简介
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.^[3] 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.^[2] 
P 和E的实际
Lewin held that the variables in the equation (e.g. P and E) could be replaced with the specific, unique situational and personal characteristics of the individual.

受格式塔理论的影响
Lewin emphasizes that the desires and motivations within the person and the situation in its entirety, the sum of all these competing forces, combine to form something larger: the life space. This notion speaks directly to the gestalt idea that the "whole is greater than the sum of its parts."
整体大于部分之和

心理环境（准事实）
the psychological reality encompasses everything that an individual perceives and believes to be true.
个人感觉和个人相信的现实（心理现实） 直接决定 行为
the only reality that is contained within the life space is the psychological reality, as this is the reality that has direct consequences for behavior. 

心理学的现实和实际无关
The psychological reality...does not depend upon whether or not the content...exists in a physical or social sense....The existence or nonexistence...of a psychological fact are independent of the existence or nonexistence to which its content refers.

Lewin, Kurt (1936). Principles of Topological Psychology. New York: McGraw-Hill. pp. 38

In Lewin's theoretical framework, the whole situation—or the life space（人P与环境E的总合）, which contains both the person and the environment—is dynamic（动态的）.
In order to accurately determine behavior, Lewin's equation holds that one must consider and examine the life space at the exact moment when the behavior occurred.
当时当地当刻的状态决定一切，过去只能是间接的作用。