We've already indicated that the aim of psychology is to describe the states of consciousness and develop a typology of them.

But the phenomena that psychology studies are almost indistinguishable from certain other phenomena that don't fall within its proper domain. Leaving for later the question of whether there's merit to the philosophical viewpoint known as materialism, it's clear that the body and soul are closely related to one another. We might almost say that nothing happens in the body that doesn't find its echo in the soul, and vice versa. For this reason, we have to be clear about the difference between physiology and psychology.

Physiological phenomena have the following characteristics:

1. They occur in space, occupy a certain part of extension, and can all be reduced to movements. In addition, we can depict them by means of figures – for example, to depict a stimulation of the nerves, it's sufficient to draw its different phases.

2. Because physiological phenomena occur in space, they can be measured. We can mathematically estimate the quantity of space they occupy.

3. Physiological phenomena are unconscious. We're often conscious of their result, of course, but not of the phenomena themselves – for example, we're not aware of the nervous stimuli that flow through our bodies when we injure ourselves. We only know the result – pain.

4. Finally, we don't attribute physiological phenomena to the self. We might say, “I suffer,” but suffering is just the psychological manifestation of a physiological wound.