The case of the orange illustrates what is usually called Perception. It is the case in which the result is the knowledge of an actual object in the outside world. When the same process goes on after the actual object has been removed it is Memory. When it goes on again in a way which is not controlled by reference to such an outside object--usually it is a little fantastic, as in dreams or fancy, but often it is useful as being so well done as to anticipate what is really true in the outside world--then it is Imagination. If it is actually untrue, but still believed in, we call it Illusion or Hallucination. When it uses mere symbols, such as words, gestures, writing, etc., to stand for whole groups of things, it is Thinking or Reasoning. So we may say that what the mind arrives at through this its one great way of acting, no matter which of these forms it takes on, except in the cases in which it is not true in its results to the realities, is Knowledge. Thus we see that the terms and faculties of the older psychology can be arranged under this doctrine of Apperception without the necessity of thinking of the mind as doing more than the one thing. It simply groups and combines its material in different ways and in ever higher degrees of complexity. Apperception, then, is the one principle of mental activity on the side of its reception and treatment of the materials of experience. There is another term very current in psychology by which this same process is sometimes indicated: the phrase Association of Ideas. This designates the fact that when two things have been perceived or thought of together, they tend to come up together in the mind in the future; and when a thing has been perceived which resembles another, or is contrasted with it, they tend to recall each other in the same way. It is plain, however, that this phrase is applied to the single thoughts, sensations, or other mental materials, in their relations or connections among themselves. They are said to be "associated" with one another. This way of speaking of the mental materials, instead of speaking of the mind's activity, is convenient; and it is quite right to do so, since it is no contradiction to say that the thoughts, etc., which the mind "apperceives" remain "associated" together. From this explanation it is evident that the Association of Ideas also comes under the mental process of Apperception of which we have been speaking.