3. Mental Causation
3.1 Descartes on the Union of Mind and Body
According to Descartes, the modal argument that I could exist without my body shows that the mind is different from the body. Reflection shows that they are different substances. The body is an extended and not a thinking substance. The mind is a thinking and not an extended substance. Despite their fundamentally different natures, there is interaction between mind and body. As Descartes notes in the Sixth Meditation, ‘I am not merely present in my body as a sailor is present in a ship, but I am very closely joined and, as it were, intermingled with it, so that I and the body form a unit.’ (Descartes (1984): 56) It is remarkable that Descartes rejects exactly the kind of metaphor for the interaction of mind and body that one might expect after reading the Second Meditation. How is this ‘union’ and ‘intermingling’ of the two fundamentally different substances of mind and body possible? Descartes concluded that there is causal interaction between the two substances.
3.2 Princess Elisabeth’s Objection
In a letter to Descartes in May 1643 Princess Elisabeth of Bohemia formulated a powerful objection to the possibility of causal interaction between thinking and extended substances:
‘I beseech you, tell me how the soul of man (since it is but a thinking substance) can determine the spirits of the body to produce voluntary action. For it seems every determination of movements happens from an impulsion of the thing moved, according to the manner in which it is pushed by that which moves it, or else, depends on the qualification and figure of the superficies [i.e. surface] of this latter. Contact is required for the first two conditions, and extension for the third. You entirely exclude extension from your notion of the soul, and contact seems to me incompatible with an immaterial thing.’ (Elisabeth of Bohemia (1994): 11)
On a very plausible understanding of causation, it is very implausible that a Cartesian mind can stand in causal relations with the body it is connected to.
In his response to Elisabeth, Descartes explains that we can understand the interaction of the body and the soul analogously to the way we understand forces like gravity or magnetism. There are other ways of understanding causation than the billiard ball model that Elisabeth appeals to. Forces can act upon bodies without the bodies touching each other.
Hume analysed causation as constant conjunction: ’we may define a cause to be an object, followed by another, and, where all the objects similar to the first are followed by objects similar to the second. In other words, where, if the first object had not been, the second never had existed.’ We can observe constant conjunction even in cases where we can observe neither contact nor extension, for instance when my body is set in motion because my mind wants something, or when the content of the mind is modified when something is perceived through the body’s sense organs. It would be anachronistic to attribute Hume’s view to Descartes, but there is a connection between constant conjunction and forces: forces may explain why constant conjunctions occur, and conversely, constant conjunctions may point to forces at work.
The question is whether Descartes can give a plausible account of how immaterial souls and material bodies can exert forces on each other. Descartes was fairly unmoved by Elisabeth’s objection. This may strike us as odd, as it sounds so decisive, but it shows a deep facet of Descartes’s philosophy. Pure reason led him to conclude that there are material as well as thinking substances. It is obvious that there is mental causation, just as it is obvious that there is perception, so these two substances, different as they are, interact with each other. These are amongst the most fundamental truths discovered in his Meditations. The problem of mental causation is a challenge for Descartes’s physics rather than a refutation of his metaphysics.
Descartes thought that there is a fixed amount of motion in the universe, where the motion of an object is the speed at which it is moving multiplied by its mass (or size, assuming matter is homogenous). Descartes’s physics contained a law of the conservation of motion: no motion is ever added to or disappears from the universe. Thus the mind cannot move an object that is at rest, as that would add motion to the universe. However, it is consistent with this principle that the mind can change the direction of existing motion.
Unfortunately, Descartes’s law of the conservation of motion is false. In Newtonian physics, what is preserved is not motion, but momentum: the product of mass and velocity, where velocity is a vector of speed and direction. Suppose the mind could change the direction of a moving object. Then it would change the quantity of momentum in the universe. Even though mental causation as Descartes envisaged it is consistent with his physics, it is inconsistent with Newtonian physics, which is the theory that describes the physical realm more accurately. This was pointed out by Leibniz. (For details, see Woolhouse (1993), chapters 6 and 8.)
3.3 The Closure of Physics
Leibniz’s argument against the possibility of causation between thinking and extended substances is based on the closure of physics.
A fairly weak formulation of this principle is that the physical realm is closed in the sense that nothing can be added to or taken away from it. Matter or energy or momentum do not appear out of nothing or disappear into nothing. Descartes subscribed to a version of the closure of physics in this sense with his law of the conservation of motion.
A stronger principle is that to explain the occurrence of a physical event, we need not look outside the physical realm. Explanations of physical events appeal to other physical events and natural laws. What supports this version of the closure of physics is yet another version, namely that physical events occur only because other physical events have occurred. Put in terms of causation, every physical event has a physical cause that suffices to bring it about (or completely determines the chance of its happening, if there is chance in nature).
This allows for the overdetermination of events so that, although the movement of your arm to pick up a cup of tea is caused by physiological events, it is at the same time caused by volitions and desires in your mind, so that it has two sufficient mental and physical causes. To exclude this option, we can add a principle that physical events are not overdetermined: the causes that suffice to bring about a physical event exclude any other events from also being its causes. Notice that this rules out overdetermination in general, i.e. also by physical events, not just overdetermination by physical and mental events. This is fairly plausible. Causes make a difference to what happens. If the occurrence of an effect was overdetermined by two or more causes, then the absence of all but one of them would make no difference to the course of events. Even if we allowed some cases of overdetermination, it is plausible that, for there to be genuine mental causation, events caused by mental phenomena should not be systematically overdetermined by mental and physical causes. This would make the mental cause appear to be redundant. If overdetermination is excluded, it follows from the closure of physics that there could be no such thing as mental causation, if the mental is non-physical.
The billiard ball model of causation may be overly simplistic, even when supplemented by forces, but it raises another question relevant to mental causation. It seems as if anything that happens on a billiard table when a ball is cued is due only to the initial impact of the cue on the ball, the distribution of balls on the table and the properties of the surface of the billiard table. Now think of the billiard balls as fundamental particles and the billiard table as space. Is anything that happens in the universe caused by what happens at the fundamental level or is there downwards causation, where conglomerations of fundamental particles have causal effects on fundamental particles? Is it possible for your body or your brain to cause events to happen, rather than that whatever happens is caused by the ongoings of the fundamental particles your body or your brain are composed of?
The closure of physics and the rejection of overdetermination do not exclude the existence of mental phenomena that, while themselves not physical, are caused by physical phenomena and have no further causal effects. T.H. Huxley gave classical description of epiphenomenalism:
‘The consciousness of brutes would appear to be related to the mechanism of their body simply as a collateral product of its working and to be as completely without any power of modifying that working as the steam-whistle which accompanies the work of a locomotive engine is without influence upon its machinery. Their volition, if they have any, is an emotion indicative of physical changes, not a cause of such changes. […] It seems to me that in men, as in brutes, there is no proof that any state of consciousness is the cause of change in the motion of the matter of the organism.’ (Huxley (1992): 136)
According to epiphenomenalism there is an essential difference between the mental and the physical. If the view is coupled with materialism, it has to be a property dualism. All objects are material, but there are two different kinds of properties, physical and mental ones.
The question remains whether epiphenomenalism can give a plausible account of how mental states come about. The view cannot hold that causation involves transmission of energy, for instance, as then any mental state, once it has been caused, would have to remain in existence forever, as should it cease to exist the energy that was transmitted onto it would cease to exist with it, contradicting the conservation of energy. If the mental is not causally efficacious, it cannot transmit energy onto anything either.
Epiphenomalism faces one half of the problem Descartes has when it comes to the interaction of the mental and the physical. How are those causally inert mental events caused by physical events? How can a thing with a certain physical property (e.g. reflecting light of a certain wavelength) bring about that another thing has a certain mental property (e.g. seeing red)? Epiphenomenalism also excludes agency of minded creatures, in the sense that the mental states of a creature explain or lead to or influence the creature’s behaviour.
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