Even among philosophers, themselves no intellectual slouches, Descartes was a formidably clever man. A mathematician of genius, he discovered the sine law of refraction and came up with the notation of 'Cartesian co-ordinates' (hence the name). As well as optics, he worked on matters as diverse as fluid dynamics, dissection and the ultimate constituents of the Universe. He was also a soldier, a lover and a courtier. He did not like to rise before midday, would not eat an egg more than a day old, and had a dog called Mr. Scratch. Despite all this, it is through his contribution to the philosophy of the mind that he is principally remembered today.
To understand this, consider a number of things that might be true of a person (call him Fred). Fred might be six feet tall, weigh 12 stone, have two legs and two arms. Fred might also have toothache, adore his girlfriend, and love listening to Mozart. In my experience, most people believe that having a body accounts for the first set of things and not the second. In order for the second set of things to be true of us, we not only need a body we also need a mind (or soul, or whatever). That is, something that was only composed of body-substance (meat, blood and so on) could not experience pain or love; indeed, could not even think. A machine could not experience pain or love and the difference between a machine and us is that we have a mind and the machine does not.
Although I think most people believe this, it is not a view that is held by many who have thought a lot about it. That is, informed opinion nowadays is that a human being (which includes, of course, the human brain and nervous system) is composed wholly of body-substance. We could put the point by saying that minds are not really a separate thing; they are rather just the way (or a product of the way) that the brain and nervous system function. One argument for believing this goes roughly as follows:
We all accept that some mental events can cause some physical events. For example, it is my decision to pick up the glass that causes my arm to move. It is my pain that causes me to move my hand out of the fire.
We also believe that only a physical event can cause another physical event. We can't get a ball to move simply by thinking about it: in some way we have to physically interact with the ball.
So, if we believe that there are mental events that cause physical events and that only a physical event can cause a physical event, then we have to conclude that those mental events are physical events. They must be: if they cause physical events then they have to themselves be physical.
So things such as decisions and pains are physical events: probably states of the brain and nervous system.
Descartes, however, held a different view – he thought that mental events were genuinely mental and not physical. His argument is rather complicated, so I will present a version that captures something of the point he was trying to get across.
Pick up an object that is lying next to you – I shall assume it is a pen. Now try to imagine that that pen is absolutely destroyed (completely vaporised into atoms) and yet remains in front of you as a pen.
You can imagine that a part of it is vaporised and then a part would remain, but you cannot imagine the whole thing is vaporised and yet the whole thing remains.
Now take something else: a couple of peanuts. You can easily imagine one peanut being destroyed (you eat it, for example) and the other remaining as a peanut. What could be simpler?
So far, so good. But, says Descartes, now think about yourself. Can you imagine yourself being vaporised (that is, your body being destroyed) and your continuing to exist?
It seems as if we can. After all, religious people have been imagining life after the death of their bodies for centuries. So, and this is the fascinating thing, we are more like the peanuts than the pen.
That is, we could not imagine the pen being destroyed and continuing to exist because there is only one pen. We can imagine that of the peanuts, because there are two peanuts and we have only destroyed one of them. So, when we think about ourselves, the fact that we can imagine life after death shows that there must be two bits to us and we have only destroyed one of them – the body bit. The fact that we can even imagine this of ourselves shows that really, we must be two bits and not just one. We are body and mind.
It is important to note that Descartes is not arguing that because we can imagine something it must be true. That would be hopeless: I can imagine unicorns grazing on my back garden but it is not true that there are unicorns grazing on my back garden. Rather, he is arguing that the fact that we can imagine this at all shows something about ourselves: that we are like the peanuts rather than the pen – there are two bits to us and not just one.
Of course, Descartes then has to go back and refute the argument that we are only body-substance. He does this by disputing the claim that only a physical event can cause another physical event – but this is something we shall leave for another time.
This argument, or something like this argument, is troubling enough to still convince some philosophers that what is known as 'Cartesian dualism' is correct. However, most people think that it goes wrong somewhere. Perhaps the problem is that sometimes people can imagine something being destroyed, and yet continuing to exist even when there is only one thing. Here is a possible case:
Consider someone who pays no attention to the news, and does not know that Tony Blair is the Prime Minister. This rather insular person actually lives in Mr. Blair's constituency – indeed, let us suppose, he knows Mr Blair quite well. Such a person would be able to imagine that Mr. Blair was vaporised, yet the Prime Minister continue to exist.
This, however, would not be like the peanut case: there are not two things here, there is just one. The problem seems to be that there is one entity, with two descriptions: 'Mr Blair' and 'The Prime Minister'. Our rather insular person can imagine a situation in which one of the descriptions does not apply ('Mr Blair') and the other one does ('The Prime Minister').
So, the story goes, all Descartes is doing is imagining one description does not apply ('the body') and another one does ('the mind'). In neither case is it true that there are two entities in question rather than one. There are only two descriptions and we are able to imagine different things of the same object depending on which description applies. Hence, the argument does not prove dualism: we could just be one thing after all.
So, what happened to Descartes? In 1649 he was invited to Sweden, were he taught the rather impressive Queen Christina. Although the facts are not certain, it is generally thought that he contracted pneumonia as a result of her insisting he get up in the morning in order to teach her before she went riding, and died in 1650.
If thinking about the nature of the mind interests you, you might want to think about taking up philosophy. What this problem – and many other problems – show is that philosophy is not a useless, abstract subject. On the contrary, unless we have some idea what the mind is, we are not going to be able to solve the really interesting questions in fields such as neurobiology and artificial intelligence. This is why you will often find that the research projects into those areas include a philosopher on the team.