Turn on the radio. Turn the volume control, now clockwise, now anticlockwise. The music is now loud, louder, now soft, softer. There is a precise correlation between the turning of the dial and the volume of the sound heard.
The dial is part of the radio, an artefact made of physical components. Turning the dial, then, causes changes to a physical object, to part of the radio, to something with mass, shape, size etc.
In complete accordance with how far the dial is turned in one direction or the other, so varies the loudness of the sound experienced. But that sound does not have mass, shape, size etc. The dial turning exactly and regularly shadows the experience. How can this be? How can something with all the properties of the physical be in this constant relationship with something so different, so alien as conscious experience of sound?
The fact that experiences of sound correlate with physical changes in a radio in not the slightest way helps us to understand why, when sound waves meet the ear and part of the brain is stimulated, there is the actual experience of hearing in the first place. There does not seem to be any music out there. Bows drawn over strings and air blown through perforated tubes send waves through the air. But what have waves in air to do with what it is like to hear music. The music happens when the brain is stimulated by data from the ear, but, the same question again, what have brain changes to do with what it is like to hear a tune?
Consciousness and Other Matters 