Unlike somatic medicine, psychology has failed to develop an adequate model of the healthy individual and particularly of the range of variations of normal experience in daily life. Consequently, psychiatry lacks what physiology contributes to pathology in medical sciences. The task of studying normal consciousness should be the concern of psychology, so that a theoretical discipline could become available to psychiatry in the same fashion that physiology is available to pathology. However, psychology has not sufficiently developed either the study of normal states of consciousness in everyday life or the knowledge of fluctuations over time between states of optimal and aversive experience.

The study of the daily experience of normal subjects became possible with the use of the Experience Sampling Method (ESM) (Csikszentmihalyi et al., 1977; Csikszentmihalyi & Larson, 1984, 1987; deVries, 1987; Hormuth, 1986). This method allows repeated assessment of the experience of subjects in their natural environment. At the University of Chicago it has been used to describe the phenomenon of peak experiences, also called ‘flow’, and as a measurement of current subjective wellbeing (Csikszentmihalyi, 1975, 1982). Through this line of research it became clear that when one's perceived challenges, that is, the intrinsic demands experienced when engaged in an activity, are greater than one's perceived skills, that is, the individual's perception of capacity to meet the demands of the activity, the person experiences worry or anxiety. In the reverse situation (skills greater than challenges), boredom and apathy are experienced (Csikszentmihalyi, 1975).