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http://dx.doi.org/10.1037/11633-020
In the present chapter we shift our scene of interest from vision to the sense of touch. In this modality we can very clearly demonstrate space-time relations. For this purpose, we go to the investigations of Helson and King, who studied a phenomenon that Helson had previously called the tau effect. The effect was given its name to prevent it from being passed off as an illusion. We have already been dealing with many phenomena that the man-in-the-street would call illusions. The common implication is that illusions are phenomena that are not real, or at least not so real as other phenomena, or perhaps not real in the same way or sense of the word. Illusions are thus taken to be phenomena in a distinct functional class by themselves. If illusoriness is to be defined as a discrepancy between "knowledge" and sensation, then there are many degrees of illusoriness, and many of the phenomena that have never been classed as illusions partake of this discrepancy in some form or degree. For this reason, if for no other, the demarcation of some phenomena as illusions, and some as not, is based upon an incomplete knowledge of sensory phenomena. (PsycINFO Database Record (c) 2016 APA, all rights reserved)