In his introduction to De Ente et Essentia St. Thomas Aquinas remarks that “a little error in the beginning leads to a great one in the end.” He is here rephrasing an observation made by Aristotle in De Caelo, I, 5: “The least initial deviation from the truth is multiplied later a thousandfold.”
The insight thus expressed is applicable to mathematics and the experimental sciences and, in fact, to all human enterprises as well as to philosophy. . . . Methodologically, the rule would appear to be a simple one to follow. When you disagree with a philosopher’s conclusions, regard them as untenable, or find them repugnant to common sense, go back to his starting point and see if he has made a little error in the beginning.