Perception has been likened to the process of creating a theory, and perceptual sets are the representations or images that we make use of to test our perceptual theories.
The fact that a door appears to have the same shape whether we look at it from the side or front-on is an example of what is called a perceptual constancy.
The Young-Helmholtz trichromatic theory of colour vision is based on the laws of additive colour mixture and argues that each of the three types of cones is most sensitive to one wavelength of light.