Читать книгу Philosophical Foundations of Neuroscience - P. M. S. Hacker - Страница 20

Aristotle’ s conception of the psuchē

Aristotle (385–322 bc) is the first great biologist many of whose treatises and observational data survive. His philosophical world picture shaped European thought until and, in certain respects, beyond the scientific revolution of the seventeenth century. So although his knowledge of the nervous system was almost non-existent, his fundamental conceptions of animate life are indispensable to an understanding of the reasoning of the early scientists, such as Galen and Nemesius, who probed the nature of the nervous system and its role in determining the cognitive, cogitative, affective and volitional powers of man. Moreover, as we shall see, Aristotle’ s conception of the nature of man, of the relation between organs and functions, between the body and the distinctive capacities that constitute what he called ‘the psuchē ’ was profound. The Aristotelian conception of the psuchē and the Cartesian conception of the mind, which displaced it in the seventeenth century, constitute in certain respects two fundamentally different ways of thinking about human nature, which have informed neuroscientific reflection on the integrative action of the nervous system throughout the ages.

The psuchē as the form of the natural body

Aristotle ascribed to each living organism a psuchē. The psuchē was conceived to be the form of a natural body that has life.² It was also characterized as the first actuality of a natural body that has organs (DA 412^b5–6). Aristotle’ s technical terminology needs elucidation.

In its common meaning, ‘psuchē’ signified ‘breath’ or ‘life breath’ (which one ‘expires’ at the moment of death or in a faint), as did the later Latin term ‘anima’, by which it was translated. It is linked with the idea of wind and of vital power. It was a pre-Aristotelian philosophical innovation to detach psuchē from such associations. It was an Aristotelian innovation to link it firmly to all organisms as the principle of life that informs each living being. Although ‘psuchē’ is commonly translated as ‘soul’, it is important to realize that, as used by Aristotle, ‘psuchē’ has none of the religious and ethical connotations of our term ‘soul’. psuchē is ‘the principle of animal life’ (DA 402^a7–8), and indeed of vegetal life too. For plants, no less than animals, have a psuchē. It would be equally misleading to translate ‘psuchē’ as ‘mind’, since the mind and mental powers are not associated, as psuchē is, with growth, nutrition or reproduction, which characterize all forms of living things. Nor is psuchē essentially linked with consciousness, as is the Cartesian conception of the mind. The term ‘psuchē’, which, in conformity with tradition, we shall in the sequel translate as ‘soul’, is a biological concept – not a religious or an ethical one. It will be important to keep this in mind not only in regard to Aristotle, but also in respect of neuroscientific debates in the seventeenth and eighteenth centuries on the existence of a ‘spinal soul’ (see §1.4).

The psuchē or soul is neither part of a living being nor an additional entity related to it

The phrase ‘to have a soul (psuchē )’ does not signify a relation of possession between an agent and an entity, as does ‘to have a car’. It signifies a set of characteristic powers of living things. These may be first-order abilities, i.e. the abilities to do certain things, or second-order abilities, i.e. the ability to acquire first-order abilities. The most common human form of second-order ability is the ability to learn to do various things. The ability to learn to speak a language is a second-order ability, the ability to speak a given language is a first-order ability (which, when not being exercised, Aristotle calls hexis or ‘first actuality’) and speaking is the exercise or actualization of the first-order ability (which Aristotle calls energeia or ‘second actuality’). What abilities a living creature has is exhibited in what the creature does. Powers or abilities (we shall use these terms interchangeably) are potentialities that are exhibited in actual performances, given appropriate conditions or opportunities. Potentialities are not physical parts of the object that possesses the potentiality. What a creature can do is determined by reference to what it does. Hence the Aristotelian psuchē does not stand to the body as the brain does, for it is not a part of the body. The soul or psuchē is constituted by the distinctive powers of the living creature as a whole, not of its parts that have functions. The proper exercise of the functions of parts of the body contributes to the good or welfare of the creature.

Nutritive, sensitive and rational soul distinguished

Aristotle distinguished a hierarchy of three kinds of soul in nature. The nutritive soul is the fundamental principle of biological life as such. It ‘is the most primitive and widely distributed power of soul, being indeed that one in virtue of which all are said to have life’ (DA 415^a23–6). It consists of the powers of growth, nutrition and reproduction. Plants possess only a nutritive soul. Their various organs (roots, leaves, stamens, etc.) enable them to exercise the essential functions of vegetal life. Animals have not only nutritive powers, but also powers of perception, desire and locomotion. They are accordingly said to possess a sensitive soul. The possession of a sensitive soul presupposes possession of the powers of a nutritive soul, but not vice versa. Mankind, however, is unique in nature in possessing not only the powers of a nutritive and a sensitive soul, but also the powers of a rational soul. These are thought (reasoning) and will (rational volition).

The psuchē or soul is neither an agent nor an entity, but the essential, defining powers of a living thing

The psuchē, therefore, is not an ‘inner agent’ – the subject of experience and the originator of action, animating the body but independent of it. It is not a substance or part of a substance. The soul consists of the essential, defining functions of a living thing with organs. The essential functions of a living being can be exercised only because of its possession of organs, which confer upon it the potentiality of exercising the functions of life appropriate to the kind of living being it is. It is, Aristotle insists, ‘not a body but something of a body’ (DA 414^a20–1). Body and soul ‘make up’ an animal, not as chassis and engine make up a car, but ‘just as the pupil and sight make up an eye, so in this case the soul and body make up an animal’ (DA 413^a1–2).³ To have a soul is not to possess some thing or to be related to some thing, it is to be, as it were, ‘en-souled’ (empsuchos ) – endowed with the powers of life.

The Aristotelian and Cartesianconceptions of the soul contrasted

Precisely because Aristotle did not conceive of the soul as a separate entity from the body, but rather as the powers of the living being, he did not make the mistake of attributing to the soul the exercise of the distinctive powers of the creature whose soul it is. Indeed, he noted that ‘to say that the soul is angry is as if one were to say that the soul weaves or builds. For it is surely better not to say that the soul pities, learns or thinks, but that the man does these with his soul’ (DA 408^b12–15).⁴ This observation marks a crucial divide between the Aristotelian conception and the later Cartesian one, inasmuch as Descartes ascribed all psychological functions to the mind (see §1.2). It also marks a crucial divide between Aristotelian thought and contemporary conceptions, inasmuch as current cognitive neuroscientists (and others) ascribe a multitude of psychological (especially cognitive and volitional) functions to the brain (see §3.1). To do so is in effect to ascribe to a part of an animal attributes which it makes sense to ascribe only to the animal as a whole.

Aristotle’ s conception was altogether different from that of his teacher Plato, who did indeed conceive of the soul as an entity separate from the body. Within the framework of Platonic, Augustinian and, much later, of Cartesian dualism, the pressing – and indeed insoluble – problem is to give a coherent account of the relationship between these two entities, and also to explain the essential unity of a human being. These questions cannot arise within the Aristotelian biological framework of thought. Indeed, Aristotle sapiently explains, ‘we can dismiss as unnecessary the question whether the soul and the body are one: it is as though we were to ask whether the wax and its shape are one, or generally the matter of a thing and that of which it is the matter’ (DA 412^b6–7). In short, to speak of the soul or psuchē of a creature is to speak of that creature’ s essential powers. A thing of a given kind will retain its soul as long as it can continue to exercise its characteristic functions. To destroy its powers to engage in its essential activities is to destroy the thing itself.⁵

Aristotle’ s conception of the sensus communis

In his account of perception, Aristotle distinguished the five senses (sense-faculties) and sense-organs that correspond to (four of ) them. Of course, the different sensory powers are all powers of a single unified being, the animal, with a single unified perceptual power. The senses are, Aristotle wrote, ‘inseparable, yet separate in account’ – that is, a different account is needed of the operations and mechanisms of each, but they are all constitutive elements of a unified perceptual power, and the sense-organs are all parts of a connected apparatus that he thought to be centred on the heart. The animal actually perceives only when the impulses initiated by impact on a sense-organ or any part of the body sensitive to touch are transmitted by the blood to the central sensorium in the heart. Aristotle in effect allocates to the heart the unifying functions which we allocate to the brain.⁶ He characterized this locus of our perceptual capacities as a ‘controlling sensory organ’⁷ – wrongly so, for we do not perceive with the ‘sensorium’ (no matter whether it be the heart or the brain) in the sense in which we see with our eyes and hear with our ears, and it is not a sense-organ in the sense in which the eyes and ears are.

Aristotle emphasized that although we have different sense-faculties, and although some objects of sense (the so-called proper sensibles, e.g. colour, sound, smell, taste) are uniquely discovered by a given sense-faculty (e.g. colour by sight, sound by hearing), others, the so-called common sensibles (e.g. motion, shape, size, number, unity), are discernible by more than one sense. The latter fact is one among a number of not wholly perspicuous reasons for postulating a ‘primary faculty of perception’ or ‘primary capacity for sense’, which he occasionally refers to as the ‘common sense’⁸ (and which later writers right down to the eighteenth century refer to as the ‘sensus communis’).

The sensus communis – a master organ for unifying sensibles in perception

The sensus communis is required, it seems, to explain how we can perceive common sensibles by perceiving proper sensibles (we could not see shape if we could not also see colour), and perceive them as unified attributes. For although we perceive shape, for example, both by sight and by touch, sight and touch are severally ways of detecting one and the same attribute: namely, shape (for we do not perceive, as it were, two distinct attributes, visual shape and tactile shape). Moreover, although when we perceive, say, a rose, we perceive different proper sensibles with our different sense-organs and their corresponding faculties, we perceive them as unified qualities of a single object.⁹ This, according to Aristotle, is a further reason for assuming a ‘single sense-faculty’ and a ‘master organ’ – the heart. Arguably, what we have here is a foreshadowing of contemporary neuroscientists’ preoccupation with the so-called binding problem, but without their Cartesian and Lockean confusion of supposing that the sensorium must produce an internal image or representation.¹⁰ Other reasons for which a sensus communis is held to be needed are as follows:

1 We do not see that we are seeing or hear that we are hearing. Nevertheless, Aristotle held, we do perceive that we are seeing or hearing – and that is one of the functions of the common sense.¹¹ (Some contemporary neuroscientists and neuropsychologists who are engaged in the investigation of ‘blind-sight’ assign to some part of the brain the function of a self-monitoring device to fulfil the same function.¹²) However, the reasoning is faulty, since we do not perceive that we see or hear; rather, when we see or hear, we can say that we do so – but not because we in any sense perceive that we do. This form of self-awareness needs elucidation, but not by this route.

2 By means of the sense of sight, we discriminate white from red, and by means of taste, we distinguish sweet from sour. But, Aristotle curiously observes, we also discriminate white from sweet and red from sour – and that neither by sight nor by taste.¹³ So, he infers, there must be some master faculty of perception which is employed to fulfil this function (DA 426b).

3 Since sleep affects all the sense-faculties (i.e. we do not see, hear, taste, smell or feel while asleep), waking and sleeping must be affections of one single unifying sense-faculty and controlling sense-organ.¹⁴

Finally, Aristotle also allocated to the sensus communis the functions of (a) apprehension of time; (b) image formation by the imagination, or fantasia; (c) memory (which, in his view, presupposes both (a) and (b)); and (d) dreaming.¹⁵ Items (b) – (d) are all functions which presuppose antecedent perception, but do not require any current use of a perceptual organ. They are, as it were, processes involving ‘decaying sense’ (or, as we might put it, ‘brain traces’ or ‘engrams’).

In these early reflections on the human faculties, on the conceptual structure necessary to describe them and their exercise, and in these arguments on the need for a sensus communis, we can see the beginnings of systematic scientific thought on the integrative action of the nervous system.

Two further points before we leave Aristotle:

The conception of pneuma

First, like Empedocles, Aristotle believed that there were four sublunary elements: earth, water, air and fire. To this list he added a further supralunary element, ‘the first element’ or ‘the first body’, subsequently called ‘the aether’, from which heavenly bodies are constituted. The sublunary elements naturally move rectilinearly (upwards or downwards). The motion of the first element, or aether, differs: it is (a) eternal, and (b) circular. There is some suggestion that Aristotle may have given some sublunary role to the first element in his biology. In De Generatione Animalium, he wrote:

Now it is true that the faculty of all kinds of soul seems to have a connection with a matter different from and more divine than the so-called elements … All have in their semen that which causes it to be productive; I mean what is called vital heat. This is not fire nor any such force, but it is the breath (pneuma ) included in the semen and the foam-like, and the natural principle in the breath, being analogous to the element of the stars. (736^b29–737^a1)

It is difficult to know what to make of this (let alone how it is meant to be applied to the soul of plants). Cicero, writing some centuries later on the basis of lost works of Aristotle, claimed that

He thinks that there is a certain fifth nature, of which mind is made; for thinking, foreseeing, learning, teaching, making a discovery, holding so much in memory – all these and more, loving, hating, feeling pain and joy – such things as these, he believes, do not belong to any one of the four elements. He introduces a fifth kind, without a name, and thus calls the mind itself ‘endelecheia’, using a new name – as it were, a certain continual, eternal motion.¹⁶

Aristotle appears, therefore, to have associated the capacities which constitute the soul with a ‘divine’ element that is incorruptible, which is a kind of vital heat or breath (pneuma ) present in semen and responsible for generation. This pneuma was converted in the heart into vital pneuma, which was then conducted along blood vessels to muscles, where it effected contraction. The concept of pneuma was to have a long, confused history throughout the struggle to clarify the integrative action of the nervous system.

Second, it is noteworthy, and relevant to the conception of the ‘spinal soul’ that preoccupied neuroscientists in the eighteenth century (see §1.4), that Aristotle observed that ‘certain insects go on living when divided into segments’. This shows, he argued, that ‘each of the segments has a soul in it identical in species, though not numerically; for both of the segments for a time possess the power of sensation and local movement. That this does not last is not surprising, for they no longer possess the organs necessary for self-maintenance’ (DA 411^a17ff.) The reasoning is that since the two halves of the insect continue to display sensitivity and capacity for motion, each half must have its own sensitive (and motor) soul. It is important (and relevant to the eighteenth-century debate on the spinal soul) that Aristotle did not think that the whole insect consists of a single ‘master soul’, as it were, and apart from that, two additional souls, one in each half of its body. Rather, the whole insect has a certain range of capacities, and if it is cut in half, the two halves will then have a certain more limited range of capacities.

Herophilus

The revolutionary writings of Herophilus (325–255 bc) are now lost, so knowledge of his discoveries on the nervous system are due to later authors, especially Galen (130–210 ad) who placed Herophilus as the most prominent among his forebears, commenting that Herophilus ‘increased anatomical theory the most’¹⁷ for ‘the neuroanatomical knowledge of the brain remained scanty until Herophilus performed his dissections’.¹⁸ These dissections confirmed Aristotle’ s distinction between the cerebrum and the cerebellum¹⁹ and revealed the nature of the ventricles, including the fourth ventricle of the cerebellum.²⁰ But perhaps most importantly Herophilus identified nerves and their emergence from the brain and spinal cord, for, as Galen records, Herophilus holds that ‘all the nerves throughout the body below the head grow either from the cerebellum (parenkephalis) or from the spinal marrow’.²¹ Herophilus also distinguished between purposive (motor) and sensory nerves, probably because of his extensive vivisection, and followed the optic nerve from the brain to the eye.²² On the other hand, Herophilus failed to distinguish between nerves on the one hand and tendons/ligaments on the other, considering these to be the terminal extensions of the nerves in muscles.²³ It was not until the beginning of the twentieth century that the ending of nerves in muscle, at the synapse formed by the neuromuscular junction, was finally resolved.²⁴

Galen: motor and sensory centres

It was Galen who apprehended that nerves arising from the brain and spinal cord are necessary conduits for initiation of muscle contraction. The Aristotelian account was changed accordingly. Vital pneuma was held to be delivered by blood vessels to the brain and converted to psychic pneuma (whose composition was unclear), whence it was conducted along the nerves to the muscles. Thus, ‘The nerves which in consequence enjoy the role of conduits, carry to the muscles the forces that they draw from the brain as from a source.’²⁵ This allowed the muscles to contract, probably as a consequence of their ballooning as they filled with the psychic pneuma.²⁶

Galen distinguished motor from sensory nerves

Hence Galen introduced the idea of motor nerves derived from the spinal cord.²⁷ As a consequence of his observations on injured chariot drivers, he also distinguished sensory from motor nerves. These were differentiated in terms of their relative ‘hardness’, motor nerves being ‘hard’ and sensory nerves ‘soft’.²⁸ The hard motor nerves were uniquely associated with their origins in the spinal cord, the soft sensory nerves with the brain.²⁹

Using the term ‘soul’ in an Aristotelian sense, Galen argued 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.³⁰ According to this conception, and given the assumed relationship between the relative hardness/softness of nerves, the motor/sensory differentiation and the fact that the hardest nerves are found uniquely associated with the spinal cord (caudal), the idea of two separate souls or principles of activity, one associated with the spinal cord and one with the brain is natural (although it is unlikely that Galen succumbed to this temptation). It was to have considerable vogue in the eighteenth century to explain the existence of continuing spinal cord reflexes in decorticate animals. However, by then the conception of the soul (the anima ) had changed and had become entwined with that of consciousness.

Galen: the functional localizationof the rational soul in the ventricles

This association of the pure sensory nerves with the brain, and the fact that these nerves were very soft, carried with it important implications for brain function.³¹ It is clear that Galen associated the whole brain, and not just the ventricles, with the mental capacities of humans. In On the Usefulness of the Parts of the Body he states: ‘In those commentaries I have given the demonstrations proving that the rational soul is lodged in the enkephalon; that this is the part with which we reason; that a very large quantity of psychic pneuma is contained in it; and that this pneuma acquires its own special quality from elaboration in the enkephalon’ (‘enkephalon’ is a cognate of ‘enkephalos’, meaning ‘that which is in the head’). So Galen attributed to the brain those functions in perception that Aristotle had attributed to the heart. However, this did not distinguish between the respective roles of the cortex and the ventricles. Galen did not ascribe to the cortex any special function with regard to the higher mental powers such as reasoning, for he observed that donkeys have a highly convoluted brain. Consequently, he thought that the cerebral convolutions could not be associated with intelligence. Instead, he identified the ventricles, rather than the cortex, as the source of such powers as reasoning.³² Galen enjoyed absolute authority for more than a millennium. It therefore comes as no surprise that the association of the ventricles with higher mental functions was elaborated in detail in the following centuries.

Nemesius: the formal attribution ofall mental functions to the ventricles

It was Nemesius (c. 390), the bishop of Emesa (now Homs) in Syria, who developed the doctrine of the ventricular localization of all mental functions, rather than just the intellectual ones. Unlike Galen, he allocated perception and imagination to the two lateral ventricles (the anterior ventricles), placing intellectual abilities in the middle ventricle, reserving the posterior ventricles for memory. Hence the idea that imagination/perception, reasoning and memory are to be found in the lateral, third and fourth ventricles respectively. Nemesius claimed that this localization was based not on a whim but on solid evidence, for he states that

The most convincing proof is that derived from studying the activities of the various parts of the brain. If the front ventricles have suffered any kind of lesion, the senses are impaired but the faculty of intellect continues as before. It is when the middle of the brain is affected that the mind is deranged, but then the senses are left in possession of their natural function. If it is the cerebellum that is damaged, only loss of memory follows, while sensation and thought take no harm. But if the middle of the brain and the cerebellum share in the damage, in addition to the front ventricles, sensation, thought, and memory all founder together, with the result that the living subject is in danger of death.³³

In relation to the anterior ventricles, he states:

Now, as organs, the faculty of imagination³⁴ has, first, the front lobes of the brain and the psychic spirit contained in them, then the nerves impregnated with psychic spirit that proceed from them, and, finally, the whole construction of the sense-organs. These organs of sense are five in number, but perception is one, and is an attribute of the soul. By means of the sense-organs, and their power of feeling, the soul takes knowledge of what goes on in them.³⁵

This localization of the various mental functions in the ventricles became known as the ventricular doctrine.

The soul taken as spiritual sub-stance rather than as first actuality

It is noteworthy that Nemesius conceived of the soul in very different terms from Aristotle and his followers. Nemesius, as a Christian, was more influenced by Neo-Platonism than by Aristotelian philosophy (he was attracted by the doctrine of the pre-existence of the soul, and also by metempsychosis). He did not conceive of the soul as the form of the body, but as a separate, indestructible spiritual substance, linked with the body in a ‘union without confusion’ in which the identity of each substance is fully preserved. Consequently, he did not attribute perception and cognition to the human being (i.e. to the whole animal), but rather to the soul. The attribution of psychological attributes to the soul of a living creature rather than to the living creature as a whole, deviates importantly from the Aristotelian conception. As we shall see, the tendency to explain how a living being perceives, thinks, feels emotions, etc. by reference to a constituent substance’ s or subordinate part of that being’ s perceiving, thinking, feeling emotions, etc. runs like a canker through the history of neuroscience to this very day.

One thousand years of the ventricular doctrine

The ventricular doctrine for the localization of psychological functions, established in the first centuries of the first millennium, was still accepted and promulgated by scholars at the beginning of the second millennium. Thus Avicenna (Abu Ali al-Husain ibn Abdullah ibn Sina), a great physician and philosopher working in the years 980–1037, could write that:

[T]he sensus communis is located in the forepart of the front ventricle of the brain. It receives all the 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, 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 in relation to the animal soul. This faculty is located in the middle ventricle of the brain. Then there is the estimative faculty located in the far end of the middle ventricle of the brain. Next there is the retentive and recollective faculty located in the rear ventricle of the brain.³⁶

Ventricular localization still held sway during the Quattrocento in Italy. Physicians like Antonio Guainerio (d. 1440) ascribed problems with memory in some patients to ‘an excessive accumulation of phlegm in the posterior ventricle so that the “organ of memory” was impaired’.³⁷ The doctrine was still being taught in the best teaching centres at the beginning of the sixteenth century, as is indicated by illustrations produced in the 1494 edition of Aristotle’ s De Anima. Leonardo da Vinci (1452–1519) went to great trouble to determine the first accurate description of the ventricles, given their presumed importance in the mental life of humans. In order to achieve this (c. 1506), he injected molten wax into the cavities in cattle. His drawings provide detail of a kind unmatched in accuracy, although they still ascribe the mental faculties to the different ventricles. In these drawings Leonardo’ s only deviation from the doctrine laid down by Nemesius 1,100 years earlier was to localize perception and sensation in the middle ventricle rather than in the lateral ventricles, a change which Leonardo made on the ground that most sensory nerves converged on the midbrain rather than more anteriorly.

Andreas Vesalius (1514–1564) comments on the dominance of the ventricular doctrine when he was at the University of Louvain. He gave detailed, accurate drawings of the human ventricles in 1543 and accompanied these in his Fabrica with descriptions of how psychic pneuma is generated in the ventricles and then distributed to the nerves in ways that are not much different from those suggested by Galen.³⁸ Although Vesalius subscribed to the conception of ventricles as the origins of the psychic pneuma, following Galen, he was sceptical about the idea that psychological functions originate in the ventricles. His writings on this are important, for they prepare the way for Thomas Willis, who, a century later, was to provide the definitive shift of attention away from the ventricles to the substance of the brain itself in the search for the physical basis of psychological functions. Vesalius noted that as the shape of the ventricles is much the same in a variety of mammals, including humans, it is difficult to associate psychological powers such as reasoning, which demarcate humans from other mammals, with the ventricles.³⁹