Tension systems is a useful mental model explaining that almost always there are some forces supporting each other and some forces opposing each other. As the previous post discussed, one serious obstacle to effective thinking is our tendency to oversimplify causal nexus, and we can improve our strategic and analytic thinking skills by taking heed of the causal complexity. The model of tension systems not only recognizes the inevitable causal complexity, but also suggests that there is always a tension between driving forces and restraining forces; in any situation, there are some forces supporting each other and some forces opposing each other.
Tension system is a conceptual pillar of modern social psychology. Kurt Lewin, the founder of modern social psychology, thought that any psychological environment is shaped by various driving forces, many of them opposing one another. Lewin pointed out that this is a dynamic field, in which any part depends on every other part in the field. Such equilibrium and interdependence means that, among other things, a small change in one force can produce large changes in the system. For example, numerous experiments in social psychology have found that seemingly trivial changes in time or environment can produce large changes in behavior.
All of this means that if we want to understand whether certain events are likely to happen (or why they happened) or to predict people’s actions, it is necessary to analyze both restraining and impelling forces, i.e. what is preventing them and what is moving them forward.
 Kurt Lewin, Defining the ‘Field at a Given Time’, Psychological Review, 50, 292-310 (1943); Kurt Lewin, Field Theory in Social Science (D. Cartwright ed., New York: Harper, 1951).