The books

Plato and other thinkers of antiquity were not much interested in mind. They were interested in "the soul," conceived as a non-material composite of reason, passion and appetite. Plato wished to demonstrate in his Phaedo and Phaedrus that the soul is immortal, and that ethical perfection consists in the harmony of its parts.

Scientific interest in mind qua organ of thought owes itself to René Descartes, who argued in his Meditations (1641) that mind is thinking substance and matter extended substance or space. This dualism looks plausible because mental and material properties seem exclusive; we do not describe thoughts as having, say, colour or weight. But dualism involves a difficulty that stumped Descartes: how, if mind and matter are so different, can they interact?

The intractability of this problem led his successors to abandon dualism. There can be only one kind of substance, they said, either mental (idealism) or material (materialism). Idealism is a minority view; its most famous exponent is Bishop Berkeley in Principles of Human Knowledge (1710). Materialism is the dominant thesis; it says that however we understand minds, they must fit into our theories about the physical world.

The toughest version of materialism is BF Skinner's Science and Human Behaviour (1953); it claimed that all mental phenomena (thought, thirst and so on) can be explained in terms of behaviour. Gilbert Ryle's The Concept of Mind (1949) argued that all theories of mind since Descartes have been bedevilled by the myth of a "ghost in the machine," in which mind is a mysterious non-physical inhabitant of the body. Like Skinner, he thought that such notions should be eliminated in favour of behavioural descriptions.

These "behaviourist" views were very influential but they did not provide any answers to the central puzzle of consciousness: how can sensations, moods, experience and thought emanate from matter? Since the 1970s, this has become the hottest endeavour in philosophy, psychology and the neurosciences.

In his A Materialist Theory of the Mind (1968), DM Armstrong argued that mental entities are identical to physical occurrences in the brain. Although there is a great difficulty about how we should understand this identity, all materialists agree that without some kind of physical basis in the brain's neural activities, there would be no mental phenomena. Mental phenomena fall into two broad categories: (a) "intentional" states such as thoughts and desires; (b) "qualia," which are the "what it feels like" of subjective experiences: the way salt tastes, the way red looks. Not all philosophers think that intentional states have to be conscious, but qualia are paradigmatically so. So the problem of consciousness can be stated thus: how can parts of the physical world -our brains-experience qualia (and be intentional)?

Paul and Patricia Churchland argue that when neuroscience is perfected, we will see that there is no such thing as consciousness (Patricia Churchland Neurophilosophy 1986; Paul Churchland Matter and Consciousness 1988). A complete theory of the brain will do away with the problem of explaining mental phenomena (both intentional states and qualia). Such a view is known as "eliminativism."

A less swingeing view is held by Daniel Dennett (Consciousness Explained 1991), who argues that although we are "intentional systems" (as computers might also be), qualia can be analysed away. But Dennett believes that the concept of consciousness is still useful: it helps to define what we are explaining.

"Reductionists" hold that consciousness can ultimately be understood in terms of brain activity (DJ Chalmers The Conscious Mind 1996). Talk of intentional states and qualia is not eliminated but explained through neuroscience.

But others are sceptical about whether consciousness will ever be explained; Thomas Nagel's essay "What Is It Like To Be A Bat?" in his Mortal Questions (1979) argues that we cannot give an objective, third person account of subjective, first person experience; Colin McGinn in The Problem of Consciousness (1991) claims that the human mind cannot understand how matter gives rise to consciousness. In The Rediscovery of the Mind (1992), John Searle argues that our problems about consciousness arise from misunderstandings about concepts.

While philosophers struggle to make sense of these concepts, neuroscientists continue their empirical work. One extraordinary finding concerns "blindsight:" partially blind patients with missing patches in their visual fields can nevertheless detect objects there without knowing that they do, thus being conscious of something without being conscious that they are conscious. A good discussion of this is found in M Davies and G Humphreys (eds) Consciousness (1993).

Old views never completely fade away; one still finds forms of dualism surviving. But the clever money lies with the Dennetts, Churchlands, and Searles. A canter through their pages will-to use an idiom they might one day render redundant-make one fully conscious of how matters stand.