Neural nets in the visual system of human beings learn how to make a very efficient interpretation of 3D scenes. That is why when somebody goes away from us, we do not perceive them as getting shorter. And when we stretch one arm and look at the two hands we do not perceive one hand smaller than the other. Visual illusions are sometimes held to show us that what we see is an image created in our brain. Our brain supposedly projects the image of the smaller hand to its correct distance in our internal 3D model. This is what is called the size constancy mechanism hypothesis.