But it is meant that, as the other sciences improve upon the knowledge of the plain man without wholly recasting it, as they accept the world in which he finds himself and merely attempt to give us a better account of it, so the psychologist may accept the world of matter and of minds recognized by common thought, and may devote himself to the study of minds, without attempting to solve a class of problems discussed by the metaphysician.

For example, he may refuse to discuss the question whether the mind can really know that there is an external world with which it stands in relation, and from which it receives messages along the avenues of the senses.

He may claim that it is no more his business to treat of this than it is the business of the mathematician to treat of the ultimate nature of space.
Thus the psychologist assumes without question the existence of an external real world, a world of matter and motion.

He finds in this world certain organized bodies that present phenomena which he regards as indicative of the presence of minds.

He accepts it as a fact that each mind knows its own states directly, and knows everything else by inference from those states, receiving messages from the outer world along one set of nerves and reacting along another set.