In the perception of a tree we can distinguish the act of experiencing, or perceiving, from the thing experienced, or perceived.

Psychology is the science of the act of experiencing, and deals with the whole system of such acts as they make up mental life.

Such being the nature of mental life, the business of psychology is primarily to describe in detail the various forms which attention or conation assumes upon the different levels of that life.

It is more difficult to designate this form of conation on its practical side by a satisfactory name.

Mental life is indeed practical through and through. It begins in practice and it ends in practice.

It is convenient to distinguish the two kinds of experience which have thus been described, the experienc-ing and the experienc-ed, by technical words.

The mental act of sensation which issues in reflex movement is so simple as to defy analysis.