In evolutionary biology and ecology, ontological and epistemological perspectives based on the replicator and the interactor have become the background that makes it possible to transcend traditional biological levels of organization and to achieve a unified view of evolution in which replication and interaction are fundamental operating processes. Using the transactional perspective proposed originally by John Dewey and Arthur Fisher Bentley, a new ontological and methodological category is proposed here: the transactor. The transactional perspective, based on the concept of the (...) transactor, bridges the dichotomy between organisms and environment that characterizes the interactional perspective on evolution and provides epistemological support for the emergentist, systemic view of evolutionary and developmental processes. (shrink)