The Homeostat is, of course, grossly different from the brain in many respects, one of the most obvious being that while the brain has a very great number of component parts the Homeostat has, effectively, only four. This difference does not make the theory of the ultrastable system totally inapplicable, for much of the theory is true regardless of the number of parts, which is simply irrelevant. Nevertheless, we are in danger, after spending so much time getting to know a system of four parts, of developing a set of images, a set of working mental concepts, that is seriously out of proportion if considered as a set of working concepts with which to think about the real brain. Let us therefore consider specifically the properties of the ultrastable system that has a very great number of parts. We shall find that one difficulty becomes outstanding, and we shall spend the remainder of the book dealing with it, for it is the main problem in the adaptation of large systems.