Spencer, H. (1892). First principles. D. Appleton.
Lua i Stíl Chicago (17ú heag.)Spencer, Herbert. First Principles. New York: D. Appleton, 1892.
Lua MLA (9ú heag.)Spencer, Herbert. First Principles. D. Appleton, 1892.
Rabhadh: Seans nach mbeach na luanna seo go hiomlán cruinn i ngach uile chás.