1. The first such truth is that the mind is not the possession of man alone. Other creatures have minds. Psychology no longer confines itself, as it formerly did, to the human soul, denying to the animals a place in this highest of all the sciences. It finds itself unable to require any test or evidence of the presence of mind which the animals do not meet, nor does it find any place at which the story of the mind can begin higher up than the very beginnings of life. For as soon as we ask, "How much mind is necessary to start with?" we have to answer, "Any mind at all"; and all the animals are possessed of some of the actions which we associate with mind. Of course, the ascertainment of the truth of this belongs--as the ascertainment of all the truths of nature belongs--to scientific investigation itself. It is the scientific man's rule not to assume anything except as he finds facts to support the assumption. So we find a great department of psychology devoted to just this question--i.e., of tracing mind in the animals and in the child, and noting the stages of what is called its "evolution" in the ascending scale of animal life, and its "development" in the rapid growth which every child goes through in the nursery. This gives us two chapters of the story of the mind. Together they are called "Genetic Psychology," having two divisions, "Animal or Comparative Psychology" and "Child Psychology." 2. Another general truth to note at the outset is this: that we are able to get real knowledge about the mind. This may seem at first sight a useless question to raise, seeing that our minds are, in the thought of many, about the only things we are really sure of. But that sort of sureness is not what science seeks. Every science requires some means of investigation, some method of procedure, which is more exact than the mere say-so of common sense; and which can be used over and again by different investigators and under different conditions. This gives a high degree of verification and control to the results once obtained. The chemist has his acids, and reagents, and blowpipes, etc.; they constitute his instruments, and by using them, under certain constant rules, he keeps to a consistent method. So with the physiologist; he has his microscope, his staining fluids, his means of stimulating the tissues of the body, etc. The physicist also makes much of his lenses, and membranes, and electrical batteries, and X-ray apparatus. In like manner it is necessary that the psychologist should have a recognised way of investigating the mind, which he can lay before anybody saying: "There, you see my results, you can get them for yourself by the same method that I used."