The field theory in psychology by #KurtLewin 1943 derives from classical field theory (viz. electromagnetism and gravitation), predating quantum field theory (viz. subatomic particles). For psychology, Lewin wrote in 1943 how history (and a subjective view of the future) matters.
It is correct that field theory emphasizes the importance of the fact that any event is a resultant of a multitude of factors. The recognition of the necessity of a fair representation of this multitude of interdependent factors is a step in the direction toward field theory. However, this does not suffice. Field theory is something more specific.Lewin (1943), p. 293
To use an illustration: Success in a certain sport may de- pend upon a combination of muscular strength, velocity of movement, ability to make quick decisions, and precise perception of direction and distance. A change in any one of these five variables might alter the result to a certain degree. One can represent these variables as five dimensions of a diagram. The resultant of any possible constellation of these factors for the amount of success can be marked as a point in the diagram. The totality of these points then is a diagrammatic representation of this dependence, in other words, of an empirical law.
Physics frequently makes use of such representation of a multitude of factors influencing an event. To each of certain properties, such as temperature, pressure, time, spacial position, one dimension is coordinated. Such a representation in physics is called ‘phase space.’ Such a phase space may have twenty dimensions if twenty factors have to be considered. A phase space is something definitely different from that three-dimensional ‘physical space’ within which physical ob- jects are moving. In the same way the psychological space, the life space or psychological field, in which psychological locomotion or structural changes take place, is something different from those diagrams where dimensions mean merely gradations of properties.
Lewin probes the question of what is a field theory, and what might not be a field theory.
Field theory, therefore, can hardly be called correct or incorrect in the same way as a theory in the usual sense of the term. Field theory is probably best characterized as a method: namely, a method of analyzing causal relations and of building scientific constructs. This method of analyzing causal relations can be expressed in the form of certain general statements about the’ nature’ of the conditions of change. To what degree such a statement has an ‘analytical’ (logical, a priori) and to what degree it has an ’empirical’ character do not need to be discussed here.Lewin (1943), p. 294
By section 2, “The Principle of Contemporaneity and the Effect of Past and Future”, Lewin aims for rigour by using mathematical notation (which isn’t beyond high school Grade 12 level).
The equivalent to (dx/dy) in physics is the concept ‘behavior’ in psychology, if we understand the term behavior to cover any change in the psychological field. The field theoretical principle of contemporaneity in psychology then means that the behavior b at the time t is a function of the situation S at the time t only (S is meant to include both the person and his psychological environment) … and not, in addition, a function of past or future situations … (Fig. 2).Lewin (1943), p. 297
In section 3, “How to Determine the Properties of a Field at a Given Time”, Lewin compares the science of psychology with medicine (and engineering, physics and biology).
If one has to derive behavior from the situation at that time, a way has to be found to determine the character of the ‘situation at a given time.’ This determination implies a number of questions which are, I think, interesting both psychologically and philosophically.Lewin (1943), p. 297
To determine the properties of a present situation or — to use a medical terminology — to make a diagnosis, one can follow two different procedures: One may base one’s statement on conclusions from history (anamneses}, or one may use diagnostic tests of the present,
Medicine, engineering, physics, biology are accustomed to use both methods, an inquiry into the past and a test of the present. But they prefer the latter whenever possible. Lewin (1943), p. 298
-  There are cases where a historical procedure is preferable. For instance, the hunger of a rat can probably be better determined by the duration of starvation than by a physiological or psychological test of the hunger at the time t. This conclusion from the past to the present can be made, however, only during periods and in settings where a ‘closed system’ (no interference from outside) can be enforced; e.g., for animals which during this period do the same amount of work, which have been on a known diet, etc. The difficulties of this type of control have lead Skinner (19) to link the problem of drive strength to properties of present consumption.
Psychology has used diagnosis by anamneses rather excessively, particularly in classical psychoanalysis and other clinical approaches to problems of personality. Psychology of perception and psychology of memory have been relatively free from the historical type of diagnosis. Experimental psychology, on the whole, has shown a progressive trend toward testing the present situation.Lewin (1943), p. 298
In section 4, “The Psychological Past, Present, and Future as Parts of Psychological Field at a Given Time” departs from physics.
The clarification of the problem of past and future has been much delayed by the fact that the psychological field which exists at a given time contains also the views of that individual about his future and past. The individual sees not only his present situation; he has certain expectations, wishes, fears, daydreams for his future. His views about his own past and that of the rest of the physical and social world are often incorrect, but nevertheless constitute, in his life space, the ‘reality-level’ of the past. In addition, a wish- level in regard to the past can frequently be observed. The discrepancy between the structure of this wish- or irreality- level of the psychological past and the reality-level plays an important role for the phenomenon of guilt. The structure of the psychological future is closely related, for instance, to hope and planning (2).Lewin (1943), p. 302-303
Following a terminology of L. K. Frank (6), we speak of ‘time perspective’ which includes the psychological past and psychological future on the reality-level and on the various irreality-levels. The time perspective existing at a given time has been shown to be very important for many problems such as the level of aspiration, the mood, the constructiveness, and the initiative of the individual. Farber (4) has shown, for instance, that the amount of suffering of a prisoner depends more on his expectation in regard to his release, which may be five years ahead, than on the pleasantness or unpleasantness of his present occupation.Lewin (1943), p. 303
It is important to realize that the psychological past and the psychological future are simultaneous parts of the psychological field existing at a given time t. The time perspective is continually changing. According to field theory, any type of behavior depends upon the total field, including the time perspective at that time, but not, in addition, upon any past or future field and its time perspectives.
The the last section, 5, on “Psychological Ecology”, Lewin responds to some issues brought up by Brunswik.
Within the realm of facts existing at a given time one can distinguish three areas in which changes are or might be of interest to psychology:Lewin (1943), p. 306
1. The ‘life space’; i.e., the person and the psychological environment as it exists for him. We usually have this field in mind if we refer to needs, motivation, mood, goals, anxiety, ideals.
2. A multitude of processes in the physical or social world, which do not affect the life space of the individual at that time.
3. A ‘boundary zone’ of the life space: certain parts of the physical or social world do affect the state of the life space at that time. The process of perception, for instance, is intimately linked with this boundary zone because what is perceived is partly determined by the physical ‘stimuli’; i.e., that part of the physical world which affects the sensory organs at that time. Another process located in the boundary zone is the ‘execution’ of an action. [….]
The essence of explaining or predicting any change in a certain area is the linkage of that change with the conditions of the field at that time. This basic principle makes the sub- jective probability of an event a part of the life space of that individual. But it excludes the objective probability of alien factors that cannot be derived from the life space.Lewin (1943), p. 309
Lewin, Kurt. 1943. “Defining the ‘Field at a Given Time.’” Psychological Review 50 (3): 292–310. https://doi.org/10.1037/h0062738. Alternate search at https://scholar.google.com/scholar?cluster=16344209988031220269
- republished at Chapter 3, in Lewin, Kurt. 1951. Field Theory in Social Science: Selected Theoretical Papers. Edited by Dorwin Cartwright. Research Center for Group Dynamics. https://archive.org/details/in.ernet.dli.2015.138989/page/n1.