It is clear that everybody interested in science must be interested in world 3 objects. A physical scientist, to start with, may be interested mainly in world 1 objects—say crystals and X-rays. But very soon he must realize how much depends on our interpretation of the facts, that is, on our theories, and so on world 3 objects. Similarly, a historian of science, or a philosopher interested in science must be largely a student of world 3 objects.
Karl Popper (1902–1994), Austrian British philosopher of science. Unended Quest, p. 183 (1976). Originally published in The Library of Living Philosophers: The Philosophy of Karl Popper, "Autobiography of Karl Popper," ed. P. Schilpp (1974).