 This kind of knowledge development, Popper argues, cannot be explained either by physicalism, which seeks to reduce all mental processes and states to material ones, or by dualism, which usually seeks to explicate knowledge by means of psychological categories such as thought, perception and belief. Consequently, he proposes a novel form of pluralistic realism, a “Three Worlds” ontology, which, while accommodating both the world of physical states and processes (world 1) and the mental world of psychological processes (world 2), represents knowledge in its objective sense as belonging to world 3, a third, objectively real ontological category. That world is the world