Lewin, a social psychologist, believed the "field" to be a Gestalt psychological environment existing in an individual's (or in the collective group) mind at a certain point in time that can be mathematically described in a topological constellation of constructs.
When fully constructed, an individual's "field" (Lewin used the term "life space") describes that person's motives, values, needs, moods, goals, anxieties, and ideals.
For Lewin, the development (or regression) of an individual occurs when their "life space" has a "boundary zone" experience with external stimuli.
Lewin took these same principles and applied them to the analysis of group conflict, learning, adolescence, hatred, morale, German society, etc.
He used theory, mathematics, and common sense to define a force field and hence to determine the causes of human and group behaviour.