Development of the Concept of Mind

Abstract

A short account is given of the development of concepts of soul, mind and brain in order to place in historical context the subject of neuropsychiatry. A selection of primary and secondary historical sources is used to trace development of these concepts. Beginning with the spirits of Animism in the 3rd millennium BC, the Greek invention of the soul and its properties, of thymos (emotion), menos (rage) and nous (intellect) are then traced from the time of Homer, in which the soul does not last the death of the body, to Plato in the 4th century BC who argued that the soul, incorporating the nous (now called mind) is incorporeal and immortal. Plato's pupil, Aristotle, commented on the impossibility of an incorporeal soul interacting with a corporeal body. He instituted a revolution in the concept of mind. This involved pointing out that ‘mind’ is a manner of speaking about our psychological powers as in thinking and remembering. Given that such powers are not a thing the problem does not arise as to the relation between mind and a corporeal body. These ideas of Plato and Aristotle were held by competing scholars and theologians during the next 2000 years. Plato was favoured by many in the Church who could more readily grasp the concept of an immortal and incorporeal soul within the context of Christian thought. Galen established in the 2nd century AD that psychological capacities are associated with the brain, and argued that the fluid-filled ventricles were the part of the brain involved. This argument stood for over 1500 years until the 17th century when Willis, as a consequence of the new blood perfusion techniques developed by Wren following Harvey, showed that blood did not enter the ventricles but the cortex, thereby transferring interest from the ventricles to the cortex. The hegemony of Plato's ideas was broken about this time by Descartes when he argued that the incorporeal soul does not consist of three parts (thymos, nous and menos) but is solely identical with the mind, which is not just concerned with reasoning but with perception and the senses, indeed identical with consciousness ‘taken as everything we are aware of happening within us’. The shadow cast by this concept, necessitating as it does relating the Cartesian mind to the cortex, stretches from the time of Willis, through to the foundation figures of neurophysiology and psychiatry in the early 20th century, namely Sherrington and Kraepelin, and beyond. This history is traced in detail because the Cartesian paradigm provides the main resistance to Kraepelin's argument that mental illness has biological concomitants. It is argued that the modern tendency to equate the mind with the brain does not illuminate the problem that was solved by Aristotle. The mind is not as either Plato of Descartes would have it, nor is it equivalent to the brain, for talk of the mind is a manner of talking about human psychological powers and their exercise, as in ‘mind your step’ (watch where you are going), ‘keep that in mind’ (remember it). It is suggested that the history of the concept of mind shows that a human being has a corporeal body and a mind, that is, a range of psychological capacities. It is the role of neuropsychiatry to identify the changes in the corporeal that need to be put aright when these psychological capacities go awry.

This essay is concerned with the history of our beliefs in the Western world concerning soul, mind and brain. It is written in order to provide a historical perspective on the gradual emergence of the idea that what has gone awry in mental illness is due to abnormalities in brain function.

Human spirit: Siberia (4000–2400 BC)

Animism arose in Siberia in the Neolithic period (4000–2400 BC) and is humanity's first attempt to make sense of experience. It involves belief in spirits that do not possess any individuality although their activities are confined to particular aspects of the natural environment. Shamanism first formalized the concept of this spirit world [1].

What was the evidence that such spirits exist? This seems to have been based primarily on the experience of dreams, on apparitions of the dead, and on hallucinations. Also important were experiences of shadows, reflections and echoes. The recurrence of a daily period of sleep, accompanied by bizarre dreams and often containing elements experienced during the waking hours, seemed to have no other explanation than that during sleep something left the body and entered another world quite different to that experienced during the waking hours. That which went forth came to be called the person's spirit. This spirit, although uniquely related to a particular individual, possessed none of the psychological attributes of the individual, their particular abilities to think, feel, remember, perceive, etc. The world this spirit inhabited seemed to possess a high content of shadows and other elusive and transitory phenomena. The sleeper's spirit journeyed into this world, which was also visited by the spirits of other humans, by animals and even by objects. The presence of dead friends or enemies in dreams was considered proof that an incorporeal part of a person survived degeneration of the body at death. Hence a human's spirit was immortal.

Where did this immortal spirit, lacking an individual's psychological attributes, reside in the body when awake? Among the peoples of southwest Asia it was taken as important to shrink the heads of one's enemies as soon as they had been killed. The basis of this ritual was fear that once the enemy's spirit had left the body at death it could act out revenge on the living. This could only be prevented by restraining the enemy's spirit from leaving the body through head shrinkage. Thus even at these early times it seems that the immortal spirit, devoid of psychological attributes, was taken as residing in the head.

Human soul: Archaic Greece (8th–5th century BC)

In the Archaic period at the time of Homer, in the 8th century BC, Greece was in contact with the shamanistic culture of the Black Sea from which was inherited the concept of a spirit wandering away from the body during sleep and during trances [2]. Homer refers to this spirit as the soul or psyche, peculiar to the individual and located in the head, but without the individual's psychological attributes for remembering, thinking, perceiving, and feeling [2]. If this had been Homer's sole contribution to the idea of the soul or psyche it would have amounted only to the renaming of the spirit of Animism and Shaminism. But Homer next made a most profound suggestion, namely that a soul exists, consisting of three parts, that is largely unique to the awake and living. One of these parts, the thymos, is considered to be the source of the emotions as in anger, courage and zeal; it can express hope and may even urge one to action; most importantly these attributes are unique to the person whose thymos it is and so this soul is often referred to as the ‘ego soul’; it resides in the chest because it is a substance that is related to breath. A second part of the soul of the living and awake is the nous (or noos), which is related specifically to one's intellectual activities; it is said to be an absorber of images but does not reside in a particular part of the body. A third part of the living soul is the menos, which is related to martial rage but not to any particular organ of the body [2].

What happens to the soul of the dead, the psyche, and of the soul of the living (thymos, nous and menos) at death? According to Homer the immortal and impersonal psyche leaves the head to reside in Hades, of which more will be said in the present review, a story not much different to that of earlier times concerning the human spirit [3]. Two components of the soul of the living die immediately with degeneration of the body, namely the nous and menos, whereas the thymos (ego soul) does not but is breathed out from the chest at death to escape the decay of the body. Because the thymos carries some of the personal psychological powers of the individual whose thymos it is, capacities associated with the emotions and the will to action, it is of considerable interest that Homer gave these ego-centred powers to a component of the living soul that was almost immortal, that escaped the degeneration of the body, without specifying further what happened to the thymos after it was breathed out at death.

In summary, the thymos was the one component of the life soul that had the potential to confer unique psychological attributes on a person. At death the psyche was thought to go to Hades, where it led a ghost-like existence as the shade or spectre of the deceased. However, of the three components of the life soul only the thymos leaves the body, is actually breathed out at death, with both the nous and menos remaining. It is not clear what happens to the thymos for it is only the psyche, which is not associated with the life soul and its psychological capacities, that outlasts the death of the body. The immortal soul, the psyche, lacked the psychological attributes of the ego soul of the living. Furthermore, because the souls of the dead do not possess a thymos, nous or menos they are incapable of speech. The immortal souls of the dead are still thought of by the Archaic Greeks as shadows (described by the term eidolon), reflecting that there had been no substantial change in the concept of the immortal soul since early Animism.

If the soul goes to Hades where is this place? The earth is encircled by a river that feeds the oceans in the cosmology of Homer. Earth, river and oceans are covered by the inverted bowl of the heavens within which the moon, sun and stars move. The sun does not sink below the level of the oceans, a place that therefore remains unlit and is called Hades.

Attribution to an immortal soul of a wide variety of psychological (cognitive and emotional) activities and responses of the individual: transition from the Archaic to the classical Greek period (5th–4th century BC)

Towards the end of the archaic age the immortal psyche took on the personal psychological activities of the individual that had previously been associated with the thymos. This most important shift of activities that were previously associated with the thymos to the immortal psyche has not been convincingly attributed to any individual or group at the end of the Archaic period. According to Claus the emergence of the concept of a personal soul cannot be taken to result from competition between the words for thymos, located in the chest and associated with the life force, with breath, and that for the psyche, located in the head but not associated with any psychological activity even though surviving as a shade after death in Hades [4]. Because if this had been the case one would have expected thymos to prevail rather than the activities of thymos to be attributed to the psyche, which then carries these into the afterlife. The problem concerns the transition from the Homeric concept of the psyche as a ‘shade’ in the tradition of Animism to that of the Platonic tradition in which the psyche becomes a personal soul, the immortal and divine part of man. Thus the Greek term for psyche becomes in the Platonic tradition that part that joins the body to make the complete human being.

The first writer to feature this new concept of the soul, that is, of the psyche bearing personal psychological activities into the afterlife, is Heraclitus (540–475 BC), although it is possible that Anaximines (ca 525 BC) also wrote in this way at about this time. It is Heraclitus who first calls the soul of living man the psyche and suggests that man consists of body and soul and that the psyche is a ‘thinking thing’ [3]. He says that ‘the human will and the ethical disposition of man are signs of the state of the soul that animates him’. Heraclitus claims that ‘you could not find the ends of the soul that you travelled every way, so deep is its logos’ [4]. With Heraclitus the psyche takes on a highly personalized form that, according to Claus, leads Heraclitus to discuss the psychological value of the psyche. Intelligence and the emotional life now depend on the psyche [4].

Snell considers that this reference to the profundity of the soul, involving a dimension that cannot be applied to a physical organ or its function, indicates a link to the Archaic lyric poets before Heraclitus [3]. They were the first to suggest that intellectual and spiritual matters could have depth, for they refer to ‘deep pondering’ and to ‘deep pain’ and to ‘deep thinking’. The thymos takes on quite personal traits, for example the lyric poet Archilochus says that his thymos ‘is stirred after suffering’. It is then to the lyric poets of the Archaic period that one must turn to find the transition of the personal psychological properties of the thymos to the immortal psyche.

Claus considers Plato's claim that Pythagoras is responsible for this conceptual shift of the psychological identity of a human from the thymos to an immortal psyche or soul is simply a device that Plato uses for the purpose of his own dialogues and is not based on historical fact [4]. Thus Plato's description in his Georgias of the Pythagorean psyche as possessing personal psychological attributes is given without any documentary evidence. Rather Pythagoras seems to have been a shaman who believed in the form of what is called metapsychosis or reincarnation in which psyche, devoid of any personal attributes, wanders aimlessly through the bodies of animals and humans. Although on some occasions Pythagoras attributed a somewhat richer existence to the psyche after death, this was simply to allow that psyche to punish a living person who had committed a grave offence.

Onians suggests that the transition of the concept of the psyche or shade that survives death in Hades to that of the psyche spoken of as possessing the psychological capacities previously associated with the thymos in the chest, might be attributed to Alcmaeon of Croton (450–500 BC) [5]. Alcmaeon in his ‘Concerning nature’ identifies the ‘passages’ leading from the eyes to the brain, that is, the optic nerve. This is said by Onians to have led Alcmaeon to the idea that the brain is the organ required for perception as in sight, sound and smell, and therefore that the brain is the seat of thought. In this way, it is surmised, there is a shift away from the conceptual scheme in which the immortal psyche or soul is located in the head and does not possess psychological powers although it persists after death in Hades as a visible but impalpable semblance of a once living being, that is, a shade, like in a dream. In this scheme the psyche and thymos, with its rich emotional and other psychological attributes, leave the body at death with the thymos then being destroyed. The new post-Archaic scheme is one in which these attributes reside with the psyche and hence with a soul that can be identified with the individual and survives death. It is interesting in this regard that the substantial disagreements between Aristotle and Plato as to the location of the organ that must function normally in order for us to express our psychological powers should revolve around these being located either in the head or the chest [5].

Attribution to an immortal soul of a wide variety of psychological (cognitive and emotional) activities and responses of the individual: consolidation and elaboration by Plato (428–348 BC)

Plato is the first to have given a fully realized psychological description of the soul in his dialogues named the Charmides and the Gorgias [4]. For Plato the soul consisted of three parts, in much the same way as that envisaged during the late Archaic period: the thymos (the ego soul concerned with the emotions and the will; associated with the organs of the chest, including the heart); the nous or logos (concerned with reason; associated with the head); and the id or pathos (concerned with the appetites; associated with the liver). The difference with the late Archaic period is that all three components of the soul are made of an immortal incorporeal substance. Plato's dualistic philosophy of a corporeal body and an incorporeal immortal soul had a major effect on the Neoplatonists and via St Augustine came to dominate all Christian thought. Platonic dualism became the most natural conception for popular Christianity and it was this dualism that was to become a characteristic of the Renaissance form of Neoplatonism.

Plato introduced the word mind for the logos soul, that is, the part of the soul concerned with reason. Plato in his dialogues puts into the mouth of Pythagoras his ideas concerning the continued existence of a person after death [4]. Here the term ‘psyche’ or ‘soul’ is used to denote that person who continues to exist after death. The psychological, cognitive and emotional activities that have been attributed to a particular person had now to be attributed to their soul. By the end of the 5th century BC Antiphon has a defendant say that he is sure of his innocence ‘for though his body may surrender, his soul saves him by its willingness to struggle, through knowledge of its innocence’.

Appetites are also activities of the soul, so pleasure taken in drinking is referred to the soul, which may also be satisfied with rich food. Souls become the bearers of moral qualities. Euripides now has Ajax saying, before he commits suicide, that ‘nothing binds the soul of man more than dishonour’. Plato institutes a train of thought in which the soul is taken to be something that can engage in activities such as thinking and planning. By the beginning of the 4th century BC Plato has consolidated a view in which one attributes to the psyche or soul a wide variety of cognitive and emotional activities and responses as well, as it being the bearer of such attributes as courage and justice. The central problem with this conception, as Aristotle was first to point out, is how could interaction occur between these two very different kinds of substances, that of an incorporeal soul and that of a corporeal body?

The soul is an individual's psychological powers: revolutionary concept of Aristotle (384–322 BC)

Unlike Plato, who considered that the soul was composed of three parts, Aristotle distinguished three different kinds of soul (De anima 415^a23–6). First, a rational soul that conferred powers of thought (reasoning) and is unique to humans, a distinction like that of nous in the Archaic period and accepted by Plato as one of his three parts of the soul; now, however, this component of the soul is a power or attribute and not a thinking incorporeal substance. Second, a sensitive soul, which confers powers of perception, locomotion and desires, possessed by all animals; like that of the thymos of the Archaic period and one of the three parts of the Platonic soul but claimed by Aristotle to be associated with the heart; again a power or capacity and not an incorporeal perceiving and desiring substance. Finally, a nutritive soul conferring powers of growth, nutrition and reproduction, possessed by all living things, that is, both flora and fauna; like the ‘id’ component in Plato's conception of the soul except that again Aristotle considers this is a power or capacity and not an incorporeal substance. The Aristotelian soul was then an array of psychological powers and capacities and therefore not a substance at all, be it a shade or other ghost-like substance (De anima 412^b6–7). Hence the problem did not arise as to the form of interaction between these two very different kinds of substances, that of the incorporeal soul and that of the corporeal body [6].

For Aristotle the word ‘mind’ is a manner of speaking, a façon de parler, about the powers of intellect, thought, and reasoning (i.e. the Aristotelian ‘rational soul’). So we say ‘keep that in mind’, remember it; ‘to have something in one's mind’, is to be thinking about something. If one has a powerful mind than the reference is generally to one's powers of thinking. To ‘lose one's mind’ is usually reserved to those occasions when the rational faculties are not correctly exercised, as in schizophrenia (this does not mean of course that one has lost all the psychological abilities, viz perceiving etc). And so on. Each such use of the word ‘mind’ is therefore readily paraphrased into a phrase that does not include the word ‘mind’, but only a psychological predicate predicable of a human being. In this sense reference to the mind is eliminable without any loss in the informational content of the sentences. Talk of the mind is then a convenient way of speaking about certain human faculties and their exercise.

The mind is not then a kind of entity and it makes no sense to say it is made up of a particular substance, be it the corporeal brain or an incorporeal soul. Certainly it makes no sense to ascribe psychological predicates (thinks, believes, sees, remembers, etc) to the mind for it is a mere façon de parler. The word ‘mind’ cannot stand in relation to anything. In phrases such as ‘I will keep that in mind’, ‘mind what you say’, the word ‘mind’ can be replaced with phrases such as ‘remember that’, ‘be careful’. It is in this sense that the ‘mind’ is a manner of speaking and so cannot stand in relation to something like a material substance. Mind is then related to the powers, in the examples given, of perception, memory and emotion, which may or may not be exercised by human beings and, depending on the power concerned, by other animals [7].

The word ‘mind’ is etymologically derived from expressions in Indo-Germanic associated with memory, thought and attention, namely, ‘to bear something in mind (to remember), ‘to turn one's mind to something’ (to begin thinking about it), ‘to have it in mind’ (intend to do it). These examples are paraphrasable into a phrase that does not include the word ‘mind’. The mind is not a thing nor is it a nothing. In speaking idiomatically of ‘mind’ we are speaking of a wide range of human character traits and powers [8]. Aristotle contends then that he cannot identify ‘mind’ with the body or any part of the body because psychological attributes can be applied to persons, that is, to human beings.

The word ‘body’ is a manner of speaking about the corporeal attributes of a human person, just as the word ‘mind’ is a manner of speaking about the psychological attributes of the human person. So that to speak of the body is to speak of a certain range of corporeal characteristics: weight, height etc. Body and soul make up an animal, not as a chassis and engine make up a car, but, as Aristotle states, ‘just as the pupil and sight make up an eye, so in this case the soul and body make up an animal’. To have a soul is not to possess something.

Association of the soul with the ventricles of the brain: from Galen (129–200) to Avicenna (980–1037)

Nerves were discovered by Heraclitus in the 2nd century BC but he had no idea as to their function. It was Galen in the second century AD who discovered that some nerves derived from the spinal cord were connected to muscle, and he called these the motor nerves. As a consequence of his observations on injured charioteers, he also distinguished sensory nerves from motor nerves [9]. Using the term ‘soul’ in the Aristotelian sense, Galen considered that there was a ‘motor soul’ and a ‘sensory soul’, which were not to be considered as two different entities but as two different functions or principles of activity [10]. The motor nerves are uniquely associated with their origins in the spinal cord. It is clear that Galen associated the brain with the mental capacities of humans. In his work On the usefulness of the parts of the body he states: ‘in those commentaries I have given the demonstration proving that the rational soul is lodged in the enkephalon; that this is the part with which we reason’. Galen attributed to the brain both rational functions and the perceptual functions that Aristotle had attributed to the heart. Galen, an Aristotelian, argued that the nous or rational soul, that is, the powers of reasoning, were dependent on the brain.

Galen did not ascribe to the cortex any special function with regard to the higher mental powers such as reasoning, because he observed that donkeys have a highly convoluted brain [11, 12]. Consequently, he thought that the cerebral common convolutions could not be associated with intelligence. Instead, he identified the ventricles, rather than the cortex, as the source of such powers as reasoning [13]. He argued, ‘if the entire anterior part of the brain is injured, its upper ventricle (i.e. the lateral ventricle) is necessarily also affected by sympathy, and the intellectual functions are damaged’. Thus according to Galen the lateral ventricle is required for intellectual activity and therefore is responsible for our being able to express our rational powers. Galen also placed the sensitive soul in the brain, and not in the heart as had Aristotle. With these exceptions Galen was a thorough-going Aristotelian, regarding the rational, sensitive and nutritive souls as powers and capacities and not as substance(s).

In contrast to Galen, Nemesius (ca 390), the bishop of Emesa, was a Neoplatonist, who argued that the logos or nous of the Platonic soul, concerned with reason and intellectual functions, was located in the ventricles. Nemesius went on to develop the doctrine of the ventricular localization of all mental functions, rather than just the intellectual ones [14]. Unlike Galen he allocated perception and imagination to the two lateral ventricles (the anterior ventricles), placing intellectual abilities in the middle ventricle, and preserving the posterior ventricles for memory. Hence the doctrine was accepted that imagination/perception, reasoning and memory are to be found in the lateral, third and fourth ventricles, respectively. This became known as the ventricular doctrine.

Galen, as an Aristotelian, regarded the ventricles as allowing the expression of psychological attributes, of the rational and sensitive souls. Nemesius, a Neoplatonist, considered the ventricles as housing the rational and sensitive parts of the soul, considered as a substance, and an immortal substance at that. These alternative views were to hold sway with different scholars up to and beyond the time of St Thomas Aquinas who attempted to reconcile them in the 13th century. Thus ventricular localization still held sway at the beginning of the second millennium. So the great physician Avicenna, working in the years 980–1037 could write that:

the sensus communis is located in the fore part of the front ventricle of the brain (now the anterior horn of the lateral ventricle). It receives all forms which are imprinted on the five senses and transmitted to it from them. Next is the faculty of representation located in the rear part of the front ventricle of the brain (now the body of the lateral ventricle), which preserves what the sensus communis has received from the individual five senses even in the absence of the sensed object. Next is the faculty of sensitive imagination, located in the middle ventricle of the brain (now the third ventricle). Then there is the estimative (rational) faculty located in the far end of the middle ventricle of the brain. Next there is the retentive and recollective faculty (memory) located in the rear ventricle of the brain (now the fourth ventricle near the cerebellum) [15].

Here the word ‘faculty’ can be read in terms of a power or capacity in the Aristotelian sense although it is not entirely clear as to whether Avicenna is alluding to components of the soul as a substance with its different parts (rational and sensitive) located in different ventricles. However, the word ‘mind’ is a façon de parler for the rational soul in Aristotle, that is, the powers of intellect and will, whereas it is the rational part of the immortal soul in Plato. This gives rise to potential confusion as to which interpretation is being followed by a particular scholar, such as in the case of Avicenna.

St Thomas Aquinas (1225–1274)

For Aristotle possession of the rational soul (i.e. a soul that includes the powers of intellect and will) is the distinctive power of man. Aquinas attempted unsuccessfully to take this Aristotelian concept of the rational soul of man, and marry it to Christian doctrine, ultimately derived through Augustine from Plato, of an immortal soul. In this attempt Aquinas radically changed the Aristotelian formulation by separating human capacities and powers from matter, thus confusing incorporality of these powers, which are abstractions, with the alleged incorporality of the soul, conceived of as the non-physical part of the human body. Aquinas argued for a mixture of Aristotelian and Neoplatonism in which the sensitive (thymos) and nutritive souls were taken as powers or capacities (as Aristotle would have it) and the rational (nous) soul was taken as a substance, and an immortal one at that (as Plato would have it) [16]. As we shall see, the mind–brain dualism of Descartes coupled with the success of Galileo's science in the face of Aristotelian teleology insured the dominance of Platonic dualism over the rational Aristotelian psychology up to the present day.

Ventricles and the soul in the century before Descartes: Leonardo da Vinci (1452–1519) and Vesalius (1514–1564)

Leonardo da Vinci, realizing the great importance of the ventricle in relation to the soul and mind, went to great trouble to give the first accurate description of the ventricles. In order to achieve this (ca 1506) he injected molten wax into the ventricular cavities in cattle. His drawings provide detail of a kind unmatched in accuracy. The drawings still ascribe the mental faculties to different ventricles. The only deviation from the doctrine laid down by Galen and Nemesius more than 1000 years earlier involved localization of perceptions and sensations to the middle ventricle (the third ventricle) rather than in the lateral ventricle.

Vesalius was sceptical about the idea that psychological functions originated in the ventricles because he noted that ‘all our contemporaries, so far as I can understand them, denied to apes, dogs, horses, sheep, cattle and other animals, the main powers of the rational soul’ – but ‘not only is the number (of ventricles) the same, but also other things (in the brain) are similar, except only in size’ [17]. Vesalius was taught that the anterior horn of the lateral ventricle contained the sensus communis because the five senses (vision, taste, smell, hearing, and touch) are brought into this ventricle by the aid of the sensory nerves [18, 19]. This, as well as the attributions to the other ventricles were the same as those of Nemesius 1100 years earlier.

Descartes (1596–1650): the soul now conceived of as only constituting the mind, no longer uniquely concerned with reasoning but now identified with consciousness, taken as ‘everything we are aware of happening within us’; this soul located in the pineal gland.

Descartes replaced the ventricular doctrine with a radically different doctrine that has dominated to the present day. He departed both from Aristotelianism and Neoplatonism as well as from the attempted synthesis of these by Aquinas and his scholastic followers. Descartes held, first of all, that perception physiologically conceived and locomotion (the ‘sensitive soul’ of Aristotle) as well as nutrition, growth and reproduction (the ‘nutritive soul’ of Aristotle) are essentially functions of the body and not of the soul, that is, essential functions of animal life, of the material substance of which animals are composed, and therefore open to purely mechanistic interpretations. Descartes departed from Plato in separating off the emotional (thymos) and appetitive (id or pathos) parts of the immortal soul, leaving the reasoning (nous or logos) part or mind as immortal and separable from the body. He then expanded this notion of the rational soul or mind to include not just intellect alone but also thought or consciousness, understood as ‘everything which we are aware of as happened within us, in so far as we have awareness of it’. Here thinking is to be identified not merely with understanding, willing, imagining, but also with sensory awareness [20]. Thought was thereby defined in terms of consciousness, that is, as that of which we are immediately aware of as happening within us. Consciousness is by this means assimilated to self-consciousness in as much as it was now held to be impossible to think and to have experiences (i.e. to feel, to perceive, to will, to imagine and cogitate) without knowing or being aware that one does. Identification of the mental with consciousness remains the dominant view of the present time, with a person taken as being identical with their mind.

Whereas, as Aristotle pointed out, Plato could not be right in having an immaterial and immortal soul (with three parts related to reason, emotion (ego) and appetites) interacting with a material body, Descartes shifted the problem to a material body interacting with an immaterial and immortal mind, identified with consciousness, in fact with feeling, perceiving, imagining, cogitating and willing. Descartes thought he had solved the problem of how non-mechanical perceptual qualities (such as colours, taste, smells etc.) arise by having these produced in the mind in the form of ideas consequent upon interactions between body and mind. Likewise he thought he had solved the problem of how voluntary movement comes about, namely through an act of will by the mind that then acts on the body. But, of course, all of these ideas are dependent on a means by which an immaterial and a material substance can interact, albeit deposited by Descartes in the pineal gland, but as much subject to Aristotle's criticisms as was the original formulation by Plato of a soul and a body.

In summary we have noted that the question of how the mind connects with the body is not a question that can arise for Aristotle. Within the framework of Aristotelianism, the very question is as senseless as the question ‘how can the shape of the table interact with the wood of the table?’ Aristotle manifestly did not leave this as a problem within his philosophy. The problem arises within the framework of Plato's dualistic philosophy, which was refuted by Aristotle, but nevertheless informed neo-Platonism and, via St Augustine, came to dominate Christian thought. To be sure, Thomas Aquinas adopted Aristotelian psychology and strove, with questionable coherence, to adapt it to Christian theology. But Platonic dualism remained the most natural conception for popular Christianity, and informed the Renaissance form of neo-Platonism. The relationship between mind and body is highly problematic to any form of dualism, and with the 17th century dominance of Descartes and the corresponding decline in the influence of Aristotelian philosophy, the problem of interaction came onto the agenda again, and has remained there ever since.

Willis (1621–1675): the idea of a soul, conceived of in Cartesian terms, located in the cortex.

Thomas Willis was an English anatomist and physician who displaced interest in the ventricles as the part of the brain concerned with the expression of our psychological attributes to that of the cortex. When Descartes died Willis was 29 and he makes explicit reference in his work to Descartes, clearly taking on much of the Cartesian concept of the soul in terms of consciousness. Thus Willis believed in a rational soul that is immortal with ‘the prerogatives of the rational soul and the difference from the other corporeal may be further noted, by comparing the acts of judgement and discourse or thought, which puts forth more perfectly and oftentimes more demonstrably than this power in the brutes’ [21]. Furthermore, Willis identifies the rational soul in the brain as doing the sensing. He suggests ‘first, that the sensible species be expressed, so that it may be impressed on the sensory; secondly that the idea of the same impression, be carried thence, by like affection and motion, by the ‘animal spirits’ flowing in the intermediate passages, to the sensus communis for otherwise sensation is not performed’ [22]. Willis held that an internal representation was formed upon the corpus callosum, the nerve bundle connecting the two hemispheres of the brain. This is reminiscent of Descartes’ idea that an image of what is seen must be produced on the surface of the pineal gland, where it is ‘presented to the soul’. Willis was then left with the problem of explaining interactions between the immaterial rational soul and the material corporeal soul in the corpus callosum, a problem of the same form as that of Descartes with the pineal gland, and harking all the way back to Plato.

Robert Boyle (1627–1691) was responsible for the transformation of alchemy into chemistry [23]. He was a close friend of Willis, whom he much influenced during their joint attempts to distil blood in order to break it down into its chemical components. Unlike Descartes they did not consider the particles that made up the blood or indeed make up the body as mechanical corpuscles [24]. Rather they took these particles to have special properties like those of salt, sulphur and spirit. Thus mixing sulphur and spirit changed the motion of the particles, producing a continual motion and agitation, so making them the active principle of life. Both Boyle and Willis called the transformation that occurred on mixing spirit and sulphur ‘a ferment’. Fermentation occurred not only in the blood but could also be seen in the process by which bread dough was transformed into a loaf of bread by leaven. Just as yeast produced heat in dough so did blood produce heat. Willis thought that fermentation in the heart released heat as a consequence of loosening the binding of the particles of spirit, salt, earth and sulphur together. Willis speculated that the brain looked like ‘a glassy alembic (i.e. a device that distils or purifies), with a sponge laid upon it, as we use to do for the highly rectifying of the spirit of wine’. In this scheme the spirits were distillations out of the blood once it had reached the brain. The sponge-like consistency of the brain then soaked up the spirits. Most importantly, Willis hypothesized that these spirits did not then pass into the ventricles but rather into the surrounding cortex. In the cortex the spirits escaped as vapours that made their way through the very small interstices of the cortex to pass from there into the nerves that leave the brain.

The critical question here is what experimental evidence did Willis have for these conjectures? His emphasis on the blood and its passage through the brain arose as a consequence of the recent discovery by William Harvey (1578–1657) of the circulation of the blood that established the subject of experimental physiology [25]. This fascination with the circulation of blood and its chemistry was accompanied by use of the technique established by Willis’ friend, Christopher Wren, of injecting ink and saffron into the circulating blood in order to trace the passage of blood vessels. Willis made such injections into one of the two carotid arteries that supply the brain with blood. Carrying out such an experiment on a dog revealed a wonderful pattern of blood vessels, many of them of very fine diameter, over the surface of the brain. It was this complex array of branching blood vessels, together with the concept that the blood they carried could, under suitable conditions, undergo fermentation to release spirits through a chemical reaction [26], that substantiated for Willis the importance of the cortex. This was further emphasized by Willis’ discovery that blood did not go into the ventricles. It followed that the ventricles could not contain spirits and so were not relevant to the capacity of the brain to sustain the expression of our psychological capacities.

Willis traced the blood vessels both over the surface of the cortex and, as they became finer, penetrating into the cortex itself. This prompted him to explore the structure of the cortex in some detail. In this he was most fortunate that his friend Boyle had discovered that spirit of wine and other substances acted as preservatives that altered the consistency of the cortex, indeed of the whole brain, from that of butter to a boiled egg. This preserved the brain so that it could then be cut into sections for a detailed examination using the microscopes recently designed by Christopher Wren and his friend Hooke [27]. The development of these techniques allowed for an unprecedented investigation into the anatomy of the brain and in particular that of the cortex. The books of Willis, such as on The anatomy of the brain and nerves, with magnificent illustrations by Christopher Wren, swept away all interest in the ventricles and focused the attention of future generations on the relationship between the structure of the cortex and the ability of animals, including humans, to express their psychological capacities [21, 22]. But it should be noted that Willis provided no direct experimental evidence to support his hypothesis that it is the cortex and not the ventricles that support our psychological abilities. His discovery that the cortex was highly vascularized but that the ventricles did not receive a blood supply, and that the cortex had a most complex structure, does not provide evidence as to which part of the brain is necessary for the expression of the psychological powers of animals. Nevertheless this was taken to be the case by future generations of researchers.

Robert Whytt (1714–1766); Marshall Hall (1790–1857) and Charles Sherrington (1857–1952): the idea of a spinal soul and reflex acts

Whytt, in the early 18th century, having shown that frogs without heads possess bodies and limbs that move in the absence of the head, was unable to conceive of such reflexes on the mechanical principles considered by Descartes and Willis. Whytt proposed that intervention of the soul was required to initiate the actions of the decapitated animals. He comments:

the motion performed by us in consequence of irritation, are owing to the original constitution of our frame, whence the soul or sentient principle, immediately, and without any previous ratiocination, endeavours by all means, and in the most effectual manner, to avoid and get rid of every disagreeable sensation conveyed to it by whatever hurts or annoys the body. If the soul were confined to the brain, as many have believed, whence is it that a pigeon not only lives for several hours after being deprived of its brain, but also flies from one place to another?

Marshall Hall in the early 19th century carried out experiments on salamanders, frogs and turtles in which parts, like the tail, move when separated from the body on being excited by the point of a needle – with this ceasing on destroying the spinal marrow [28]. He arrived at the conclusion that sensory nerves exist that do not produce sensations and that motor nerves exist that do not merely mediate volitional acts. This led Hall to the idea of a reflex act that is dependent on three components: a nerve leading from the point or part irritated to and into the spinal cord marrow; the spinal marrow itself; and a nerve or nerves passing out from the spinal marrow. All of these he conceived of as being in essential relation or connection with each other [29, 30]. In this scheme there is no need of a spinal soul. Nevertheless, even at the end of the 19th century the idea of a spinal soul was still being considered. Eduard Pflüger (1829–1910) suggested in 1853 that the spinal cord itself is sentient and possesses consciousness [31]. Michael Foster (1836–1907) in his Textbook of physiology still entertained the problem of ‘whether the frog spinal cord could be said to be conscious or even intelligent. It is to the work of Charles Sherrington, at the end of the 19th century, in laying down the conceptual scheme for the analysis of the role of the spinal cord in stepping and standing, that one must turn for completion of the research programme initiated 80 years earlier by Marshall Hall [32]. As a consequence of this work the notion of ‘a spinal soul’ no longer figured in neurophysiology.

Sechenov (1829–1905)and Pavlov (1849–1936), Fritsch (1837–1927) and Hitzig (1838–1907) and Sherrington (1857–1952): the Cartesian soul of the cortex revisited

Attempts were made to redeem the soul from the brain by treating the brain as a reflex centre along the lines that Bell had followed in refuting the idea of a spinal soul. Sechenov in 1863 suggested that under definite conditions the brain may ‘act like a machine, its functioning being manifested in so-called involuntary movements’ [33]. Also ‘by means of absolutely involuntary learning of consecutive reflexes, in all spheres of the senses the child acquires a multitude of more or less complete ideas of objects, i.e. elementary concrete knowledge. The latter occupies in the integral reflex exactly the same place as the sensation of fright in the involuntary movement; hence it corresponds to the activity of the central element of the reflex apparatus’. Ivan Pavlov took his work on the reflex stimulation of the salivary glands, or the conditioned reflex as he called it, as supporting Sechenov [34].

Fritsch and Hitzig published their work Uber die elektrische Ermgbarkeit des Grosshirns in 1870. They described the results of their experiments on stimulating the brains of dogs with galvanic currents that led them to the idea of a ‘motor cortex’ [35]. Areas were found on the surface of the cortex that gave muscular contractions involving the face and neck, on the opposite side of the dog to the hemisphere being stimulated, as well as forepaw extension and flexion. This led them to the idea that discrete areas of the cortex possess motor functions and they generalized this idea by suggesting that other specific functions might be found in other areas of the cortex. In 1902 Grunbaum and Sherrington discovered the motor area on the free surface of the hemisphere as well as the relative topography of some of the chief subdivisions of the main regions of the motor cortex [36].

These wonderful physiological discoveries did not, however, illuminate the question of whether a ‘cortical soul’ existed or, to put it more perspicuously, the question of the relationship between the mind and the cortex remained deeply puzzling to Sherrington, his contemporaries and proteges. Sherrington read extensively in the works of philosophers from Aristotle onwards but his grasp of philosophical problems was infirm. Despite acquaintance with Aristotle's De anima he failed to see the depth and fruitfulness of the Aristotelian conception of the soul and its bearing on the conceptual questions that plagued him. He noted Aristotle's ‘complete assurance that the body and its thinking are just one existence’, and that the “oneness’ of the living body and its mind together seem to underlie the whole (Aristotelian) description as a datum for it all” [37]. Sherrington did not probe the Aristotelian philosophical doctrine properly. Instead, he moved towards a Cartesian dualist conception of the relation between mind and body, unsurprisingly encountering the same insoluble problem as beset Plato and Descartes.

Charcot (1825–1893): development of the clinicopathological approach for correlating brain diseases with neurological symptoms (multiple sclerosis)

Cruveilhier in his two-volume work Anatomie pathologique du corps humain ou descriptions avec figures lithographies morbides dont le corps humain est susceptible (1829–1842) pioneered what came to be known as the clinicopathological approach for correlating brain diseases to clinical symptoms, namely the correlation of changes in behaviour with pathological changes in the cellular constituents of the brain and spinal cord [38]. In this work he emphasized both the living patient and his symptoms along with their pathological anatomy. He gave the first case of multiple sclerosis that has been documented using the clinicopathological approach, depicting the lesions of multiple sclerosis in the brain of the patient Josephine Paget. But it is to Charcot that one must turn for the first comprehensive description of disseminated sclerosis.

Charcot, the father of clinical neurology, emphasized the importance of collecting detailed clinical information as well as detailed pathological information on the same patient. He gives numerous examples of this in his Lectures on the diseases of the nervous system (1868–1877). One of these shows lesions in the upper lumbar region of the spinal cord of a patient who had suffered from multiple sclerosis, indicating clearly the posterior columns of the spinal cord invaded throughout, with lesions consisting of axons, some of very small diameter, all deprived of their medullary sheaths [39]. Although Paul Broca had before these lectures of Charcot in 1861 shown an association between aphasia and damage to the frontal cortex in his patient M. Leborgne, otherwise known as Tan [40], it is Charcot who established the clinicopathological approach as a powerful tool for teasing out the biological basis of neurological symptoms.

Aloysius Alzheimer (1864–1915): discovery of a correlation between the degeneration of cells in the cortex and the loss of psychological capacities that characterize dementia

There were several observations in the second half of the 19th century that related changes in behaviour to major endogenous lesions of the brain. In 1863 Virchow discovered amyloid (starchy) degeneration and devoted a great deal of work to the pathology of brain tumours, which he erroneously attributed to conversion of connective tissue. It is, however, to Alzheimer that one must turn in order to find a correlation between fine cellular changes in the cortex of a patient and changes in their behaviour. By the time of Alzheimer's work Santiago Ramon y Cajal (1852–1934) had discovered the individual cellular constituents of the cortex and identified these as neurons and glial cells, opening up the study of pathological changes in these cellular constituents of the brain and their correlation with changes in psychological capacities, that is, in behaviour [12, 41]. In 1896 Alzheimer published his thesis on the histopathology of general paralysis of the insane [42, 43]. This is a complication that arises after syphilis infection, with approximately 10% of all hospitalized psychiatric patients suffering from this disease at the time of Alzheimer. He showed that there were histological changes in the brain that accompanied the diseased state. In 1901 Alzheimer identified a patient in the Frankfurt Asylum with a loss of short-term memory and other psychological capacities whom he called Mrs Auguste D. This patient died in 1906 at the age of 56 and Alzheimer had access to her brain for histological purposes. Using the newly invented silver-staining technique, perfected by Nissl, Alzheimer showed in 1907 that the cortex of the brain of Mrs Auguste D contained accumulations of amyloid plaques [44]. Subsequent post-mortem examination of the brain of a 56-year-old demented patient (Johann F), after silver staining, showed the widespread presence of amyloid plaques (without any indication of neurofibrillary tangles) [45]. This was the first indication that this psychiatric condition, namely loss of memory accompanied by the loss of other psychological powers, was associated with morphological changes in the cellular constituents of the cortex.

Curiously the claim of an association between cortical degeneration and dementia was not resisted, even though loss of capacities that clearly fall into the psychological such as memory and rational thought are involved in addition to changes in motor performance, that is, in a neurological condition that, since Descartes, had been attributed to abnormalities in the ‘machinery’ of the body. Contemporary textbooks of neurological illness highlight this ‘machinery’, emphasizing that neurological patients are those who present with symptoms of a disease of the nervous system. A typical list concerns diseases of cranial nerves (involving tests on smell, face sensations, reaction to light and auditory phenomena), of motor function (maintaining limb posture), of reflex function (reflex activity of the spinal cord viz. biceps, triceps etc.), of sensory function (skin of face, legs and neck), of gait (standing and walking) and most interesting for the present purposes, of higher cortical function (memory, aphasia). Loss of memory is certainly loss of a psychological capacity, yet has been taken as falling in the purview of neurology, that is, to be considered in terms of something going awry with the machinery of the body. In contrast, contemporary manuals of psychiatry, such as Diagnostic and statistical manual of mental disorders (4th edn), provide a startlingly unhelpful and incoherent definition for the diagnosis of a mental illness as:

a clinically significant behavioural or psychological syndrome or pattern that occurs in an individual and that is associated with present distress (e.g. a painful symptom) or disability (i.e. impairment in one or more important areas of functioning) or with a significantly increased risk of suffering death, pain, disability, or an important loss of freedom. Whatever its original cause, it must currently be considered a manifestation of a behavioural, psychological or biological dysfunction in the individual [46].

So the psychological powers of memory, thinking, perceiving, feeling and so on, which if they go awry inevitably show up in behavioural changes, are separated out from such changes, and biological dysfunction is not credited with always being an accompaniment of a mental illness. A fault in the biological machinery is not then a necessary concomitant of such an illness whereas it is for a neurological illness (putting memory aside). The association of the foundations of neurology with the Cartesian machinery of the corporeal body has often left psychiatry, still working in the shadow of Descartes, with treating the incorporeal Cartesian soul, taken as the mind and equivalent to consciousness.

Emil Kraepelin (1856–1926): championing the concept that the loss of psychological capacities that characterize psychiatric illness is due to the loss of normal cellular functions in the cortex

The discovery of correlations between the loss of cells in the cortex and both classical neurological diseases such as multiple sclerosis together with the neurological and psychiatric changes accompanying dementia, as well as the discovery of localized brain areas for different psychological capacities such as language, unleashed a wide-ranging research programme in clinicopathology. Such programmes were not accompanied by concerns about the soul, identified now with consciousness as defined by Descartes, and its relation with the brain. This was because the clinicopathological approach was taken, as noted here, to be consistent with the now accepted Cartesian view that mechanistic biology was responsible for those functions that neurology now covered, so that questions concerning the relationship between the brain and mind are not relevant. This was of course not the case when considering problems concerned with mental illness. Here the Cartesian paradigm determined that these involved the mind, identified with consciousness, which had gone awry, not the biological mechanisms of the body.

Kraepelin's suggestion in around 1904 that mental illness is caused by biological changes conditional on genetic and environmental factors was revolutionary [47]. For Kraepelin each psychiatric disorder has some specific underlying biological cause. This was part of the reason why he championed Alzheimer's discovery of amyloid plaques in dementia. Kraepelin was responsible for delineating schizophrenia from manic depression, now recognized as composed of major depression and bipolar disorder. In so doing he laid the definitive foundations for psychiatry and neuropsychiatry in the 20th century [48].

Kraepelin's emphasis on searching for what has gone awry in brain function that leads to a loss of normal psychological capacities raises insurmountable difficulties for the concepts of mind adopted by either Plato or Descartes, because according to them mind is an incorporeal substance. Such difficulties are not removed by identifying the mind with the brain rather than with an incorporeal substance. This has been termed the ‘mereological fallacy’, namely of attributing to a part (in this case the brain, which does not possess psychological capacities) powers that can be logically attributed only to the whole (in this case the person whose brain it is, and who does possess psychological capacities) [7, 49]. The ‘insurmountable difficulties’ facing Kraepelin in the shadow of Plato and Descartes were not present during the 1500 year period in which Aristotelian thought was considered highly significant. For Aristotle the mind is but a manner of speaking about our psychological powers in thinking, feeling, perceiving, remembering etc. Contrary ideas of the concept of mind that are held by contemporary philosophers and psychiatrists cannot be maintained in the face of Aristotle's criticisms and ideas [8]. The continuing resistance to Kraepelin's proselytizing efforts on behalf of the search for biological changes that underlie what has gone awry in our psychological abilities, which have been traditionally treated as belonging to the domain of psychiatry, cannot be sustained in the light that Aristotle casts on the relation between the biological and the psychological.

Conclusion

This essay traces, over a period of more than 3000 years, the evolution of the concepts of soul, mind and brain and how they are conceived as being related. This places contemporary views on this subject in a historical context. What we can at least conclude is that human beings are language-using, self-conscious animals that have a mind and a body. The word ‘mind’ is used when one is talking of our psychological capacities, espically those pertaining to thinking, the will and affections consequent on being a language-using animal. The word ‘body’ is used when talking of corporeal characteristics pertaining to appearance, physique, health, and sensation. A fundamental assumption of neuropsychiatry is that when the mind goes awry there is a concomitant pathological change in the function of the body, in particular of that part of the body called the brain. A principal emphasis of this essay was to allay the concerns of many that this assumption of neuropsychiatry in some way detracts from their humanity. One of the two greatest biologists of all time, Aristotle, has shown that this is not the case.