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Rh in the brain is the physical adjunct of a sensation in the mind, such as visual perception, we may regard the mind's interpretation of the sensation as a transformation of the actual physical process as it would be described by external observers if observed directly by them.

If now the exact nature of this transformation depends upon the physical process taking place as in the case of the transformations just considered, but at the same time leaves invariant the fundamental laws on which a description of the process depends, the description of the process by means of the transformation will be a correct qualitative description but will not be a true quantitative one, since the transformation varies for each independent event and so there are no fixed units of measurement. It is possible that when the mind is concentrated on a subject the type of transformation is practically constant, and so our interpretations of sensations become clearer as they become more of a quantitative nature, but it is dangerous to speculate and so I shall leave the subject at this point.