I have borrowed this phrase from the Hungarian-British writer Arthur Koestler, who in turn borrowed it – for use as a book title – from the English philosopher Gilbert Ryle, who devised it in the first place. Ryle used it as a derogatory term for criticizing the view that the mental and physical worlds are essentially separate. However, sometimes the meanings of clever phrases evolve from negative to neutral or even positive. This is true of ‘the Big Bang’, introduced by British astronomer Fred Hoyle to ridicule the idea of an expanding universe of finite age but now widely used by supporters of this view. Although an identical shift cannot be said to have happened in relation to ‘the ghost in the machine’, this phrase is often now used in a more neutral way than Ryle intended. I use it here because I like the phrase from a literary perspective; and it seems appropriate as a title for a chapter in which the main focus is animal consciousness. But I do not wish to prejudge at the outset the nature of the link between the brain and the consciousness of a human or any other kind of animal.

In previous chapters we have looked briefly at ideas about animal intelligence. For example, in Chapter 12 we considered whether the octopus might be the most intelligent of all invertebrates; and in Chapter 24 we examined the difference in intelligence between humans and our closest relatives, chimpanzees. Intelligence and consciousness are not the same thing, though they are related in the sense that both depend on the existence of a nervous system.