We said just now, that a nerve begins in a sense organ and ends in a muscle. This statement represents the general scheme well enough, but leaves out an important detail. The nerve does not extend directly to a muscle, but ordinarily goes by way of the brain. The brain is merely a great group of nerve cells and fibers which have developed as a central organ where a stimulation may pass from almost any sense organ to almost any muscle. But another importance attaches to the brain. When a sense organ is stimulated and this stimulation passes on to the brain and agitates a cell or group of cells there, _we are conscious_. Consciousness shifts and changes with every shift and change of the stimulation. The brain has still another important characteristic. After it has been stimulated through sense organ and nerve, a similar brain activity can be revived later, and this revival is the basis of _memory_. When the brain is agitated through the medium of a sense organ, we have _sensation_; when this agitation is revived later, we have a _memory idea_. A study of consciousness, or mind, the conditions under which it arises, and all the other problems involved, give us the other part of the field of psychology. We are not merely acting beings; we are _conscious_ acting beings. Psychology must study human nature from both points of view. We must study man not only from the outside; that is, objectively, in the same way that we study a stone or a tree or a frog, but we must study him from the inside or subjectively. It is of importance to know not only how a man _acts_, but also how he _thinks and feels_. It must be clear now, that human action, human behavior, is the main field of psychology. For, even though our main interests in people were in their minds, we could learn of the minds only through the actions. But our interests in other human beings are not in their minds but in _what they do_. It is true that our interest in ourselves is in our minds, and we can know these minds directly; but we cannot know directly the mind of another person, we can only guess what it is from the person's actions. =The Problems of Psychology.= Let us now see, in some detail, what the various problems of psychology are. If we are to understand human nature, we must know something of man's past; we must therefore treat of the origin and development of the human race. The relation of one generation to that preceding and to the one following makes necessary a study of heredity. We must find out how our thoughts, feelings, sensations, and ideas are dependent upon a physical body and its organs. A study of human actions shows that some actions are unlearned while others are learned or acquired. The unlearned acts are known as _instincts_ and the acquired acts are known as _habits_. Our psychology must, therefore, treat of instincts and habits.