This book describes a lost tradition that can be called reasonableness. The tradition began with Aristotle, was recommended to Western education by Augustine, flourished in the schools of the Renaissance through the nineteenth century, then got lost in the academic and philosophic shuffles of the twentieth century. The modern critical thinking movement has tried to reclaim some of the tradition, but the central idea of reasonableness—the part that makes it broader than mere reasoning—remains gutted. For Aristotle and the subsequent tradition of Western education the difference between reasoning and reasonableness was partly a matter distinguishing three sources of information, the methods for handling those three sources, and understanding the levels of certainty available in each. The three sources can be generically called intuition, experience, and testimony. The first two were available to an individual reasoner, but the third was social. Testimony required the individual to trust information gained from other people. The first two were the stuff of reasoning, but the third was the key to a broader reasonableness.

One of the most provocative parables used to teach the subtleties of this aspect of reasonableness was first offered by John Locke in An Essay Concerning Human Understanding (1689). Locke has a King of Siam, while listening to a Dutch ambassador tell of the far north, suddenly recoil at the report that water gets so cold in Holland that it turns hard enough for an elephant to walk on it. Astonished, the king replies, “Hitherto I have believed the strange Things you have told me, because I look upon you as a sober fair man, but now I am sure you lye.” The story’s goal is to help readers understand that reliance on one’s own experience and reason is limited and that assent to testimony, even highly improbable testimony, from a credible witness is important for right reasoning. Although the story became the fodder for people on both sides of the eighteenth-century debate about the reasonability of belief in miracles, Locke had more mundane issues in mind. He was exploring the levels of certainty and guidelines of trust inherent in the slippery realm of information we gain from sources outside ourselves.