Cita APA (7a ed.)

Santayana, G. (1918). Philosophical opinion in America. [s.n.].

Cita Chicago Style (17a ed.)

Santayana, George. Philosophical Opinion in America. [S.l.]: [s.n.], 1918.

Cita MLA (9a ed.)

Santayana, George. Philosophical Opinion in America. [s.n.], 1918.