It may be a matter for astonishment that two complicated and effective modes of reaction are arrived at by animals in the absence of abstract ideas about forces. Yet so it is; before any speculation on power, before any induction or deduction, before any abstract notion of the nature of spirit and matter, animals have learned to deal quite well with what we call physical and personal forces. How did they do it? The study under experimental conditions of the establishment of new reactions in animals reveals the process very clearly. Imagine a cat shut up in a cage, the door of which can be opened by pressing down a latch. When weary of confinement the cat begins to claw, pull, and bite, here, there, and everywhere. After half an hour, or an hour of this purposive, but unreasoned, activity, he chances to put his paw upon the latch and escapes. If again put into the cage, he does not seem to know any better than before how to proceed. Yet something has been gained by the first experience. For now he directs his clawing, pulling, and biting more frequently towards the part of the cage occupied by the latch. Because of this improvement he finds himself released sooner than the first time. The repetition of the experiment shows the cat learning to bring his movements to bear more and more exclusively upon the door or its immediate surroundings. Ultimately he will have learned to make just the necessary movement and no other. In this gradual exclusion of useless movements, the cat is guided entirely by results. The psycho-physiological endowment required for acquisitions of this kind involves no abstract ideas but only (1) the desire to escape; (2) the impulse and ability to perform the various movements we have named; (3) an indefinite remembrance of the position occupied when success was achieved, combined with a tendency to repeat the same movements when in the same situation.