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

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.

Hence, in desiring, the more the enjoyment is delayed, the more fancy begins to weave about the object images of future fruition, and to clothe the desired object with properties calculated to inflame the impulse.

The sensory acts are accordingly distinguished by their objects.

But though cognition is not an element of mental action, nor even in any real sense of the word an aspect of it, the distinction of cognition and conation has if properly defined a definite value.

The interval between a cold expectation and a warm desire may be filled by expectations of varying degrees of warmth or by desires of varying degrees of coldness.

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.