We think of perception as like a camera recording the world as it is. But neuroscience reveals perception is more like interpretation: the brain actively constructs its model of reality from ambiguous sensory data. Visual illusions demonstrate this vividly — the Müller-Lyer illusion makes equally long lines appear different lengths, the Necker cube flips between two interpretations, and after-images appear when no light stimulus is present. These are not tricks that fool perception; they reveal how perception normally works.

The brain has a "blind spot" — a small region of the retina with no photoreceptors where the optic nerve exits. Yet we see no hole in our visual field. The brain fills it in with what it expects to be there based on surrounding context. Similarly, we never notice our eyes' constant micro-movements (saccades) because the brain suppresses that information. What we perceive is not the raw sensory signal but a cleaned-up, interpreted, gap-filled model that the brain has constructed.

Color perception reveals construction dramatically. The wavelengths of light hitting the retina are objective physical facts. But what color we see depends on context: the famous "dress" photo that went viral in 2015 appeared blue-black to some and white-gold to others — the same pixels, different brains constructing different colors based on different assumptions about lighting. Color constancy — our ability to see a red apple as red whether in sunlight or shadow — requires the brain to actively subtract its estimate of the illumination. Color is not in the world; it is constructed by the brain.