The second reason that this method of procedure is most important is found in the fact that all the other departments of psychology--and with them all the other sciences--have to use introspection, after all, to make sure of the results which they get by other methods.

For example, the natural scientist, the botanist, let us say, and the physical scientist, the electrician, say, can not observe the plants or the electric sparks without really using his introspection upon what is before him.

The light from the plant has to go into his brain and leave a certain effect in his mind, and then he has to use introspection to report what he sees.

The astronomer who has bad eyes can not observe the stars well or discover the facts about them, because his introspection in reporting what he sees proceeds on the imperfect and distorted images coming in from his defective eyesight.
So a man given to exaggeration, who is not able to report truthfully what he remembers, can not be a good botanist, since this defect in introspection will render his observation of the plants unreliable.
In practice the introspective method has been most important, and the development of psychology has been up to very recently mainly due to its use.

As a consequence, there are many general principles of mental action and many laws of mental growth already discovered which should in the first instance engage our attention.