In the 1920's (everyone's favorite psychologist) Sigmund Freud developed a theory of personality. In this theory he employs the ideas of psychic determinism or the assumption that all psychological events have a cause (pg. 547). In his 1923 book The Ego and the Id he described the psyche as having three parts: the id, ego and super-ego. The id is impulsive and seeks pleasure without taking possible consequences into account. The id operates off the pleasure principle or the tendency of the id to strive for immediate gratification (pg. 547). The ego is rational and tries to balance out the irrational thinking of the id. The ego then tries to balances the id by postponing gratification until it can find an apprpriate outlet for a certain behavior, this is called the rality principle (pg. 547). Finally, the super-ego is the moral compass out of the three and bases it's thinking with the ideal in mind.
The author of our book mentions that much of the time the id, ego and super-ego interact harmoniously and only sometimes to the wills of these entities within us collide.
I also found this cool 1990's video on YouTube that details Freud's Psychoanalytic Theory. Enjoy!