Briefly, informal reasoning requires clearly linking your general claims with concrete, specific data. Much of the clear thinking we do in our everyday lives follows logical principles, but in a less formal and systematic way than the thinking of a research scientist. And for most occasions this informal reasoning is adequate. Aristotle points out that it would be just as much a mistake to expect certain proofs in argument as to expect only probable proofs in mathematics. That's not to say your argument can be illogical, only that you shouldn't confuse formal logic with clear thinking or good sense, the essential qualities your argument should display. Briefly, informal reasoning requires clearly linking your general claims with concrete, specific data.
When our thinking begins with specifics and moves toward a generalization, we are moving inductively.
When our thinking begins with specifics and moves toward a generalization, we are moving inductively. That is, if you were to taste several hard, green apples and then draw the general conclusion that all hard, green apples are sour, you would be using inductive reasoning. And, of course, the more apples tasted and the greater the variation in the times and conditions of tasting, the greater the likelihood that your general conclusion would be valid.
In your writing, then, when you reason inductively, ask whether you've examined the evidence carefully, whether it justifies your general conclusion, and whether you've given readers enough specific evidence to persuade them that your thinking is sound and your general conclusion is true.
Reasoning that moves in the opposite direction (from general to specific) is called deductive reasoning.
Reasoning that moves in the opposite direction (from general to specific) is called deductive reasoning. Here, you take a general principle that you know to be true and use it to understand a specific situation. For instance, you may know from experience that as a general rule bad weather reduces business at the golf course. You may also learn that today's weather will be cold and rainy. From these two pieces of knowledge, you can produce a third, more specific piece: Business at the golf course will be slow today. In writing, deductive reasoning most often appears in a shortened version (called an enthymeme) that may be hard to recognize. That's because one or more links in the chain of reason have not been stated directly but only implied. Consider the following example:
Bill never turns in his assignments, so he'll fail the course.
What is not directly stated but only implied is the general principle that students who don't turn in their assignments will fail the course.
Such shortened forms are perfectly acceptable, but only if the underlying links and claims are sound. An opponent may want to refute you by challenging some underlying assumptions in your thinking; likewise, you'll want to look for faulty reasoning when you refute your opposition.
8.6 Read the following statements and comment on their use of informal reasoning. What details would you need to see in order to be convinced? Can you find any unstated assumptions that need to be examined?
a. Coach Ratcliffe should be fired because a coach's job is to win ballgames.
b. I know he's popular because he drives a Corvette.
c. The president hasn't done anything about welfare reform, so he has no sympathy for the poor.
d. The Sun Belt continues to be the fastest-growing part of the country.
e. Too much smoking ruins a person's health, so you know Louisa's in bad shape.
f. Today's prisons are practically like country clubs.
g. Because several new schools have been built in the past few years, Chicago has an outstanding school system.
h. Imported cars are higher in quality than American cars.
i. Mr. Price got the contract, so you know he paid a few people off.
j. Arthur Jensen should be elected to the city council because he is a successful real estate developer.
8.7 Look over the following examples, fill in any missing links in the reasoning chain, and comment on the uses of informal logic:
Claim: Coach Ratcliffe should be fired.
Link: A coach's job is to win ballgames.
Data: The team had a 4 and 6 record this year. They had a 3 and 7 record last year. They had a 1 and 9 record the previous year.
Claim: Arthur Jensen Should be elected to the City Council.
Link: The best person is the most experienced.
Data: Arthur has served two terms on the council. His opponent has never been on the council. Arthur is a successful real estate developer.
Claim: Omaha has an outstanding school system.
Data: The buildings are well-maintained. Most schools have computers. Several new schools have been built in the past few years.
Link: Fair grading policies give every student an equal opportunity to succeed.
Data: Pop quizzes in H240 discriminate against students who prefer to cram for tests. Attendance policies in H240 discriminate against students who must work during class times. Writing assignments in H240 favor students with access to word processors.