The encyclopedia of philosophy; volume 7 and 8
editor in chief, Paul Edwards
Macmilan , 1972
 Buku Referensi
The encyclopedia of philosophy volume 7 and 8
Paul Edwards, editor in chief
Macmilan , 1972
 Buku Referensi
The Encyclopedia Americana : the international reference work, volume 7
Americana Corporation, 1960
 Buku Referensi
The Encyclopedia Americana : the international reference work, volume 8
Americana Corporation, 1960
 Buku Referensi
Singh, U.K.
International encyclopedia of professional management Vol. 7, sales management
New Delhi : Anmol Publications, 1998,
 Buku Referensi