Philosophical Foundations of Neuroscience. P. M. S. Hacker
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humans in particular. As we shall see, the roots of current conceptual difficulties in cognitive neuroscience are buried deep in the past. Grasping this aspect of our intellectual and scientific heritage will help to bring current conceptual problems into sharp focus. These problems are the principal concern of this book.
It might be asked why we do not concentrate more on the role of the great sensory systems, such as vision, in our historical sketch of the integrative action of the nervous system. The reason is that the early neuroscientists took up the challenge of understanding the motor system first, for it allowed experimentation which they could undertake to test their ideas with the techniques then available. This was not the case with the sensory systems. These pioneers saw the need to integrate their account of the sensory systems into their evolving knowledge of muscular contraction and movement. This led them to speculate on the relationship between vision and motor performance. It did not, however, add much to our understanding of how vision occurs, a subject that had to wait for techniques that became available only in the nineteenth and especially the twentieth century.1
1.1 Aristotle, Galen and Nemesius:The Origins of the Ventricular Doctrine
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 412b5–6). 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 402a7–8), 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 415a23–6). 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 414a20–1). 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 413a1–2).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