A dualist acknowledges a fundamental distinction between matter and mind. A monist, by contrast, believes that mind is a manifestation of matter — material in a brain or perhaps a computer — and cannot exist apart from matter. A dualist believes the mind is some kind of disembodied spirit that inhabits the body and therefore conceivably could leave the body and exist somewhere else. Dualists readily interpret mental illness as 'possession by devils', those devils being spirits whose residence in the body is temporary, such that they might be 'cast out'. Dualists personify inanimate physical objects at the slightest opportunity, seeing spirits and demons even in waterfalls and clouds.

F. Anstey's 1882 novel Vice Versa makes sense to a dualist, but strictly should be incomprehensible to a dyed-in-the-wool monist like me. Mr Bultitude and his son mysteriously find that they have swapped bodies. The father, much to the son's glee, is obliged to go to school in the son's body; while the son, in the father's body, almost ruins the father's business through his immature decisions. A similar plotline was used by P. G. Wodehouse in Laughing Gas, where the Earl of Havershot and a child movie star go under the anaesthetic at the same moment in neighbouring dentist's chairs, and wake up in each other's bodies. Once again, the plot makes sense only to a dualist. There has to be something corresponding to Lord Havershot which is no part of his body, otherwise how could he wake up in the body of a child actor?

Like most scientists, I am not a dualist, but I am nevertheless easily capable of enjoying Vice Versa and Laughing Gas. Paul Bloom would say this is because, even though I have learned to be an intellectual monist, I am a human animal and therefore evolved as an instinctive dualist. The idea that there is a me perched somewhere behind my eyes and capable, at least in fiction, of migrating into somebody else's head, is deeply ingrained in me and in every other human being, whatever our intellectual pretensions to monism. Bloom supports his contention with experimental evidence that children are even more likely to be dualists than adults are, especially extremely young children. This suggests that a tendency to dualism is built into the brain and, according to Bloom, provides a natural predisposition to embrace religious ideas.

Bloom also suggests that we are innately predisposed to be creationists. Natural selection 'makes no intuitive sense'. Children are especially likely to assign purpose to everything, as the psychologist Deborah Keleman tells us in her article 'Are children "intuitive theists"?'⁸¹ Clouds are 'for raining'. Pointy rocks are 'so that animals could scratch on them when they get itchy'. The assignment of purpose to everything is called teleology. Children are native teleologists, and many never grow out of it.

Native dualism and native teleology predispose us, given the right conditions, to religion, just as my moths' light-compass reaction predisposed them to inadvertent 'suicide'. Our innate dualism prepares us to believe in a 'soul' which inhabits the body rather than being integrally part of the body. Such a disembodied spirit can easily be imagined to move on somewhere else after the death of the body. We can also easily imagine the existence of a deity as pure spirit, not an emergent property of complex matter but existing independently of matter. Even more obviously, childish teleology sets us up for religion. If everything has a purpose, whose purpose is it? God's, of course.

But what is the counterpart of the usefulness of the moths' light compass? Why might natural selection have favoured dualism and teleology in the brains of our ancestors and their children? So far, my account of the 'innate dualists' theory has simply posited that humans are natural born dualists and teleologists. But what would the Darwinian advantage be? Predicting the behaviour of entities in our world is important for our survival, and we would expect natural selection to have shaped our brains to do it efficiently and fast. Might dualism and teleology serve us in this capacity? We may understand this hypothesis better in the light of what the philosopher Daniel Dennett has called the intentional stance.

Dennett has offered a helpful three-way classification of the 'stances' that we adopt in trying to understand and hence predict the behaviour of entities such as animals, machines or each other.⁸² They are the physical stance, the design stance and the intentional stance. The physical stance always works in principle, because everything ultimately obeys the laws of physics. But working things out using the physical stance can be very slow. By the time we have sat down to calculate all the interactions of a complicated object's moving parts, our prediction of its behaviour will probably be too late. For an object that really is designed, like a washing machine or a crossbow, the design stance is an economical short cut. We can guess how the object will behave by going over the head of physics and appealing directly to design. As Dennett says,

Living things are not designed, but Darwinian natural selection licenses a version of the design stance for them. We get a short cut to understanding the heart if we assume that it is 'designed' to pump blood. Karl von Frisch was led to investigate colour vision in bees (in the face of orthodox opinion that they were colour-blind) because he assumed that the bright colours of flowers were 'designed' to attract them. The quotation marks are designed to scare off mendacious creationists who might otherwise claim the great Austrian zoologist as one of their own. Needless to say, he was perfectly capable of translating the design stance into proper Darwinian terms.

The intentional stance is another short cut, and it goes one better than the design stance. An entity is assumed not merely to be designed for a purpose but to be, or contain, an agent with intentions that guide its actions. When you see a tiger, you had better not delay your prediction of its probable behaviour. Never mind the physics of its molecules, and never mind the design of its limbs, claws and teeth. That cat intends to eat you, and it will deploy its limbs, claws and teeth in flexible and resourceful ways to carry out its intention. The quickest way to second-guess its behaviour is to forget physics and physiology and cut to the intentional chase. Note that, just as the design stance works even for things that were not actually designed as well as things that were, so the intentional stance works for things that don't have deliberate conscious intentions as well as things that do.

It seems to me entirely plausible that the intentional stance has survival value as a brain mechanism that speeds up decision-making in dangerous circumstances, and in crucial social situations. It is less immediately clear that dualism is a necessary concomitant of the intentional stance. I shan't pursue the matter here, but I think a case could be developed that some kind of theory of other minds, which could fairly be described as dualistic, is likely to underlie the intentional stance — especially in complicated social situations, and even more especially where higher-order intentionality comes into play.

Dennett speaks of third-order intentionality (the man believed that the woman knew he wanted her), fourth-order (the woman realized that the man believed that the woman knew he wanted her) and even fifth-order intentionality (the shaman guessed that the woman realized that the man believed that the woman knew he wanted her). Very high orders of intentionality are probably confined to fiction, as satirized in Michael Frayn's hilarious novel The Tin Men: 'Watching Nunopoulos, Rick knew that he was almost certain that Anna felt a passionate contempt for Fiddlingchild's failure to understand her feelings about Fiddlingchild, and she knew too that Nina knew she knew about Nunopoulos's knowledge…' But the fact that we can laugh at such contortions of other-mind inference in fiction is probably telling us something important about the way our minds have been naturally selected to work in the real world.