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Logic and the Reasonable Person

Logic is a mandatory course for all freshmen at The Heights, and not all of them are happy about it. They all begin with at least some idea of what logic is, and many of them are skeptical that they could possibly need a course on the subject, complete with a longish textbook. Don’t we all know the difference between the logical and the illogical without studying it? Don’t we know logic almost instinctively? 

As it happens, I think the analogy between human reason and animal instinct is a pretty good one. The Greeks defined man as the rational animal: we are similar to the other animals insofar as we are alive in the material world, but different from them precisely because we have the capacity for reason whereas the other animals act only by instinct. Of course, humans sometimes act from instinct as well—compelled by emotions or delusions so powerful that we sometimes do not think of ourselves as free to choose. But that is the exception that proves the rule. As a rule, we tend to rely most on our reason when we have the freedom to choose our actions, and to define ourselves and our character by those choices. Logic, then, is an essential part of our human nature. 

But if we are logical by nature, doesn’t that imply that there is no need—and perhaps no possibility—of further instruction? That’s a good question, to which a good answer might be: What do you mean by “imply”? And now we’re doing logic.

A dictionary can tell us that a question about whether A “implies” B is normally a question about whether B must be truewhenever A is true. But a dictionary can’t tell us whether A really does imply B. For that, we need to understand how any fact can ever imply another fact, so we can apply that general understanding to the relationship between A and B. We need a general understanding of how logical reasoning works. 

Logic, then, is both an art—a thing we do while reasoning from A to B—and a science of how and why that reasoning works (or doesn’t). Logic is the art and the science of sound reasoning. 

As an art, logic is extremely practical. Students use logic to answer true/false and multiple-choice questions. Detectives use logic to solve crimes. Mathematicians use logic to prove useful theorems. Mechanics use logic to figure out which part of a machine or system is malfunctioning. Doctors use logic to diagnose illnesses. Investors use logic to predict whether some new fact will make a particular stock go up or down. Even politicians (occasionally) use logic to propose public policies. Logic isn’t everything—it’s not how we appreciate a flower’s beauty or a mother’s love, for example—but there are few areas of human activity in which we could succeed without logic. 

The ubiquity of logic in our lives is neither a Western eccentricity nor a modern development. Psychologist Steven Pinker, in his book Rationality, describes the daily lives of the San, a hunter-gatherer tribe in the Kalahari Desert of Southern Africa, who use logical reasoning to track their prey and choose the appropriate hunting strategy. They know, for example, that porcupines have two pads on each foot whereas honey badgers have only one—which suggests that the identity of the animal leaving the print can be inferred from whether the print has one pad or two. But the San also know that, when the ground is hard, a two-padded foot may leave a one-padded print because the second pad may have left no impression on the hard ground. So the San correctly reason that if they see two padprints, they are very likely to be on the trail of a porcupine; but if they see only one it could be either a complete honey badger print or an incomplete porcupine print. Logically, this hunter-gatherer logic is essentially indistinguishable from the process of medical diagnosis in the most advanced hospitals, as Pinker shows.

Logic also has communicative significance. Anyone who uses language to communicate with another human—in other words, virtually everyone—must abide by the internal logic of that language. A native speaker instantly recognizes that “The dog ate my homework” may be a preposterous excuse, but it is not nonsensical; whereas “Green ideas sleep furiously” doesn’t make enough sense to be true or false. This is because language is really just a delivery vehicle for the logical content of the things we say. Whether we say, in English, “The stick is hard” or, in German, “Der Stock ist hart,” we are communicating the same logical content. We might say that proficiency in a second language is largely a matter of being able to “see through” the words in the second language and perceive the underlying meaning without first translating the words into one’s native tongue.

So we see that logic is well worth studying, for at least four reasons. First, our capacity for logic is perhaps the most intrinsically human characteristic, the thing that distinguishes us from the animals. Second, logic is the principal means by which we extend our knowledge of the world. Third, logic is essential for our decisions about what we ought to do in any situation. And fourth, logic is an indispensable element of our communications with each other. Altogether, that’s a pretty strong case for relevance.

But wait: there’s more! As humans, we don’t always agree about these things: who we are, or what we can know, or how we should act, or even what words mean. And when we disagree, we often try to persuade each other of A rather than B. How? Well, sometimes we “persuade” by force or by trickery, but most of the time we persuade by appealing to reason. That is, we offer reasons to believe that certain things are true; and we make arguments that, because certain things are true, certain other things must also be true (or in some cases, are probably true). The better one understands logic, the better one is likely to be at this common task of persuasion. And perhaps just as importantly: the better one understands logic, the better one is likely to be at rejecting bad arguments by other people who might mislead us, intentionally or unintentionally. 

The study of logic therefore tends to make us better at knowing ourselves, better at knowing what’s true of the world around us, better at deciding what to do, better at understanding each other, and better at making good arguments or at least resisting bad ones. But with all of these impressive benefits before us, a few words of caution are in order: knowing these things will not necessarily make you a reasonable person. Let me explain.

Abraham Lincoln is supposed to have told his audiences a story about a boy who was asked how many legs his calf would have if he called its tail a leg. “Five,” the boy is supposed to have replied. But Lincoln then delivered the correct answer: four. A calf has four legs no matter what you call the tail, because a tail is not a leg no matter what you call it. The humble but profound moral of the story is that the truth seldom depends on the words we use. With rare (and contrived) exceptions, true propositions are true whether we admit their truth or not—which means that acknowledging what is true (or not) is always a choice we make; an act of the will. We can ignore the truth, and we can often get away with it up to a point. But becoming reasonable persons requires us to want to know the truth and to act in accordance with it. Reasonable people treat make-believe as a form of imaginative play, not a viable path to a life well lived. 

Logic helps us quite a bit once we’ve made the fundamental choice to live in the truth. Logic helps us not only to avoid mispersuasion or dispersuasion by others, but to examine our own reasoning and make sure it is sound; to see whether our conclusions comport with reality. And logic tells us that if any opinion is inconsistent with reality, reality wins. Still, logic cannot compel us to follow her conclusions. Tragically, there are and have always been people who choose to live as rebels against truth, as if they could make 2 + 2 equal 5 if they only wanted it enough. There is sometimes a sort of romantic charm about such people, but their stories typically end badly. So first and foremost, the reasonable person desires not only to know the truth but to act in accordance with it.

Second, the reasonable person possesses certain intellectual virtues—habits of doing the right thing at the right time for the right reasons—that will make it possible to live out that commitment to truth in community with other people. For example, the reasonable person needs the virtue of humility, at least enough to acknowledge the limits of his own knowledge. Few people learn much when they think they already know it all. In addition, the reasonable person must possess the virtue of charity, at least enough to believe that others—including even (and perhaps especially) our critics—might be able to teach us a great deal. The reasonable person also needs courage, specifically the courage to correct his own errors when they are discovered. There is little point in desiring the truth, or in possessing the intelligence and character to learn the truth, unless we ultimately possess the character to act on the knowledge we gain. Humility, charity, and courage are all necessary if the search for truth is to be meaningful and effective, which makes these virtues indispensable to the reasonable person.

In the terms of traditional logic, we might say that logic is not a sufficient cause of these virtues, but it is a necessary cause—which, if we study logic, is a distinction that comes in handy quite often. 

Becoming a reasonable person is not easy, and some people seem to have a harder time than others. But it is very much worth the effort, and ninth grade is none too early to start.

Mark Grannis teaches Logic and History at The Heights. This essay is adapted from the introduction to his textbook, The Reasonable Person: Traditional Logic for Modern Lifepublished on September 4, 2023.

About the Author

Mark Grannis

Philosophy, History

Mark Grannis joined the faculty in 2019 to teach Logic and History, after practicing law for over thirty years and managing the firm he co-founded in 1998. He holds an A.B., cum laude, from Georgetown University, where he majored in Government and Economics. He holds a J.D., cum laude, from the University of Michigan Law School, where he served as an editor of the Michigan Law Review and won several awards for his writing. In 2023, he published The Reasonable Person: Traditional Logic for Modern LifeHe and his wife Sarah have two children, including Will (’21). They live in Chevy Chase with the majestically indifferent Cyrus, King of Purrrrsia.

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