Perception

When I read an introduction to Neural Networks a few years ago, one of the very simplest neural networks they showed as an example was one which could spot edges. It demonstrated very capably how an incredible small amount of "brainpower" could spot a difference between one area and an adjacent one. And since then I've believed that this is one of the bases of psychology (as well as animal psychology, etc.). All of perception is basd on the ability to differentiate, to seperate, to split. And it's because of the very simple method of most of this differentiation that we can frequently be fooled by slightly unusual circumstances.

For example, see these demonstrations of visual illusions. If the human eye/mind worked in any other way than comparing objects to the objects directly adjacent to them, none of these would work.