Secondly, let us consult the instances, albeit rare, of individuals who, with the best of sight, have failed to distinguish colours, and, with the sharpest hearing, to distinguish tones, while for those who have this ability the perception of an altered quality (not merely of the degree of the sensation) in the case of the different intensities in the scale of colours or tones is definite, as is also the number of those which may be intelligibly distinguished.