What causes illusions? From the perspective of the theory of in - depth processing and misapplication of constancy, illusions have cognitive roots.

When people perceive the size of three - dimensional objects in space, they always take distance into account. This is an important condition for maintaining the size constancy of objects.

When people consciously or unconsciously apply this characteristic of perceiving the three - dimensional world to perceiving flat objects, it will cause illusions.

In this sense, illusions are an exception to perceptual constancy and the result of people's misapplication of perceptual constancy.