Perceptual constancy is the tendency to perceive familiar objects as unchanging even when the retinal image changes. Size, shape, brightness, and color remain relatively stable across variations in distance, viewing angle, and illumination.

A door is seen as rectangular whether it is closed or half open. A person walking away does not appear to shrink. A white shirt still looks white under indoor lighting. The visual system achieves these constancies by using contextual information—distance cues, surrounding surfaces, and assumptions about the external world—to recover the stable properties of objects from the changing proximal stimulus.

Constancy is not perfect. Extreme conditions can break it, and certain illusions exploit the same mechanisms that normally produce stability. Yet under ordinary circumstances constancy is remarkably effective, allowing recognition and action to proceed without continual recalibration.

The world looks stable because perception works to keep it that way.