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Descartes and Mind-Body Union
Descartes divided substances into the mental (res cogitans), the physical (res
extensa) and God. Mental substance is non-extended and physical substance is
extended. So, how do mind and body communicate? Via the conarium or pineal
gland. Descartes thought that “animal spirits” travelled along the nerves to the brain
and then to the pineal gland at the centre. There, they “agitate” the gland and this
motion transmits information to the mind. The process can happen in reverse.
Descartes did not think that the mind and body are wholly separate and related like a
driver to a car. In a famous passage in Meditation VI, he writes:
Nature also teaches me, by these sensations of pain, hunger, thirst and so on, that I
am not merely present in my body as a sailor is in a ship, but that I am very closely
joined to it and, as it were, intermingled with it, so that I and the body form a unit. If
this were not so, I am who am nothing but a thinking thing would not feel pain when
the body was hurt but would perceive the damage purely by the intellect, just as a
sailor perceives by sight if anything in his ship is broken. Similarly, when the body
needed food or drink, I should have an explicit understanding of the fact instead of
having confused sensations of hunger and thirst. For these sensations of hunger,
thirst, pain and so on are nothing but confused modes of thinking which arise from the
union and, as it were, the intermingling of the mind with the body. (MM VI: CSM II: 56)
But this vague notion of intermingling didn't satisfy many. Princess Elizabeth of
Bohemia asked Descartes to tell her:
“how the human soul can determine the movement
of the animal spirits in the body so as to perform
voluntary acts — being as it is merely a conscious
substance. For the determination of movement
seems always to come about from the moving
body's being propelled — to depend on the kind of
impulse it gets from what sets it in motion, or again,
on the nature and shape of this latter thing's surface.
Now the first two conditions involve contact, and the
third involves that the impelling thing has extension;
but you utterly exclude extension from your notion of
soul, and contact seems to me incompatible with a thing's being immaterial.”
So, the problem is that if causation requires contact, then how can the mental and
the physical if the mental is non-physical and the physical non-mental? It is important
to remember that Descartes and his contemporaries understood causation in terms
of contact. For A to cause B, A must be spatiotemporally connected with B at some
point to impart its force or energy. Causation is snooker-ball causation.
Descartes never seemed too troubled by the Causation Problem. To Gassendi, who
raised the same problem as Elisabeth, he wrote:
[This problem presupposes] amongst other things an explanation of the union between
the soul and the body, which I have not yet dealt with at all. But I will say…that the whole
problem…arises simply from a supposition that is false and cannot in any way be proved,
namely that, if the soul and the body are two substances whose nature is different, this
prevents them from being able to act on each other (AT VII 213: CSM II 275).
There are two things here. Firstly, that there is an explanation of the mind-body
union. Secondly, that there is no problem with two substances are of different natures
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does not prevent them from acting on one another. What more does Descartes say
about each?
On the mind-body union, Descartes says in his letters to Elisabeth that our idea of it
is one of our three basic types of idea. He writes:
“I believe that there are some basic concepts in us, that act as templates on the basis of
which we form other concepts…for body, we have the concept of extension, for mind, we
have just the concept of thought, by which we are to understand all the perceptions of the
understanding and actions of the will ; and finally, for the body and mind together, we
have nothing more than the concept of their union, on the basis of which we have the
concepts of the force that the mind has to move the body and the body to move the mind,
in causing its emotions and passions.” (letter to Elizabeth 21/v/1643)
“We do not understand those things relating to the union of mind and body [imagination,
emotions, passions] clearly through the understanding nor through the understanding
aided by the senses; but they make themselves clearly known to us via the senses…It
through metaphysical thoughts, that exercise the pure understanding, that the concept of
mind becomes familiar; and is through the study of mathematics, which exercises
principally the imagination through the consideration of shapes and motion, that the
concepts of distinct bodies become familiar; and finally, it is just through living and
ordinary conversations, and abstaining from thinking and studying those things that
exercise the imagination, that one comes to understand the union of mind and body.”
(letter to Elizabeth 28/vi/1643)
So it seems that Descartes says we have a primitive concept of the union of mind
and body through just observing how the physical world of objects and our body
cause and are caused to change by phenomena in the mental world of thoughts.
Furthermore, we notice phenomena at the intersection: imaginings and emotions
have a mental and physical component – a foot each in both worlds.
But how do they act? Descartes gives us no clear answer. He writes:
“We need to recognize that the soul is really joined to the whole body, and that we cannot
properly say that it exists in any one part of the body to the exclusion of the others. For
the body is a unity which is in a sense indivisible because of the arrangement of its
organs, these being so related to one another that the removal of any one of them
renders the whole body defective. And the soul is of such a nature that it has no relation
to extension, or to the dimensions or other properties of the matter of which the body is
composed: it is related solely to the whole assemblage of the body's organs. This is
obvious from our inability to conceive of a half or a third of a soul, or of the extension
which a soul occupies. Nor does the soul become any smaller if we cut off some part of
the body, but it becomes completely separate from the body when we break up the
assemblage of the body's organs…But even though the soul is joined to the whole body,
nevertheless there is a certain part of the body where it exercises its functions more
particularly than in all the others. The part of the body in which the soul directly exercises
its functions is not the heart at all, or the whole of the brain. It is rather the innermost part
of the brain, which is a certain very small gland situated in the middle of the brain's
substance and suspended above the passage through which the spirits in the brain's
anterior cavities communicate with those in its posterior cavities. The slightest
movements on the part of this gland may alter very greatly the course of these spirits, and
conversely any change, however slight, taking place in the course of the spirits may do
much to change the movements of the gland.” (The Passions of the Soul §31)
Descartes says that the soul acts through the pineal gland but is not “imprisoned”
there. The pineal gland that is agitated by the soul, whose movements cause the
release of animal spirits, that flow from the ventricular system in which the pineal
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gland sits, through the brain and along the nerves. But if it is not imprisoned and the
soul is joined to the whole body, how is this so?
He writes:
“I saw that the gravity, while remaining coextensive with the heavy body, could
exercise all its force in any one part of the body; for if the body were hung from a rope
attached to any part of it, it would still pull the rope down with all its force, just as if all
the gravity existed in the part actually touching the rope instead of being scattered
through the remaining parts. This is exactly the way in which I now understand the
mind to be coextensive with the body-the whole mind in the whole body and the
whole mind in any one of its parts.” (MM R VI: CSM II: 297)
Descartes therefore says that the soul is in the body like gravity is in a body. It is
throughout the body and yet can act at one point. (When questioned about this
analogy by Elizabeth, Descartes admits it is not perfect, as the mind and body/brain
are distinct substances whereas a body and its "gravity" are not: see his letter of
28/vi/1643)
Although not envisaged by Descartes, one good feature of the gravity analogy is that
we are prepared to say that the moon causes the tides even though there is no
contact between the two. But we can also explain gravity in purely physical terms.
There is mass and there are forces that massy objects are susceptible to. We can
calculate the behaviour of massy objects under gravity. We can absorb gravity into
our theory of physics precisely because it plays a systematic (measurable,
predictable) role. But we are still in the dark about how particular activities of the soul
lead to particular activities in the body. For all that we can see scientifically is the
behaviour of the brain and body. It is only from within our mind that we can be aware
of the thoughts and sensations associated with them. We have nothing more than a
correlation of mental and physical worlds but not causation.
Hume argued (according to the standard view) that there is no real relation of
causation. For A to cause B is for there A-type events to be regularly and
spatiotemporally followed by B-type events. As long as there are robust, regular
correlations between mental and physical events, then there is nothing more we can
ask for. But most people do not accept Hume's view today.
Most scientists and philosophers today would raise the objection that the physical
world is causally closed. Roughly speaking, this is the idea that every event that
happens in the physical world has a complete physical cause. For example, we can
explain why the light comes on when I press the switch entirely in terms of physics:
the matter in my hand causes the matter in the switch to move which causes the
contact. But then if there is a complete physical explanation of every event in the
universe, there is nothing for a non-physical mind to "do". Since minds clearly do
have physical effects (my thought leading to action) and are affected by physical
events (think: drinking coffee / alcohol, brain damage, etc.), this is strong evidence
that they are physical things.
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