John Adams, Charles Francis Adams (1856). “The Works of John Adams, Second President of the United States: With a Life of the Author, Notes and Illustrations”, p.66
David Hume, Eric Steinberg (1993). “An Enquiry Concerning Human Understanding ; [with] A Letter from a Gentleman to His Friend in Edinburgh ; [and] An Abstract of a Treatise of Human Nature”, p.51, Hackett Publishing