Robert Barclay (1850). “An Apology for the Christian Divinity: As the Sameis Held Forth and Preached by the People Called, in Scorm Quakers, Being a Full Explanation and Vindication of Their Principles and Doctrines”, p.13
John Stuart Mill (2008). “Utilitarianism and On Liberty: Including 'Essay on Bentham' and Selections from the Writings of Jeremy Bentham and John Austin”, p.112, John Wiley & Sons
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"Neil deGrasse Tyson and Executive Producer Ann Druyan Talk COSMOS: A SPACETIME ODYSSEY, Public Appetite for Science, and More". Interview with Christina Radish, collider.com. March 17, 2014.