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Knowledge, for Popper, was objective, both in the sense that it is objectively true (or truthlike), and also in the sense that knowledge has an ontological status (i.e., knowledge as object) independent of the knowing subject (Objective Knowledge: An Evolutionary Approach, 1972). 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). World Three, he argued, was the product of individual human beings in exactly the same sense that an animal path is the product of individual animals, and that, as such, has an existence and evolution independent of any individual knowing subjects. The influence of World Three, in his view, on the individual human mind (World Two) is at least as strong as the influence of World One. In other words, the knowledge held by a given individual mind owes at least as much to the total accumulated wealth of human knowledge, made manifest, as to the world of direct experience. As such, the growth of human knowledge could be said to be a function of the independent evolution of World Three. Many contemporary philosophers, such as Daniel Dennett, have not embraced Popper's Three World conjecture, due mostly, it seems, to its resemblance to mind-body dualism.

Who produced the contents of World Three?
individual human beings