With a little ingenuity, he claims, consciousness can now be studied routinely: we have several ‘signatures’ of conscious thought in the activity of the brain, and a theory, descended from Baars’s ideas, of what consciousness does for us, and why it exists. His message is that there has been enormous progress. He trained in mathematics and psychology and now runs a laboratory at the Collège de France outside Paris. Dehaene is a neuroscientist with little time for philosophers. In Consciousness and the Brain, Stanislas Dehaene calls this ‘the first glimpse of a neuronal correlate of conscious experience’. What they found was that in the ‘early’ stages of visual processing, the activity of neurons mostly reflected what was being presented to the eyes, but that deeper inside the brain were neurons whose firing was associated instead with the monkey’s report (via the lever) of what it was experiencing. Monkeys were trained to use a lever to indicate which image they were seeing in just such a situation (a clever experimental feat in itself), and Logothetis and his colleagues recorded the activity of neurons in different parts of the monkeys’ brains to work out which were active when each image was being registered. If one of your eyes is shown a face and the other is shown a house, your experience will be house, then face, then house, and so on. It had been known since the 19th century that if quite different images are shown to each of your eyes at the same time, your conscious experience doesn’t blend the two but flips between them. In 1990, Francis Crick, working with Christof Koch, offered a somewhat different theory, focusing on consciousness in visual experience, and around the same time some groundbreaking experiments were undertaken by Nikos Logothetis, working with Jeffrey Schall and David Leopold.

We are conscious of whatever is currently in that workspace. But in 1988, Bernard Baars put forward his ‘global workspace’ theory, that a system in the brain functions to integrate diverse sources of information for use in a slow, attentive style of thinking. Consciousness was not a serious topic for science – it was too elusive, too much of a mess, yielding little but fruitless speculation. Before then, if you brought the subject up, the result would be like the scene in a Western when someone says the wrong thing in a saloon and the piano suddenly stops playing. It wasn’t until the 1980s that scientists’ reluctance to talk about consciousness relented.

Conscious thought might involve an ability to think about one’s own mental states, to perceive them as thoughts and feelings. Consciousness was seen as an aspect of certain sophisticated forms of experience that have both a distinctive feel and a role in intelligent thought. The others were the problem of ‘qualia’, explaining how the subjective feel of the mind could be a feature of a physical system and ‘intentionality’, the fact that thoughts can be about things, and can represent objects and events, including those far removed from us. That moment having finally passed, say forty years ago, philosophy took the problem of consciousness as one of the three major challenges faced by anyone attempting a theory of the relation between mind and body. In philosophy, the mood in the middle years of the 20th century was to deny or dissolve the problem: if we just talked about everything more clearly, Wittgenstein and Ryle believed, we’d see there was no issue. T he problem of explaining consciousness is the joint property of philosophy, psychology and neurobiology, though there have been times when none of these fields much wanted it.

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