Varieties of Dualism: Ontology

Varieties of Dualism: Ontology

There are various ways of dividing up kinds of dualism. One natural way is in terms of what sorts of things one chooses to be dualistic about. The most common categories lighted upon for these purposes are substance and property, giving one substance dualism and property dualism. There is, however, an important third category, namely predicate dualism. As this last is the weakest theory, in the sense that it claims least, I shall begin by characterizing it.

2.1 Predicate dualism

Predicate dualism is the theory that psychological or mentalistic predicates are (a) essential for a full description of the world and (b) are not reducible to physicalistic predicates. For a mental predicate to be reducible, there would be bridging laws connecting types of psychological states to types of physical ones in such a way that the use of the mental predicate carried no information that could not be expressed without it. An example of what we believe to be a true type reduction outside psychology is the case of water, where water is always H2O: something is water if and only if it is H2O. If one were to replace the word ‘water’ by ‘H2O’, it is plausible to say that one could convey all the same information. But the terms in many of the special sciences (that is, any science except physics itself) are not reducible in this way. Not every hurricane or everyinfectious disease, let alone every devaluation of the currency or every coup d'etat has the same constitutive structure. These states are defined more by what they do than by theircomposition or structure. Their names are classified as functional terms rather than natural kind terms. It goes with this that such kinds of state are multiply realizable; that is, they may be constituted by different kinds of physical structures under different circumstances. Because of this, unlike in the case of water and H2O, one could not replace these terms by some more basic physical description and still convey the same information. There is no particular description, using the language of physics or chemistry, that would do the work of the word ‘hurricane’, in the way that ‘H2O’ would do the work of ‘water’. It is widely agreed that many, if not all, psychological states are similarly irreducible, and so psychological predicates are not reducible to physical descriptions and one has predicate dualism. (The classic source for irreducibility in the special sciences in general is Fodor (1974), and for irreducibility in the philosophy of mind, Davidson (1971).)

2.2 Property Dualism

Whereas predicate dualism says that there are two essentially different kinds of predicates in ourlanguage, property dualism says that there are two essentially different kinds of property out in the world. Property dualism can be seen as a step stronger than predicate dualism. Although the predicate ‘hurricane’ is not equivalent to any single description using the language of physics, we believe that each individual hurricane is nothing but a collection of physical atoms behaving in a certain way: one need have no more than the physical atoms, with their normal physical properties, following normal physical laws, for there to be a hurricane. One might say that we need more than the language of physics to describe and explain the weather, but we do not need more than its ontology. There is token identity between each individual hurricane and a mass of atoms, even if there is no type identity between hurricanes as kinds and some particular structure of atoms as a kind. Genuine property dualism occurs when, even at the individual level, the ontology of physics is not sufficient to constitute what is there. The irreducible language is not just another way of describing what there is, it requires that there be something more there than was allowed for in the initial ontology. Until the early part of the twentieth century, it was common to think that biological phenomena (‘life’) required property dualism (an irreducible ‘vital force’), but nowadays the special physical sciences other than psychology are generally thought to involve only predicate dualism. In the case of mind, property dualism is defended by those who argue that the qualitative nature of consciousness is not merely another way of categorizing states of the brain or of behaviour, but a genuinely emergent phenomenon.

2.3 Substance Dualism

There are two important concepts deployed in this notion. One is that of substance, the other is the dualism of these substances. A substance is characterized by its properties, but, according to those who believe in substances, it is more than the collection of the properties it possesses, it isthe thing which possesses them. So the mind is not just a collection of thoughts, but is that which thinks, an immaterial substance over and above its immaterial states. Properties are the properties of objects. If one is a property dualist, one may wonder what kinds of objects possess the irreducible or immaterial properties in which one believes. One can use a neutral expression and attribute them to persons, but, until one has an account of person, this is not explanatory. One might attribute them to human beings qua animals, or to the brains of these animals. Then one will be holding that these immaterial properties are possessed by what is otherwise a purely material thing. But one may also think that not only mental states are immaterial, but that the subject that possesses them must also be immaterial. Then one will be a dualist about that to which mental states and properties belong as well about the properties themselves. Now one might try to think of these subjects as just bundles of the immaterial states. This is Hume's view. But if one thinks that the owner of these states is something quite over and above the states themselves, and is immaterial, as they are, one will be a substance dualist.
Substance dualism is also often dubbed ‘Cartesian dualism’, but some substance dualists are keen to distinguish their theories from Descartes's. E. J. Lowe, for example, is a substance dualist, in the following sense. He holds that a normal human being involves two substances, one a body and the other a person. The latter is not, however, a purely mental substance that can be defined in terms of thought or consciousness alone, as Descartes claimed. But persons and their bodies have different identity conditions and are both substances, so there are two substances essentially involved in a human being, hence this is a form of substances dualism. Lowe (2006) claims that his theory is close to P. F. Strawson's (1959), whilst admitting that Strawson would not have called it substance dualism.

3. Varieties of Dualism: Interaction

If mind and body are different realms, in the way required by either property or substance dualism, then there arises the question of how they are related. Common sense tells us that they interact: thoughts and feelings are at least sometimes caused by bodily events and at least sometimes themselves give rise to bodily responses. I shall now consider briefly the problems for interactionism, and its main rivals, epiphenomenalism and parallelism.

3.1 Interactionism

Interactionism is the view that mind and body—or mental events and physical events—causally influence each other. That this is so is one of our common-sense beliefs, because it appears to be a feature of everyday experience. The physical world influences my experience through my senses, and I often react behaviourally to those experiences. My thinking, too, influences my speech and my actions. There is, therefore, a massive natural prejudice in favour of interactionism. It has been claimed, however, that it faces serious problems (some of which were anticipated in section 1).
The simplest objection to interaction is that, in so far as mental properties, states or substances are of radically different kinds from each other, they lack that communality necessary for interaction. It is generally agreed that, in its most naive form, this objection to interactionism rests on a ‘billiard ball’ picture of causation: if all causation is by impact, how can the material and the immaterial impact upon each other? But if causation is either by a more ethereal force or energy or only a matter of constant conjunction, there would appear to be no problem in principle with the idea of interaction of mind and body.
Even if there is no objection in principle, there appears to be a conflict between interactionism and some basic principles of physical science. For example, if causal power was flowing in and out of the physical system, energy would not be conserved, and the conservation of energy is a fundamental scientific law. Various responses have been made to this. One suggestion is that it might be possible for mind to influence the distribution of energy, without altering its quantity. (See Averill and Keating 1981). Another response is to challenge the relevance of the conservation principle in this context. The conservation principle states that ‘in a causally isolated system the total amount of energy will remain constant’. Whereas ‘[t]he interactionist denies…that the human body is an isolated system’, so the principle is irrelevant (Larmer (1986), 282: this article presents a good brief survey of the options).
Robins Collins (2011) has claimed that the appeal to conservation by opponents of interactionism is something of a red herring because conservation principles are not ubiquitous in physics. He argues that energy is not conserved in general relativity, in quantum theory, or in the universe taken as a whole. Why then, should we insist on it in mind-brain interaction?
Most discussion of interactionism takes place in the context of the assumption that it is incompatible with the world's being ‘closed under physics’. This is a very natural assumption, but it is not justified if causal overdetermination of behaviour is possible. There could then be a complete physical cause of behaviour, and a mental one. The strongest intuitive objection against overdetermination is clearly stated by Mills (1996: 112), who is himself a defender of overdetermination.
For X to be a cause of Y, X must contribute something to Y. The only way a purely mental event could contribute to a purely physical one would be to contribute some feature not already determined by a purely physical event. But if physical closure is true, there is no feature of the purely physical effect that is not contributed by the purely physical cause. Hence interactionism violates physical closure after all.
Mills says that this argument is invalid, because a physical event can have features not explained by the event which is its sufficient cause. For example, “the rock's hitting the window is causally sufficient for the window's breaking, and the window's breaking has the feature of being the third window-breaking in the house this year; but the facts about prior window-breakings, rather than the rock's hitting the window, are what cause this window-breaking to have this feature.”
The opponent of overdetermination could perhaps reply that his principle applies, not to every feature of events, but to a subgroup—say, intrinsic features, not merely relational or comparative ones. It is this kind of feature that the mental event would have to cause, but physical closure leaves no room for this. These matters are still controversial.
The problem with closure of physics may be radically altered if physical laws are indeterministic, as quantum theory seems to assert. If physical laws are deterministic, then any interference from outside would lead to a breach of those laws. But if they are indeterministic, might not interference produce a result that has a probability greater than zero, and so be consistent with the laws? This way, one might have interaction yet preserve a kind of nomological closure, in the sense that no laws are infringed. Because it involves assessing the significance and consequences of quantum theory, this is a difficult matter for the non-physicist to assess. Some argue that indeterminacy manifests itself only on the subatomic level, being cancelled out by the time one reaches even very tiny macroscopic objects: and human behaviour is a macroscopic phenomenon. Others argue that the structure of the brain is so finely tuned that minute variations could have macroscopic effects, rather in the way that, according to ‘chaos theory’, the flapping of a butterfly's wings in China might affect the weather in New York. (For discussion of this, see Eccles (1980), (1987), and Popper and Eccles (1977).) Still others argue that quantum indeterminacy manifests itself directly at a high level, when acts of observation collapse the wave function, suggesting that the mind may play a direct role in affecting the state of the world (Hodgson 1988; Stapp 1993).

Varieties of Dualism: Ontology