My experience with neurological patients has underlined what ordinary life tells us: that a brain in some working order is a necessary condition for human consciousness. Unlike mind-brain identity theorists, however, I do not believe that consciousness is identical with neural activity in the cerebral cortex, the brain stem, the thalamus or wherever. This does not mean that I think I have an immaterial soul; nor do I subscribe to a 'ghost-in-the-machine' Cartesian dualism. Rather, I am a non-Cartesian atheist who just can't help noticing that however hard you look, you will not find sensations, affections and reasons in bits of the brain, or even distributed throughout the brain. So although a functioning brain is necessary for every aspect of consciousness, from the simplest twinge of sensation to the most exquisitely constructed sense of self, it is not sufficient for consciousness – and certainly not for the kind of consciousness you and I enjoy.

A necessary condition, or set of conditions, of something happening, is that without which it will not happen; a sufficient condition, or set of conditions, is that which is enough to ensure that it happens. In order for me to be knocked down by a 97 bus in London, it’s necessary for me to be in London. However, being in London is not sufficient – otherwise I might be even more inclined to avoid the place. Something else is required – for example that I should be in a certain street; that a 97 bus should be in the same street; that, preoccupied with the mind-body problem, I should walk in front of the bus; and that the driver, who may also be pondering the mind-body problem, should fail to see me in order to stop in time.