There exists a fundamental convergence between some major trends of modern epistemology—as outlined, for instance, by Filmer Northrop and Henry Margenau—and the theories actually developed within sciences of the human mind where two types of thought—one implicit and, the other, explicit—tend to refer to two different lines of development. Moreover, these theories can find in the psychology of Lev Vygotsky some seminal hypotheses of a major importance. In order to highlight this convergence, we parallel the role played by structured conceptual (...) systems in Vygotsky’s conception of intellectual development and Northrop’s epistemology. We show how these conceptual systems account for the notion of causality and can explain the success of scientific thought, i.e. its possible match with the real world, while this match falsely justified an overall biological model of intellectual development in Jean Piaget’s work. We conclude that dual process theorists should no longer neglect cognitive tools, and especially conceptual systems, which underpin the awareness and mastery of thought that are characteristic of type 2 processes. This whole analysis leads us to maintain that human psychology is not characterized in the first instance by a need to act, but a quest for meaning. (shrink)