Greek philosophers. In the present contribution, we will ﬁrst consider the early conception of soul developed in the Greece of Archaic age. Then we deal with the soul and psychic doctrines elaborated in the context of the Presocratic tradition. The early steps of the encephalocentric theory are discussed along with Plato’s contribution to the deﬁnition of the soul concept. The origin and further development of the cardiocentrism is described later, with particular emphasis on Aristotle’s biological philosophy. The great contribution of the Alexandrian anatomists, and the controversy between Stoic doc-trines and Galen’s epistemology are examined. Finally, a brief survey on the concept of psychic pneuma and the legacy of Greek scientists will be presented.

The Soul in Homeric Times

The Homeric poems were likely composed in the second half of the eighth century B.C.E. but they are representative of an oral tradition reﬂecting the views of Greeks living in former times. In a celebrated essay on the soul concept in the Greece of Archaic age, the German philologist Bruno Snell put forward the thesis that the Greeks of Homer had not yet developed a unitary concept of the psychic life [40]. Homer distinguished indeed different types of soul. There was a not localized soul – a kind of “life-soul” or “breath-soul” that animates the body, called psyche – and different body souls, called thymos or noos or menos [7]. The psyche was representative of the individual life and person’s identity. This kind of soul was not associated with any speciﬁc body part. It was silent during active life but appeared in the dreams during sleeping, left the body during swoons and abandoned the corpse when a person died. At the moment of death, the psyche ﬂed away from the limbs or through the wounds and departed to Hades where it began an afterlife. This kind of soul lacked any psychological attribute and possessed merely eschatological traits.

Remarkably, the term psyche is etymologically connected with the verb psychein, which means “to blow” or “to breathe”. By contrast, the body souls were active during the waking life [7]. The thymos was, above all, the source of emotions. It was also the potency that set the body in movement. It resided in the chest, where it was concentrated into the phrenes. These were the location of feelings, like joy and grief, pity and revenge, anger and fear. Phrenes are generally identiﬁed with the diaphragm but some scholars believe that they may also indicate the lungs [33]. The noos was more linked with intellectual and rational attitudes. The noos intervened when the subject had to reason and ponder. It began to shape the mind concept. It resided in the chest, without any relationship with distinct anatomical structures. Noos has the same root of the verb noein, which means “to understand” or “to penetrate”. The menos was the aggressive impulse, the fury, the rage in the battle. It was located in the chest but was not a physical organ. In Homer’s poems we ﬁnd also the terms kradie or etor, which are translated with “heart”. These terms were not related to intellectual functions but designated some source of feelings.

Natural philosophers and 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 [40]. The contribution of natural philoso-phers 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 [3]. These scholars made the ﬁrst attempt to interpret natural phenomena rejecting supernatural causes or mythical explanations, and introducing a new critical spirit of rational discussion [28]. 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. Anaximenes (born around 560 B.C.E.), one of the greatest exponents of the Ionian school in Miletus, maintained that the source of human thoughts was the air. Air was the originative substance and basic form of both physical world and psychic life. It was divine and changed by condensation and rarefaction. There was a close relationship between the cosmic air and the breath-soul. One of Anaximenes’ fragments so runs: “as our soul, being air holds us together and controls us, so does wind and air enclose the whole world” (DK 2B2) [11].

Of the same opinion was Diogenes of Apollonia (born around 470 B.C.E.), who attributed “thinking and the senses, as also life, to air” (DK 64 A 19) [11]. He held that “the internal air perceives a small part of the essence of god” (DK 64 A 19)[11], implicitly admitting that this element was endowed with intelligence. By contrast Heraclitus (born in the second half of the sixth century), suggested an identiﬁcation of the rational soul with ﬁre. The world, in his opinion, was an ever living ﬁre. Also human soul was composed of ﬁery aither and a dry soul was “the wisest and best” (DK 22 B 118) [11], whilst a moistened soul, for example due to excessive drinking, was in an ineffective state. This implicated different states of self-consciousness and cognitive faculties being related to the degrees of ﬁeriness in the soul. Thus, intellect was explicitly placed in the soul.

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. Empedocles of Acragas (ca. 495–435 B.C.E.) assigned to the blood, in particular the blood localized all around the heart the function to produce thoughts: “the blood around the heart is men’s thought (no¯ema)”, he said (DK 31 B 105) [11].

Thus, men think with the blood, and also sensation is a purely physical process. Thinking and perceiving are one and the same process. Empedocles was particularly interested in the mechanism of sensation. He developed the theory of pores and efﬂuences. Everything is continually giving off efﬂuences, which enter in another body through pores. Sensation is a matter of symmetry of pores. The proper object of each sense ﬁts into the sense-organ.
Parmenides of Elea (born about 515 B.C.E.) also accepted Empedocles’ theory of pores and efﬂuences. As to the soul, he localized it to the entire thorax (DK 28 B 45) [11], an opinion later shared by Epicurus (342–270 B.C.E.). Parmenides consid-ered the soul made up of igneous material and regarded it as the human intellect. Democritus (born about 460 B.C.E.), one of the fathers of the atomistic doctrine, maintained that the soul was to be identiﬁed with intellect. In his opinion, it consisted of the lightest, fastest moving and spherical atoms with igneous char-acter. Interestingly, he distinguished two parts in human soul: a rational one localized to the chest or the brain, and an irra-tional one that was “spread over the whole body” (DK 68 B 105)[11].

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 (Figs. 2 and 3). The former considered the brain as the seat of human consciousness, sensation and knowledge; the latter attributed all these faculties to the heart. Both theo-ries 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 prob-ably ﬁrst recognized by Alcmaeon of Croton no later than the early ﬁfth century B.C.E. [29]. He was a physician and, remark-ably, 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 compro-mised if the brain is moved and changes its place” (DK 24 A 5)[11], probably referring to concussions caused by head trauma. Unlike Empedocles, 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 [4]. The word Alcmaeon used to express the concept of “understanding” was “xynienai”, which literary means “to put together” [23]: all animals have sensation, but only man can make a synthesis of his sensations. In another testimony, Alcmaeon was credited with the relevant idea that the brain was “the seat, in which the highest, principal power of the soul is located” (DK 24 A 10) [11].

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.

The doctrine of soul in Plato

A small second–third c. C.E. fragment of Plato’s Phædo (P. Mich. inv. 5980)

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 (psych¯e)istobe found in Timaeus. According to Plato’s view, there are three soul species in the human body. Only the ﬁrst one was qualiﬁed as “immortal” and “divine”, the logos. It was called “the imper-ishable principle of the deathly animal” [34,35]. This soul was recognized to be bound to “the head, which is the most divine part and dominates over the rest [of the body] in us. And the gods gave to this part also the whole body as servant” [34,35]. With this sentence, Plato conferred a hierarchical primacy to the head over each other part of the body. In a following passage, Plato better speciﬁed the anatomical seat of this soul: “one part, like a ﬁeld, had to receive the divine seed in it, and this part of the marrow moulded [the gods] wholly round and named it brain (enkephalos) because … the vase that contained it was the head (kephal¯e)” [34,35]. This soul is intelligent, rational but invisi-ble. Thus Plato, following Pythagoras, developed the concept of the body as a temporary receptacle of the immortal soul, which could pass from one body into another at death.

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 [34,35]. 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 gods] made the deathly species [of soul], and in the fear of contaminating the divine … they placed the perishable one in another body site, far away from it, and established an isthmus and a boundary between the head and the breast, placing the neck in-between, in order to keep them separated. Thus in the chest and in the so-called trunk they bound the perishable species of soul. And because one part of it had a better character and one worse, they divided the cavity of the trunk in two compartments … and placed the diaphragm as a seal in the middle. And they placed that part of the soul, which takes part to bravery and rage and is belligerent, nearer the head, between the diaphragm and the neck because, being subordinate to reason and agreeing with it, this would strongly repress the greed” [34,35]. Remarkably, 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 epithym¯etikon. This “craves for food and drink” [34,35]. 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” [34,35]. At last, Plato mentioned a fourth species of soul, the soul of sexual impulse. This localized below the umbilicus and was fully irrational and unwilling to accept discipline.

Plato’s tripartite schema of human soul was not entirely original. As mentioned previously, Democritus, roughly con-temporary 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 [6].
Philolaus of Croton (ca. 470–385 B.C.E.), almost contempo-rary with Socrates, developed a four-fold system of psychic and vital principles in man. He distinguished the following anatom-ical structures, each associated with distinct psychophysical aptitudes. The head was conceived as the seat of the intellect; the heart was regarded as the foundation of life and sensa-tion; 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 [31]. 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 (n¯ous) and the ratio-nal faculty (phrenes), whilst the heart was the place of courage, bravery and audacity (thymos) (DK 58 B 1a) [11].

7. The heart and the cardiocentric theory
7.1. Aristotle

The notion that the heart was the source of emotions and thoughts was much diffused in the ancient world, in Egypt and Mesopotamia for instance [13]. By the Greeks, it can be traced back to Homeric poems. In Iliad and Odyssey indeed, we ﬁnd the soul placed either in the phrenes, usually the midriff, or in the chest, but sometimes also in the heart [7]. The importance given to the heart as the seat of soul and intellect was likely based upon the anthropologic evidence that life occurred as long as the heart kept pulsating and death supervened after cessation of heart activity. In addition, the heart was associated with the vital heat and, in Empedocles, we have seen that the blood around the heart, which was the most important vehicle of life heat, was indeed the source of human thoughts. The origin of the “scien-tiﬁc” theory assuming the heart as the seat of intelligence and sensation is obscure. It has been ascribed to Sicilian physicians, like Philistion of Locri, and to the School of Cnidos in Asia Minor but this opinion is not accepted by all scholars [24].

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 [21] (Fig. 3). According to Aristotle, the soul is deﬁned as the substance or the form of a living body [4]. It is indeed the primary cause of living, perceiving and thinking. He distinguished different soul faculties [4]. The vegetative or nourishing soul belongs to both plants and animals. Sensitive and motor souls pertain only to animal. The intellectual soul (n¯ous) is limited to man. All soul faculties reside in the heart. Only the n¯ous 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 [1,2,5]. 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) [1].

As to the brain, this was bloodless, cold and bipartite. Aris-totle made for the ﬁrst time the important anatomical dis-tinction between cerebrum (enkephalos, brain) and cerebel-lum (parenkephalis, para-brain) [1]. 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 addi-tion, 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 [1]. It has been suggested that these passages might possibly refer to the optic nerve and tract, and to trigeminal and oculomotor nerves [9,42]. Of course Aristotle had no concept of nerves, and there was for him no nervous system as such. 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” [2], 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 [2].

Despite such relevant anatomical contributions, Aristotle’s speculation about brain functions was rather disappointing. In his view, the brain played indeed only a subsidiary role. Being bloodless it had no sensory properties because “the instruments of sensation are the blood-containing parts” [2]. In addition, Aristotle recognized that the brain was insensible when touched. As the moistest and coldest of all the organs, it served merely to diminish the heat of the blood generated by heart [5].It had therefore no intellectual meaning. Notably, however, Aris-totle implicitly assigned to the brain a somewhat ambiguous and indirect role in human consciousness and psychic activ-ity insofar as this organ had the task to temper the excess of vital warmth produced by the heart [9,10]. In this perspec-tive, the brain was regarded as the inductor and generator of sleep [2,5].

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References – Footnotes

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