Why has the field of psychology not developed like the natural sciences?

The article suggests an answer to the question of why the natural sciences such as physics have been able to develop unified theories that provide satisfactory and efficient explanations for many natural phenomena, while psychology has failed to develop unified theories to explain psychological phenomena. The article’s answer is based on the observation that in physics, the units of measurement (UMs) have an expression in theoretical terms that are the equivalent of observational terms (UMs-equivalency). In contrast, in psychology, UMs have an expression only in theoretical terms. The UMs-equivalency in physics is not a sufficient condition for constructing successful unified theories, but it is a necessary con-dition. Not every physical theory that maintains UMs-equivalency becomes a successful theory, because the theory may not properly represent the processes in reality. This article develops and justifies this idea and suggests that it is difficult to imagine a successful unified theory in psychology because UMs-equivalency does not exist in this field.