Vibrations, chemicals and light-waves exist in the world; sounds, tastes, smells and colours only seem to. ‘Many sensations which are supposed to be qualities residing in external objects have no real existence save in us,’ said Galileo in the 17th century. ‘They reside only in the consciousness.’ But is consciousness itself, then, other than, and outside, physical things? We have known for millennia that it is in some way linked to the brain, yet, for all the advances of science, no scientific world view is able to accommodate it. How conscious states are generated by movements of brain tissue remains, in Thomas Huxley’s analogy, just as ‘unaccountable’ as that the genie should appear when Aladdin rubs his lamp.