Philosophy

That should sound very familiar since we’ve talked about this idea several times. What Malebranche points out is this. If these secondary qualities do not exist in matter, then they do not exist in the brain or any other part of the human body, since those things are made of matter. Yet they do exist in the mind, since they exist in our conscious awareness of the material world even though they do not exist in the material world itself.

But if these qualities exist in the mind while not existing in the brain, then it follows that the mind is not the same thing as the brain. Indeed, if these secondary qualities exist in the mind but not in the material world, then it follows that the mind is not a material thing of any kind. The mind is an immaterial or non-physical thing. And notice that if Malebranche is right, then the idea that the mind is not physical is not somehow an anti-scientific idea. On the contrary, it follows directly from the “mechanical” view of matter that inspired the founders of modern science.

Spelling out Cartesian dualism

Because it is in part grounded in the “mechanical world picture” that arose in reaction against the Aristotelian conception of nature, Cartesian dualism is a very different form of dualism from the kind we saw in thinkers like Aristotle and Aquinas. In particular, it takes a very different view of the nature of the body and the soul’s relationship to it.

Recall that a substance is an independently existing entity, as contrasted with the attributes of a substance, which do not exist independently, but only in the substance. For example, a stone would be a substance, whereas the color, shape, and size of the stone are among its attributes. They can exist only in the stone rather than in a free-floating way.

Now, Cartesian dualism holds that reality consists of two fundamental sorts of substance. The first is:

1. Material or extended substance ( res extensa in Latin): The very essence or nature of matter, according to Descartes, is extension. That is to say, he thinks that to be a material thing is essentially to be the kind of thing that is extended in space – that has geometrical properties like length, width, and breadth. Extension manifests itself through features like size, shape, position, and local motion (i.e. movement through space).

It is crucial to understand that for Descartes, there is nothing more to matter than that. Matter, as he conceives of it, is essentially an abstract mathematical structure. It is to be conceived of as a kind of Cartesian coordinate system (which you might recall from high school math). Color, odor, taste, heat, cold – remember, none of that is really “out there” in matter, according to Descartes. Even the black and white that you see in a drawing of a Cartesian coordinate system are not really “out there,” since those are colors in the relevant sense.

How to think about matter:

Neither, in Descartes’ view, should we think of matter in terms of Aristotelian notions like potentiality. To understand matter, you need in his view to strip away from your idea of it anything tied to qualities like color, sound, etc. and just focus on what you can model mathematically. In fact, you need to stop trying to imagine or picture it at all. It is really your intellect rather than your imagination or senses that tell you what matter is really like. That brings us to the other main kind of substance:

2. Mental or thinking substance ( res cogitans in Latin): The very nature or essence of the mind, Descartes says, is thought. What he means is that it’s not just that the mind has thoughts, but that it just is pure thought, it is nothing more than thought. It is what is left when you doubt away everything else except “I think, therefore I am.”

To be a mind – to be a res cogitans or a thing that thinks – is to be the sort of thing that reasons, remembers, wills, feels, wonders, and so on. It is not to be a physical object of any kind. The mind or res cogitans has no length, width, size, weight, position in space, or any other physical properties at all. Again, it is what you would be in an Evil Genius-type world: an entity of pure thought or consciousness totally divorced from matter.

Now, res extensa or matter by itself is just dead, unthinking stuff taking up space and moving through it. Hence the human body left to itself would be no more conscious or aware than a desk, a stone, or a toaster. Meanwhile, the res cogitans or mind by itself would be like an angel, a pure disembodied intellect with no connection to any sort of body or sense organs. However, that brings us to a third, non-fundamental and composite kind of substance identified by Descartes:

3. The human being: A human being, in Descartes view, is a kind of composite or “mash-up” of res extensa and res cogitans, of pure extension and pure thought, which arises when those two kinds of substance get into a causal relationship with one another. And it has its own distinctive attributes, that neither res extensa nor res cogitans have by themselves – namely, appetites, emotions, and sensations.

The idea is this. A body by itself is, again, pure extension. It’s just a geometrical structure. Hence by itself it would no more be capable of sensation than a cube or a sphere or a table or a stone would be. If you stuck a knife into the leg of a body that had no mind or res cogitans associated with it, then even if it were to respond by flinching and screaming, there would be no sensation of pain, or any other sensation, associated with it. Similarly, if you had a res cogitans or mind with no body, it would no more be capable of a sensation like pain than an angel would. It would just be a kind of “bloodless” disembodied intellect.

But when you stick a res cogitans together with a res extensa – a mind together with a bit of matter – then you get features like sensations, such as a sensation of pain. It has both a bodily aspect (the pain has a location, such as being in the leg where you were stabbed) and a mental aspect (it has a conscious feel to it). It’s a kind of hybrid of mental and physical. Appetites and emotions are like that too – they have both a mental aspect and a bodily aspect.

Those are the sorts of features that arise only when res cogitans and res extensa get together, and they are distinctive of human beings – as opposed to angels on the one hand (which are res cogitans without res extensa) and tables, chairs, stones, and the like on the other (which are res extensa without res cogitans).

Because it’s non-physical, you can’t really draw a res cogitans. But symbolically, we might represent our three substances (two of them fundamental and irreducible, and the third non-fundamental and reducible to a composite of the first two) as follows:

Pure res cogitans Pure res extensa Human being

Departure from Aristotle

Again, only two of these three substances are fundamental. You can break a human being down into two other kinds of substance, res extensa and res cogitans, but you can’t break those things down into any more fundamental sorts of thing. So, Descartes’ view is a kind of dualism (“dual” meaning “having two aspects”). But again, it is very different from the kind of dualism we saw in Aristotle and Aquinas, in several ways.

First, for Aristotle and Aquinas, a human being is one substance, even if this one substance has two sorts of activities, bodily and non-bodily. But again, for Descartes, a human being is a composite of two substances, two independently existing objects. (Descartes’ dualism is, accordingly, sometimes called “substance dualism” as well as “Cartesian dualism.”)

Now, Aristotle and Aquinas think of the body as part of you, as essential to what you are. That’s why Aquinas thinks of the soul after death as radically incomplete. But Descartes’ view seems to imply that the body is not essential to you. The real you is just the res cogitans, and the body or res extensa seems to be like the clothes you wear, a merely external and non-essential thing that you may or may not have while still being the kind of thing you are.

Second, for Aristotle and Aquinas, soul and body are the formal cause and material cause, respectively, of the one substance that is the human being. But not for Descartes. He thinks of the soul as the res cogitans, and of res cogitans and res extensa as two substances related to one another as efficient causes – like two billiard balls knocking into one another, except that one of them (the res cogitans) has no length, width, size, position in space, etc.

Third, Descartes’ form of dualism has a notorious weird consequence. Aristotle and Aquinas, like common sense, think of color, odor, taste, etc. as existing in matter in just the way the senses tell us they do. But Descartes, as an adherent of the mechanical philosophy, says that none of these things are really in matter. They really exist only in the mind, in the res cogitans.

Now, what that entails is that color, odor, taste, and sensation in general can exist only in the sort of thing that has a res cogitans – the sort of thing that can think to itself “I think, therefore I am.” But in the material world, only human beings do that. Therefore, Descartes concludes, only human beings are really conscious. Non-human animals are not! Non-human animals (dogs, cats, birds, etc.) are mere “automata,” like robots made of flesh and blood. They exhibit movements of various complex kinds, but no true conscious awareness. They behave as if they had sensations like pain, seeing, hearing, etc., but do not really have them, any more than an animatronic puppet at Disneyland’s Pirates of the Caribbean ride does.

“What the..? Who are you calling an automaton?!”

Everyone has always hated this implication of Descartes’ position. But there is a kind of logic to it, if you accept his starting assumptions. In particular, if you accept a “mechanical” view of matter, on which color, odor, sound, taste, heat, cold, etc. as common sense understands them are in no way in matter, then matter seems to be of its nature devoid of anything like conscious awareness as we know it. And in that case, it would seem to follow that purely material things (which dogs, cats, fish, birds, etc. would seem to be) lack any consciousness. Though they are dualists, Aristotle and Aquinas would not say this, because they reject that “mechanical” view of matter. Descartes sees himself as just following out the implications of the “mechanical” view of nature, on which all purely physical things are like machines.

The interaction problem

This brings us, finally, to the main objection traditionally raised against Cartesian dualism. It is called the “interaction problem.” The problem is this: If a mind is pure thought with no extension at all (i.e. no length, width, depth, size, position in space, etc.), and the body is pure extension with no thought whatsoever, then how can there possibly be any cause and effect interaction between them? They seem to be too radically different for that to be possible.

The way mind and body interact cannot be the way that two physical objects interact, e.g. by making physical contact, transmitting electrical current, etc. For a res cogitans has no physical surface or location, no electrical charge, no physical properties of any kind. The way they interact also cannot be the way that one thought might generate another thought in our stream of consciousness. For a res extensa is entirely devoid of any thought or consciousness. So, again, how can there be any causal interaction between them?

Common sense would say that there is in fact a cause and effect relation here. After all, what goes on in your mind (what you are thinking, feeling, etc.) seems clearly to affect what goes on in your body (what you say and do), and vice versa, all the time. Descartes agrees. The problem is that he was never able to explain exactly how this interaction is possible, given that he makes mind and body out to be two radically different kinds of substance.

Aristotle and Aquinas, despite being dualists of a sort, do not have this problem, because they don’t think of soul and body as two substances in the first place. Again, they think of them as the formal and material causes, respectively, of the same one substance (namely, the human being). So, there are, for them, not two things here that need to “interact,” but just one thing.

But Descartes thinks they are two radically different things, and the problem is that he makes them so radically different that he is unable to get them back together again, as it were. As we will see, this problem had a dramatic impact on the history of philosophy after Descartes. Among other things, it led some to react against Descartes and Cartesian dualism, and to try to develop some alternative way of understanding the relationship between mind and body.

OK, that’s enough for now. Here are some optional YouTube videos you might find useful:

Wireless Philosophy on Mind-Body Dualism: