He is able to investigate the mind in two ways, which are of such general application that anybody of sufficient training to make scientific observations at all can repeat them and so confirm the results.

One of these is what is called Introspection.

It consists in taking note of one's own mind, as all sorts of changes are produced in it, such as emotions, memories, associations of events now gone, etc., and describing everything that takes place.

Other persons can repeat the observations with their own minds, and see that what the first reports is true.

This results in a body of knowledge which is put together and called "Introspective Psychology," and one chapter of the story should be devoted to that.
Then the other way we have is that of experimenting on some one else's mind.