Popper, Karl R., author
Basic Books, 1959
Buku Teks
Morris, Richard, author
Simon and Schuster, 1983
Buku Referensi
Artikel Jurnal
Cohen, Morris R.
Harcourt, Brace & World, Inc., 1934
Buku Teks
Mill, John Stuart
Longmans, Green and Co., 1949
Buku Teks