This paper formalizes an implicit presupposition in all scientific theories of action dealing with all levels and subsystems of living and nonliving systems, whether they be theories of particle action, animal action, human action, or evolutionary action. Once formalized, this presupposition leads to a mathematical law of optimization (a variational principle). By specifying the functions involved in this law, one can then derive specific theories in such fields as physics, psychology, and economics. Thus, this paper answers the question: “Why are optimization principles so prevalent throughout the sciences?” It results from an implicit consistency premise in all scientific theories. We illustrate how our optimization law can be used to derive specific laws in many different scientific areas.