More recently he introduced a notion of “World 3”—a world of scientific and artistic knowledge, distinct from the physical world (World 1) and the world of mind or thought (World 2). In “Epistemology without a knowing subject” and “On the theory of the Objective Mind” he uses “World 3” to mean a world of intelligibles: ideas in the objective sense, possible objects of thought, theories and their logical relations, arguments, and problem situations (1974, p. 154). It seems Popper wants to put a Platonic world of Ideal Forms alongside the mental and physical worlds.
Peter Medawar was impressed by Popper’s World 3. “Popper’s new ontology does away with subjectivism in the world of the mind. Human beings, he says, inhabit or interact with three quite distinct worlds; World 1 is the ordinary physical world, or world of physical states; World 2 is the mental world; World 3 is the world of actual or possible objects of thought—the world of concepts, ideas, theories, theorems, arguments, and explanations—the world, let us say, of all artifacts of the mind. The elements of this world interact with each other much like the ordinary objects of the material world; two theories interact and lead to the formulation of a third; Wagner’s music influences Strauss’s and his in turn all music written since. . . . The existence of World 3, inseparably bound up with human language, is the most distinctly human of all our possessions. The third world is not a fiction, Popper insists, but exists ‘in reality.’ It is a product of the human mind but yet is in large measure autonomous. This was the conception I had been looking for: The third world is the greater and more important part of human inheritance. It’s handing on from generation to generation is what above all else distinguishes man from beast.” But the difficulty of explaining the interaction between these worlds is fatal for Popper, as for his predecessors.