It is based on the Young–Helmholtz theory that the normal human eye sees color because its inner surface is covered with millions of intermingled cone cells of three types: in theory, one type is most sensitive to the end of the spectrum we call "red", another is more sensitive to the middle or "green" region, and a third which is most strongly stimulated by "blue". The named colors are somewhat arbitrary divisions imposed on the continuous spectrum of visible light, and the theory is not an entirely accurate description of cone sensitivity. But the simple description of these three colors coincides enough with the sensations experienced by the eye that when these three colors are used the three cones types are adequately and unequally stimulated to form the illusion of various intermediate wavelengths of light.