John Locke, George Berkeley, David Hume (2013). “The Empiricists: Locke: Concerning Human Understanding; Berkeley: Principles of Human Knowledge & 3 Dialogues; Hume: Concerning Human Understanding & Concerning Natural Religio”, p.334, Anchor
Orison Swett Marden (2015). “ORISON SWETT MARDEN Premium Collection - Wisdom & Empowerment Series (18 Books in One Volume): Steps to Success and Power, How to Get What You Want, An Iron Will, Be Good to Yourself, Every Man A King, Keeping Fit, Prosperity – How to Attract It, Stepping-Stones To Fame And Fortune...”, p.832, e-artnow