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." In fulfilling this requirement the psychologist resorts to two methods of procedure.