Gregory on the Grand Illusion

It is sometimes said that all perception is a grand illusion. But this does not help. We may be driven into saying that 'everything is illusion', but this is as pointless as saying 'everything is a dream'. For when applied to everything, the words 'dream' and 'illusion' cease to have meaning. We need contrasts for seeing, and contrasts for describing and thinking. To claim that there is an illusion, there must be some contrasting non-illusion. This applies across the board. If every object were red there would be no point in seeing red, or using the word 'red'.

What are illusions?  

We may say that illusions are departures from reality-but what reality? Appearances are very different from deep realities of physics. If these are taken as reference truths we would have to  say that all perceptions are illusions. This is as pointless as saying that perception is a dream.  Illusions are judged with simple common-sense ideas of physics, and measured with kitchen instruments: rulers, clocks, scales, thermometers, and so on. We might define illusions as deviations from kitchen physics. (Gregory, 2009, pp. 9-10).

I think this is a disappointing reply to the idea of the grand illusion: "To claim that there is an illusion, there must be some contrasting non-illusion."  All of vision could be a grand illusion if all visual perception were a deviation from kitchen physics.  (Set aside the worries one might have about this definition.)  The contrast would be if a visual perception were not a deviation from kitchen physics.  I would think that the illusion/non-illusion distinct only requires a conceptual or theoretical distinction or a logical possibility of something that is an illusion and something that is a non-illusion.  We seem to have this.  We do not, however, require positive instances of both illusions and non-illusions.