Albert Einstein (2015). “Bite-Size Einstein: Quotations on Just About Everything from the Greatest Mind of the Twentieth Century”, p.32, St. Martin's Press
Immanuel Kant (2015). “The Critique of Practical Reason: Theory of Moral Reasoning: From the Author of Critique of Pure Reason, Critique of Judgment, Dreams of a Spirit-Seer, Perpetual Peace & Fundamental Principles of the Metaphysics of Morals”, p.51, e-artnow