Columbia subject headings:
Collingwood, R. G. (Robin George), 1889-1943.
Popper, Karl R. (Karl Raimund), 1902-1994.
Harvard subject headings:
Collingwood, R. G. (Robin George), 1889-1943.
Popper, Karl R. (Karl Raimund), 1902-1994.
Princeton subject headings:
Collingwood, R. G. (Robin George), 1889-1943.
Popper, Karl R. (Karl Raimund), 1902-1994.