John Stuart Mill (1862). “A System of Logic, Ratiocinative and Inductive: Being a Connected View of the Principles of Evidence, and the Methods of Scientific Investigation”, p.207
Martin Luther (1931). “Early Protestant Educators: The Educational Writings of Martin Luther, John Calvin, and Other Leaders of Protestant Thought”, Ams PressInc
Henry Tudor (1834). “Narrative of a tour in North America: comprising Mexico, the mines of Real de Monte, the United States, and the British colonies; with an excursion to the island of Cuba. In a series of letters, written in the years 1831-2”, p.233