How many different worlds or realities did Popper differentiate in Objective Knowledge?
He proposed three worlds: World One, being the physical world, or physical states; World Two, being the world of mind, or mental states, ideas, and perceptions; and World Three, being the body of human knowledge expressed in its manifold forms, or the products of the second world made manifest in the materials of the first world (i.e., books, papers, paintings, symphonies, and all the products of the human mind).