Columbia subject headings:
Philosophers, Modern.
Popper, Karl R. (Karl Raimund), 1902-1994.
Wittgenstein, Ludwig, 1889-1951.
Harvard subject headings:
Philosophers, Modern--20th century.
Popper, Karl R. (Karl Raimund), 1902-1994.
Wittgenstein, Ludwig, 1889-1951
Princeton subject headings:
Philosopher's, Modern--20th century.
Popper, Karl R. (Karl Raimund), 1902-
Wittgenstein, Ludwig, 1889-1951.