Scientific students of behavior will, with few exceptions, accept this law in theory, but in practice we have not fully used it. We have too often been content to say that a man may respond in any one of several ways to the same situation, or may attend to one rather than another feature of the same object, without insisting that the man must in each case be different, and without searching for the differences in him which cause the different reactions. The changes in an organism which make it respond differently on different occasions to the same situation range from temporary to permanent changes. Hunger, fatigue, sleep, and certain diseases on the one hand, and learning, immunity, growth and senility on the other, illustrate this range. Behavior is predictable _without recourse to magical agencies_. It is, of course, the case that any given difference between the responses of an animal to the same situation depends upon some _particular_ difference in the animal. Each immunity, for example, has its detailed representation in an altered condition of the blood or other bodily tissue. In general the changes in an animal which cause changes in its behavior to the same situation are fully enumerated in a list of the bodily changes concerned. That is, whatever changes may be supposed to have taken place in the animal’s vital force, spiritual essence, or other magical bases for life and thought, are useless for scientific explanation and control of behavior. No competent thinker probably doubts this in the case of such changes as are referred to by hunger, sleep, fatigue, so-called ‘functional’ diseases and immunity, and those who do doubt it in the case of mental growth and learning seem to represent an incomplete evolution from supernatural, or rather infrascientific, thinking. There may be in behavior a surplus beyond what would be predictable if the entire history of every atom in the body was known—a surplus necessarily attributable to changes in the animal’s incorporeal structure. But scientific thinkers properly refuse to deliberately count upon such a surplus.