Our sense of "reality" and "constancy" reflect brain processes that attempt to adjust what we see in order to compensate for variations in illumination (see Tricks of the Eye, Wisdom of the Brain, Reality Reconsidered, From a Brain Perspective). Hence what we see is an "informed guess" based on an unconscious presumption that if changes in something can reasonably be attributed to some change in the surroundings, then one should treat the something as a constant, something that exists independently of the context in which it is being observed.

Notice in the picture that the checkerboard on the right is more brightly lit than the one on the left but that they both seem to consist of similarly lighter and darker squares. What you don't notice is that the light squares of the left checkerboard are actually darker than the dark squares of the right checkerboard. You can see the relative brightness of selected squares in the two checkerboards in the lower figure, and verify these brightnesses yourself by clicking and dragging on those squares in the upper image. It is because of your brain's informed guessing that you don't notice these differences.