P. M. S. Hacker - Philosophical Foundations of Neuroscience

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). 3To 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). 4This 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