The Union of Mind and Body

by Keith Wilson

In the Meditations , Descartes’ primary concern was to draw a sharp distinction between the nature of the mind and the nature of the body. 1 Mind or ‘soul’ is conceived to be the fundamental essence of all thinking beings, and is without substance, size or form. 2 Body, on the other hand, is comprised of physical matter, extended in space 3 , and is the source of our everyday perceptions of the external world. 4 Along with God, Descartes considers mind to be the source of all movement, activity and intelligence in the cosmos, making human beings unique in that they alone possess the supreme faculties of thought and reason. Conversely, the physical world (including plants, animals and all other material entities) is seen as inert, passive and mechanical, devoid of any spontaneity or capability to act except in accordance with the demands of mind, God and the fundamental laws that He has established. This view essentially divides the world into two mutually exclusive, but complementary, parts: ( α ) the dynamic, thinking and free-willed part that is known on an individual scale as ‘the soul’, and on a cosmic scale as ‘God’, and ( β ) the complex, intricate and sophisticated piece of machinery that is the human body, or on an immensely larger scale, the physical universe. In each case, Descartes draws a bold line across the unity of existence in order to try and demonstrate the differing natures of these two aspects of reality.

In drawing this distinction, Descartes creates a puzzle. If the soul exists independently of physical space and matter as a unity in itself, how can it interact with or influence the body? When we pick up a bottle to pour ourselves a glass of wine, for example, the thought of performing such an action appears to arise first in the mind, which then by an act of will causes our arm to extend in order to grasp the bottle and pour. Similarly, how can the body, if it is of a fundamentally different nature and substance, influence the mind, as happens when we become thirsty, or perhaps drink too much wine and find ourselves unable to think clearly? Clearly, the two entities must be related, but what precisely is their relationship, and by what mechanism does it operate? These questions, put to Descartes by Gassendi 5 and Princess Elizabeth of Bohemia 6 , are fundamental to understanding the nature of mind, body and the relationship between them, and must therefore be answered before Descartes’ absolute division between mind and body can be fully accepted.

Descartes’ own response to these questions was complex and ultimately inconclusive. In the Meditations , he proposes that the mind and body interconnect and combine to form a third entity, 7 known as the union of mind and body . Moreover, the sensations of pain and thirst, which affect both body and mind, as well as the ability of the mind to move the body, are deemed to be properties of this union and not of the mind or body in themselves. He justifies this by asserting that human beings have certain inbuilt “primitive notions” 8 , some of which (e.g. number) are general enough to be applied to anything, others that only apply to mind (e.g. thought), body (extension) or to the union of mind and body. Attempting to apply of one of these notions to the wrong object is a philosophical error; for example, it would be incorrect to suppose that a stone ‘thinks’ as this is a notion that applies only to minds. Similarly, the notion that applies to the soul’s ability to move the body is incorrectly applied to understanding how intangible qualities or forces such as ‘heaviness’ act upon objects. Descartes argues that this notion should instead be applied to understanding how qualities of the union of mind and body enable the intangible soul to act upon the body, and vice versa . The trouble with this ‘explanation’ is that it doesn’t actually solve anything. Beyond stating that the ability of the mind to move the body derives from some mystical union, Descartes fails to provide an adequate explanation of the nature and functioning of that union in a way that makes it clear how two distinct and separate substances can affect one another. 9 His assertion that this is as easy to understand as the way that ‘heaviness’ moves an object towards the centre of the earth hardly simplifies the problem as Descartes himself denies that heaviness has any real existence 10 , and to this day the precise mechanism of gravity remains a matter of great debate.

The resulting gap in Descartes’ explanation lead Malebranche, a supporter of Cartesian dualism, to argue that not only did the notion of the union of mind and body fail to explain anything, but that it was unnecessary in a universe that is ruled by God. In the doctrine known as Occasionalism, he describes the mind and body as being united to God alone, who functions as an intermediary, moving the body in accordance with the mind’s wishes and influencing the mind in accordance with the physical conditions of the body. 11 Malebranche argued that no material substance, including nerves and muscles, can be moved by mind alone since the substance of mind has no physical existence and is therefore powerless over matter. All movements, no matter how small, must happen by acts of God, who alone is capable of moving the matter He has created. In other words, when we reach out to pick up some object, we merely initiate an act of will; our arm only appears to follow our command because God Himself wills it to move. In effect, Malebranche replaced Descartes’ union of mind and body with the will of God, which as Leibniz rightly claims, amounts to a deus ex machina 12 . Note that, in a clear demonstration of how the split between mind and body implies a similar division between God and matter, it is precisely Descartes’ argument for the independence of mind and body that leads Malebranche to invoke God as the missing link between thought and action.

Leibniz refused to accept Occasionalism not only because it sidestepped the problem, but because he believed that even divine influence should be comprehensible in terms of “secondary causes” that are amenable to philosophical analysis. 13 In its place, he proposed a radically different view where each substance of the universe is seen as being both independent of any external influence and yet proceeding in “perfect conformity to things outside it” 14 . Seen this way, the apparent communication between substances is an illusion, created by the pre-established harmony of nature in which each part obeys laws in accordance with not only its own nature, but also the nature of everything else around it. Thus, each individual substance in the universe simultaneously reflects the state of the entire universe , so that body and soul are united not through some mysterious third entity, but because the body actually ‘contains’ the soul, whilst at the same time the soul ‘contains’ the body. The mutually reciprocal nature of this relationship ensures that when our mind decides to reach out our arm to pick something up, the substances in our body behave in such a way that conforms perfectly to them being under the control of a mind that initiates this action, but without any actual communication having taken place . Similarly, when our mind appears to have been influenced by the body, such as when we are thirsty, for example, this sensation arises simultaneously in both entities because each contains a perfect ‘mirror image’ of the other and behaves in absolute accordance with it. In this way, Leibniz reconciles the duality between mind and body by uniting them within a common framework that behaves according to strict and universal laws of nature, which alone dictate the precise unfolding of events and the universe as a whole in accordance with every element in it.

Although this thesis may seem quite incredible to us now as it suggests a state of affairs that is in many respects more miraculous than the problem it set out to solve, Leibniz’s theory does represent a number of important advances over Descartes’. Firstly, the mind and body are seen as unified, not through some mysterious third entity or God, but directly and indivisibly: they are simply aspects of each other. This view subsequently lead Leibniz to propose that all ‘simple substances’—known as monads —contain a measure of perception and reason 15 , and as such have greater or lesser degrees of those qualities that were previously attributed to souls alone. Secondly, by removing the reliance upon God as the provider of primitive notions (as in Descartes) or the mediator of mind-body interaction (as in Occasionalism), Leibniz emphases the role of pre-established physical laws that dictate the behaviour of both living and non-living things. As such, his monadology can be seen as a forerunner of the modern scientific view that, in contrast to Descartes’ sterile mechanical universe, takes account of the spontaneity and ‘holistic’ nature of physical matter. However, despite its advantages, the monadology remains deeply problematic in its treatment of both causation and free will, the existence of which appears to undermine the very possibility of such pre-established harmony. 16 Even so, the fundamental unity of mind and body that Leibniz proposes goes a long way towards solving the problems outlined by Descartes.

That mind exists independently of the body can be seen as a fundamental mistake of philosophy that is a direct result of Descartes’ overly mechanistic conception of physical matter. As soon as one separates the active principal ( α ) from material substance ( β ), 17 this creates a false duality that forces Descartes and others to posit hypothetical entities in order to explain the relationship between these mutually interconnected elements. However, as soon as one permits ‘inert’ matter to have some measure of freedom, no matter how primitive, to act spontaneously of its own accord, we create what Leibniz refers to as “a substance which is endowed with a share of reason”. 18 Just as modern physics has begun to recognise that matter and energy or waves and particles are alternate manifestations of a single substance that combines the characteristics of both, Leibniz’s monadology points the way towards a unified theory of consciousness in which the properties of mind are seen to arise from the fundamental nature of the universe itself. Under such a theory, the properties of spontaneity, perception and free will are present in all substances, albeit in a very rudimentary form. These are somehow organised, amplified and exploited by living organisms to create what we recognise as ‘mind’, which can be seen to exist dependently on physical matter as an observable aspect of living beings, and not independently as Descartes believed.

© 2004 Keith Wilson, University of York