For example, the Müller-Lyer illusion is interpreted traditionally as exemplifying a measurement error. The perceiver sees difference in length between two lines that are equal to some standard of measure, say, a ruler. … The Establishment is tempted to say that the perceiver … falsely infers from the play of light at the eyes that the two lines are of different lengths when, in fact, they are of the same length.
What must be assumed to give legitimacy to this claim for perceptual error? The following come quickly to mind: (1) Whatever the proper basis of measurement for describing the figure is, it is one and the same as the basis of the measurement device by which the figure is described. (TSRM, p. 279).
So, I have been wrestling with TSRM for a while now, trying to figure out how they mean to explain visual illusions. They suggest that cognitivists make assumption (1) above, but I am having a tough time parsing this. I am not sure what they are attributing to cognitivists. Let me explain my confusion.
Suppose, just for expository convenience, that the proper basis of measurement for describing the Müller-Lyer illusion is length, that is, that the Müller-Lyer illusion should be described in terms of length. Is that one side of the kind of identity claim TSRM are noting?
So, then is length one and the same as "the basis of the measurement device by which the figure is described"? That doesn't make sense to me. How can length be a device? Is there some typo here?
Any help here would be appreciated.