Ernst von Glaserfelds’ Radical Constructivism has enriched and extended geographical thinking significantly. As a geographer, however, I cannot neglect material worlds. In the discourses on the great transformation towards low carbon societies or on global environmental change, for instance, materiality and its modification play a central role. Following Karen Barad’s notion of agential realism, I will argue that it is not about matter as such (understood as an object with regards to ontology), but rather about matter as part of phenomena, which are the result of intra-actions and in which matter and meaning are ontologically entangled. This will allow us to capture a world in which matter is not fixed or given and at the same time not the mere result of specific processes. It is a world, in which differences, of course, matter, but cannot be taken for granted. With this, mattering matter could enrich Radical Constructivism.