There is a vast field of knowledge in the realm of psychology. The knowledge of imagination and of imagination turning into thought, the knowledge of feeling and of feeling turning into emotion, the knowledge of passion and of passion turning into expression, the knowledge of impulse and its outlet, the knowledge of attraction and its contrary effect, the knowledge of the origin and source of sympathy and antipathy--all these belong to psychology. Thus psychology is a knowledge of perceptible things, yet not of solid things that one can touch, and that is why it is more difficult to explain the laws of psychology in words than the laws of material science.