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

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

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

The thing of which the act of perception is the perception is experienced as something not mental.

When we come to images or memories or thoughts, speculation, while always closely related to practice, is more explicit, and it is in fact not immediately obvious that such processes can be described in any sense as practical.

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.

Desire then is the invasion of the whole self by the wish, which, as it invades, sets going more and more of the psychical processes; but at the same time, so long as it remains desire, does not succeed in getting possession of the self.