Part 6c

Common Sense and the Pineal Gland

by Rene Descartes Icon

There is a problem about mistakes that I make regarding the things that nature tells me to seek out or avoid, and also regarding some of my internal sensations.

Some cases of this are unproblematic.

Someone may be tricked into eating pleasant-tasting food that has poison concealed in it. But here nature urges the person towards the pleasant food, not towards the poison, which it doesn’t know about.

All this shows is that the person’s nature doesn’t know everything, and that is no surprise.

Other cases, however, raise problems.

They are ones where· nature urges us towards something that harms us ·and this can’t be explained through nature’s not knowing something.

Sick people, for example, may want food or drink that is bad for them.

A sick man is one of God’s creatures just as a healthy one is. In each case, it seems a contradiction to suppose that God has given him a nature that deceives him.

A badly made clock conforms to the laws of its nature in telling the wrong time, just as a well made and accurate clock does. We might look at the human body in the same way. We could see it as a kind of machine made up of bones, nerves, muscles, veins, blood and skin in such a way that, even if there were no mind in it, it would still move exactly as it now does in all the cases where movement isn’t under the control of the will or, therefore, of the mind.

If such a body suffers from dropsy, for example, and is affected by the dryness of the throat that normally produces in the mind a sensation of thirst, that will affect the nerves and other bodily parts in such a way as to dispose the body to take a drink, which will make the disease worse.

Yet this is as natural as a healthy body’s being stimulated by a similar dryness of the throat to take a drink that is good for it. In a way, we might say, it is not natural. Just as we could say that a clock that works badly is ‘departing from its nature’, we might say that the dropsical body that takes a harmful drink is ‘departing from its nature’, that is, from the pattern of movements that usually occur in human bodies.

But that involves using ‘nature’ as a way of comparing one thing with another – a sick man with a healthy one, a badly made clock with an accurate one – whereas I have been using ‘nature’ not to make comparisons but to speak of what can be found in the things themselves; and this usage is legitimate.

When we describe a dropsical body as having ‘a disordered nature’, therefore, we are using the term ‘nature’ merely to compare sick with healthy. What has gone wrong in the mind-body complex that suffers from dropsy, however, is not a mere matter of comparison with something else.

There is here a real, intrinsic error of nature, namely that the body is thirsty at a time when drink will cause it harm. We have to enquire how it is that the goodness of God does not prevent nature from deceiving us in this way. This enquiry will fall into four main parts.

There is a great difference between the mind and the body. Whereas every body is by its nature divisible, the mind can’t be divided. For when I consider the mind, or consider myself insofar as I am merely a thinking thing, I can’t detect any parts within myself; I understand myself to be something single and complete. The whole mind seems to be united to the whole body, but not by a uniting of parts to parts, as the following consideration shows. If a foot or arm or any other part of the body is cut off, nothing is thereby taken away from the mind.

As for the faculties of willing, of understanding, of sensory perception and so on, these are not parts of the mind, since it is one and the same mind that wills, understands and perceives. They are (I repeat) not parts of the mind, because they are properties or powers of it. By contrast, any corporeal thing can easily be divided into parts in my thought; and this shows me that it is really divisible.

This one argument would be enough to show me that the mind is completely different from the body, even if I did not already know as much from other considerations.

The mind is not immediately affected by all parts of the body but only by the brain – or perhaps just by the small part of it which is said to contain the ‘common sense’ (as the pineal gland).

The ‘common sense’ was a supposed faculty, postulated by Aristotle, whose role was to integrate the data from the five specialized senses.] The signals that reach the mind depend upon what state this part of the brain is in, irrespective of the condition of the other parts of the body. There is abundant experimental evidence for this, which I needn’t review here.

Whenever any part of the body is moved by another part that is some distance away, it can be moved in the same fashion by any of the parts that lie in between, without the more distant part doing anything. For example, in a cord ABCD, if one end D is pulled so that the other end A moves, A could have been moved in just the same way if B or C had been pulled and D had not moved at all.

Similarly, when I feel a pain in my foot, this happens by means of nerves that run from the foot up to the brain. When the nerves are pulled in the foot, they pull on inner parts of the brain and make them move; and nature has laid it down that this motion should produce in the mind a sensation of pain as though occurring in the foot. But since these nerves stretch from the foot to the brain through the calf, the thigh, the lumbar region, the back and the neck, that same sensation of ‘pain in the foot’ can come about when one of the intermediate parts is pulled, even if nothing happens in the foot. This presumably holds for any other sensation.

One kind of movement in the part of the brain that immediately affects the mind always produces just one kind of sensation; and it would be best for us if it were always the kind that would contribute the most to keeping us alive and well. Experience shows that the sensations that nature has given us are all of just such kinds; so everything about them bears witness to the power and goodness of God.

For example, when the nerves in the foot are set in motion in a violent and unusual manner, this motion reaches the inner parts of the brain via the spinal cord, and gives the mind its signal for having a sensation of a pain as occurring in the foot. This stimulates the mind to do its best to remove the cause of the pain, which it takes to be harmful to the foot.

God could have made our nature such that this motion in the brain indicated something else to the mind – for example, making the mind aware of the actual motion occurring in the brain, or in the foot, or in any of the intermediate regions. But nothing else would have been so conducive to the continued well-being of the body. In the same way, when we need drink a certain dryness arises in the throat; this moves the nerves of the throat, which in turn move the inner parts of the brain.

That produces in the mind a sensation of thirst, because the most useful thing for us to know at this point is that we need drink in order to stay healthy. Similarly in the other cases.

All of this makes it clear that, despite God’s immense goodness, the nature of man as a combination of mind and body is such that it is bound to mislead him from time to time.

For along the route of the nerves from the foot to the brain, or even in the brain itself, something may happen that produces the same motion that is usually caused by injury to the foot; and then pain will be felt as if it were in the foot. This deception of the senses is natural, because a given kind of motion in the brain must always produce the same kind of sensation in the mind; and, given that this kind of motion usually originates in the foot, it is reasonable that it should produce a sensation indicating a pain in the foot.

Similarly with dryness of the throat= it is much better that it should mislead on the rare occasion when the person has dropsy than that it should always mislead when the body is in good health. The same holds for the other cases.

This line of thought greatly helps me to be aware of all the errors to which my nature is liable, and also to correct or avoid them. For I know that so far as bodily well-being is concerned my senses usually tell the truth.

Also, I can usually employ more than one sense to investigate the same thing; and I can get further help from my memory, which connects present experiences with past ones, and from my intellect, which has by now examined all the sources of error.

So I should have no more fears about the falsity of what my senses tell me every day; on the contrary, the exaggerated doubts of the last few days should be dismissed as laughable. This applies especially to the chief reason for doubt, namely my inability to distinguish dreams from waking experience.

The 2 are vastly different, in that dreams are never linked by memory with all the other actions of life as waking experiences are.

If, while I am awake, anyone were suddenly to appear to me and then disappear immediately, as happens in sleep, so that I couldn’t see where he had come from or where he had gone to, I could reasonably judge that he was a ghost or an hallucination rather than a real man. But if I have a firm grasp of when, where and whence something comes to me, and if I can connect my perception of it with the whole of the rest of my life without a break, then I am sure that in encountering it I am not asleep but awake.

I should not have any doubt of its reality if that is unanimously confirmed by all my senses as well as my memory and intellect. From the fact that God isn’t a deceiver it follows that in cases like this I am completely free from error. But since everyday pressures don’t always allow us to pause and check so carefully, it must be admitted that human life is vulnerable to error about particular things, and we must acknowledge the weakness of our nature.


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