If we call the world of "things"Mof physical objects—the first world, and the world of subjective experiences (such as thought processes) the second world, we may call the world of statements in themselves the third world.
Karl Popper (1902–1994), Austrian born-British philosopher of science. Unended Quest, p. 181 (1976). Originally published in The Library of Living Philosophers: The Philosophy of Karl Popper, "Autobiography of Karl Popper," ed. P. Schilpp (1974).
The famous distinction by which the author defends logic, semantics, etc., against psychologism.