: Informal Reasoning

Ordinary, Everyday Reasoning

Upon arriving at the scene of an apparent suicide, detective Keesha Kane observes a dead man slumped over his computer keyboard, a bullet hole in his right temple. A pistol lays within easy reach of his hand. Over the din of two barking dogs, the man’s wife is hysterical. “I was asleep in the bedroom when I heard a pop or a bang,” she explains. “I hurried downstairs, where I found my husband, exactly as you see him here. He’s been depressed lately, but I never imagined . . ..”

Detective Kane studies the scene, clicks a few computer files, and announces, “This was no suicide. The gun is near his right hand, the wound is to his right temple, and yet, his computer’s mouse is set up for a southpaw.”

“So?”

“Your husband was left-handed, suggesting that someone else shot him, then placed the gun near his right hand to make it appear like a suicide.”

“But. . ..” The wife hugs herself, shivering. “Who could have done such a thing?”

“Not an intruder. Just listen to your dogs. The way they’re baying at me, no doubt they’d have gone crazy over an intruder. Surely, they’d have wakened you before a shot was fired. And yet, you claim that . . ..”

“Duke! Bosco! Shut up! I’m trying to concentrate!”

The detective leans forward. “Mrs. Chandler, were you truly asleep when the shot was fired?”

“I loved my husband! If you’re insinuating that. . ..” The wife’s eyes narrow into thin lines.

Her voice goes deadly calm. “Why would I want him dead?”

“Better question: Why were some of his files deleted after you called 911? Perhaps we’ll know better when we restore them,” Kane declares. “In the meantime, you have the right to remain silent . . ..”

In making the victim’s wife a prime suspect, Detective Kane is relying on what’s called inductive reasoning. She bases her initial suspicion on a generalization that left-handed people tend to hold guns with their left hands. She also makes a causal inference that murder, not suicide, was the cause of death, based on the inconsistency between the deceased’s hand preference and the location of the gun. Furthermore, she relies on sign reasoning to infer that the dogs didn’t bark because no stranger was present, and again on sign reasoning to infer that the files deleted after the husband’s death provide clues to motives for the crime. Of course, Detective Kane may be wrong. She has made multiple assumptions and inferences. Nevertheless, her reasoning accounts for all the available evidence.

After reading this chapter, we hope you’ll agree with us that being able to use informal reasoning, as the detective does, is essential for arguing and reasoning well. Understanding these types of inductive reasoning will improve your ability to make and refute arguments. With that in mind, in this chapter we’ll take a look at several kinds of inductive reasoning, including cause—effect, analogy, sign, generalization, and example. First, however, let’s examine inductive reasoning more broadly.

Inductive Reasoning Defined: Take a Leap

Inductive reasoning, otherwise known as informal reasoning, isn’t just for detectives. Ordinary people use inductive reasoning all the time. It is the lingua franca of everyday argument. In fact, informal reasoning is so ubiquitous that we usually grasp arguments without even being aware of doing so. Simply stated, inductive reasoning is any form of reasoning that requires an inference. In the process of such reasoning, an arguer reaches a conclusion about something that is unknown on the basis of something that is known. For example, if you saw a woman’s eyes tear up, you might infer that she was sad. Keep in mind, however, that although we use inductive reasoning regularly, we don’t always use it well. Instead, we often jump to the wrong conclusions. For example, the woman in question might be crying with joy or cutting an onion. Whatever the case, her example illustrates an important characteristic of inductive reasoning. Specifically, inductive reasoning is probabilistic in nature, meaning that, when making an inference, we cannot be certain that we are correct.