bmacs27 says... I think he chose his wording a little poorly. The purpose of perception is to estimate physical properties of the environment. It's just not meant to estimate the exact intensities and so forth. For example, in the Adelson checkerboard illusion, you are reporting your perception of the surface reflectances irrespective of illuminant. That's a perfectly sensible environmental property to estimate. It just isn't a direct mapping to the image falling on your retina.