 Modelling a shift of qualities. If perceptual contents are wide and the environment is subject to change, we should expect a shift, even if the perceptual contents would not shift as readily as attitude contents would. Perhaps they would eventually shift after several centuries on Inverted Earth, if a subject could stay alive that long. But how would a distinctive quality even imaginably undergo such a shift? For example, suppose that a quality is supposed to shift from blue to yellow. A shift from blue to yellow might reasonably be supposed to be a smooth and gradual shift along the spectrum that passes through green. But it is hardly plausible that one would experience such a shift, or a period of unmistakable greenness in particular.