This equation was first presented in Lewin's book, "Principles of Topological Psychology", published in 1936. The equation was proposed as an attempt to unify the different branches of psychology (e.g. child psychology, animal psychology, psychopathology) with a flexible theory applicable to all distinct branches of psychology. This equation is directly related to Lewin's field theory. Field theory is centered around the idea that a person's life space determines their behavior. Thus, the equation was also expressed as "B = f"("L"), where "L" is the life space. In Lewin's book, he first presents the equation as "B" = "f"("S"), where behavior is a function of the whole situation ("S"). He then extended this original equation by suggesting that the whole situation could be roughly split into two parts: the person ("P") and the environment ("E"). According to Lewin, s"ocial" behavior, in particular, was the most psychologically interesting and relevant behavior.