Gretchen Rubin (2009). “The Happiness Project: Or, Why I Spent a Year Trying to Sing in the Morning, Clean My Closets, Fight Right, Read Aristotle, and Generally Have More Fun”, p.14, Harper Collins
David Brainerd (1822). “Memoirs of the Rev. David Brainerd: Missionary to the Indians on the Borders of New-York, New-Jersey, and Pennsylvania: Chiefly Taken from His Own Diary”, p.65