No conception can reach a scientific status unless it is supported by a mathematical theory. Clearly, this universal rule is as inescapable in psychology as in any other field. In psychology, a theory already exists, namely Vygotsky’s Activity Theory, which has a potential axiomatic structure. Unfortunately, until now, this structure has never been made explicit, so that it is necessary to make it apparent. In fact, moving from the units of analysis of Vygotskian theory, it is possible to give psychology the consistency of a rigorous formal system. Of course, to do that doesn’t mean avoiding the necessity, complementary and inescapable, of submitting the system thus obtained to empirical control, and to adjust it step by step according to factual evidence. However, a formal system also helps empirical and experimental verification, in that it makes concepts and their relationship clearer, more precise and more apparent.