One key strategy for unifying the discipline of psychology is to develop a meta-theoretical framework through the advancement of core concepts.  Rather than having these be strictly defined from the outset, this commentary argues for the utility of open-ended concepts for scientific advancement. This is illustrated with a brief historical review and current status of Prägnanz, assimiliation-accommodation, schema, liminality and mediation, which also show the difficulties on finding core concepts for psychology as a whole. Open-ended concepts may be useful here in that they can help to bring together converging lines of research from different approaches within psychology. Finally, a case is made for mediationas a core concept that is currently converging with notions of extended and distributed cognition.