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 developmen