  The knowledge argument asks us to imagine a future scientist who has lacked a certain sensory modality from birth, but who has acquired a perfect scientific understanding of how this modality operates in others. This scientist – call him Harpo – may have been born stone deaf, but become the world’s greatest expert on the machinery of hearing: he knows everything that there is to know within the range of the physical and behavioural sciences about hearing. Suppose that Harpo, thanks to developments in neurosurgery, has an operation which finally enables him to hear. It is suggested that he will then learn something he did not know before, which can be expressed as what it is like to hear, or the qualitative or phenomenal nature of sound. These qualitative features of experience are generally referred to as qualia. If Harpo learns something new, he did not know everything before. He knew all the physical facts before. So what he learns on coming to hear – the facts about the nature of experience or the nature of qualia – are non-physical. This establishes at least a state or property dualism. (See Jackson 1982; Robinson 1982.)