This paper is a chapter from a book by Lagache on The Unity of Psychology. He analyzes some characteristics of modern psychology which are partly the result of the influence of psychoanalysis on that discipline. In all branches of psychology, Lagache finds a certain number of common principles. He feels that the aim of the psychoanalytic observation should be to understand the behavior of the patient toward the fundamental rule. Successive psychic steps are better appraised when described not as 'psychic processes' but as hesitation, withdrawal, inhibition, and trial and error, much like those of a rat in a labyrinth. Psychoanalysis relates behavior to four coördinates: the individual history, the present environment, the psychoanalytic situation, and the body.