For the American jazz musician, Charlie Parker, it meant, "your own experience, your own thoughts, your wisdom." "Without music life would be an error," said the German philosopher Friedrich Nietzsche. 

And in the words of American pop singer Billy Joe, "Everyone loves music!"

They must be right, because we spend US$40 billion a year on music. But what do we mean by music? What defines it? And why are we so touched by i...