The most important of his edits was small but resounding. He crossed out, using the heavy backslashes that he often employed, the last three words of Jefferson’s phrase “We hold these truths to be sacred and undeniable” and changed them to the words now enshrined in history: “We hold these truths to be self-evident.” The idea of “self-evident” truths was one that drew less on John Locke, who was Jefferson’s favored philosopher, than on the scientific determinism espoused by Isaac Newton and on the analytic empiricism of Franklin’s close friend David Hume. In what became known as “Hume’s fork,” the great Scottish philosopher, along with Leibniz and others, had developed a theory that distinguished between synthetic truths that describe matters of fact (such as “London is bigger than Philadelphia”) and analytic truths that are self-evident by virtue of reason and definition (“The angles of a triangle equal 180 degrees”; “All bachelors are unmarried”). By using the word “sacred,” Jefferson had asserted, intentionally or not, that the principle in question—the equality of men and their endowment by their creator with inalienable rights—was an assertion of religion. Franklin’s edit turned it instead into an assertion of rationality.
From Benjamin Franklin: An American Life by Walter Isaacson