Inductive vs Deductive Reasoning Help (page 3)
Introduction to Inductive vs Deductive Reasoning
"You should carefully study the Art of Reasoning, as it is what most people are very deficient in, and I know few things more disagreeable than to argue, or even converse with a man who has no idea of inductive and deductive philosophy."
—William J. Wills, explorer (1834–1861)
You hear arguments of all kinds throughout the day. In this lesson, you'll learn how to recognize the components of a deductive argument and how it differs from an inductive argument.
Consider the following conversation:
- "Junior, time to go to bed."
- "But why?"
- "Because I said so!"
Only a parent can get away with giving the answer "because I said so." But even parents sometimes have trouble using this approach to make a convincing argument. It's important to provide qualifiable reasons for asking someone to accept a claim or take a certain action. Providing qualifiable reasons is the best way to support your argument.
In the next three lessons, you're going to learn about deductive arguments: what they are, how they work, and how to recognize (and make) a good deductive argument—one that supports its assertions.
First, you need to know what deductive reasoning is. To help define it, the counterpart of deductive reasoning, which is inductive reasoning, will be introduced first.
When detectives arrive at the scene of a crime, the first thing they do is look for clues that can help them piece together what happened. A broken window, for example, might suggest how a burglar entered or exited. Likewise, the fact that an intruder didn't disturb anything but a painting that hid a safe might suggest that the burglar knew exactly where the safe was hidden. And this, in turn, suggests that the burglar knew the victim.
The process described above is called inductive reasoning. It consists of making observations and then drawing conclusions based on those observations.
Like a detective, you use inductive reasoning all the time in your daily life. You might notice, for example, that every time you eat a hot dog with chili and onions, you get a stomachache. Using inductive reasoning, you could logically conclude that the chili dogs cause indigestion, and that you should probably stop eating them. Similarly, you might notice that your cat tries to scratch you every time you rub her stomach. You could logically conclude that she does not like her stomach rubbed. In both examples, what you're doing is moving from the specific—a particular observation—to the general—a larger conclusion. Inductive reasoning starts from observation and evidence and leads to a conclusion.
Using inductive reasoning generally involves the following questions:
- What have you observed? What evidence is available?
- What can you conclude from that evidence?
- Is that conclusion logical?
We'll come back to these questions in a later lesson. For now, you know enough about inductive reasoning to see how deductive reasoning differs from it.
The word syllogism means "logical appeal" and is considered a type of logical argument and an important element in formal conclusions and premises. Truth tables, on the other hand, are mathematical tables that examine numerical formulas—and are way too complicated to cover here!
Unlike inductive reasoning, which moves from specific evidence to a general conclusion, deductive reasoning does the opposite; it generally moves from a conclusion to the evidence for that conclusion. In inductive reasoning, the conclusion has to be "figured out" and we must determine whether or not the conclusion is valid. In deductive reasoning, on the other hand, we start with the conclusion and then see if the evidence for that conclusion is valid. Generally, if the evidence is valid, the conclusion it supports is valid as well. In other words, deductive reasoning involves asking:
- What is the conclusion?
- What evidence supports it?
- Is that evidence logical?
If you can answer yes to question 3, then the conclusion should be logical and the argument sound.
It's easy to confuse inductive and deductive reasoning, so here's something to help you remember which is which:
- Inductive: Evidence · Conclusion (IEC)
- Deductive: Conclusion · Evidence (DCE)
Inductive reasoning starts with the evidence and moves to the conclusion. Deductive reasoning begins with the conclusion and moves to the evidence for that conclusion. Here's a memory trick to help you: You can remember that the word Inductive begins with a vowel, as does Evidence, so in inductive reasoning, you start with the evidence. Deductive begins with a consonant, and so does Conclusion, which is where you begin in deductive reasoning.
In the field of logic, deductive reasoning includes formal (mathematical or symbolic) logic such as syllogisms and truth tables. Some practice with formal logic will certainly sharpen your critical thinking and reasoning skills, but this book won't cover that kind of logic. Instead, we will continue to focus on informal logic—that is, the kind of critical thinking and reasoning skills that help you solve problems, assess and defend arguments, and make effective decisions in your daily life.
The Parts of a Deductive Argument
Problem-Solving Strategies Help talked about the importance of identifying the main issue in order to solve a problem. You learned to ask yourself, "What is the real problem to be solved here?" Then you took that problem and broke it down into its parts.
In looking at deductive arguments, you should follow a similar process. First, you should identify the conclusion. The conclusion is the main claim or point the argument is trying to make. The various pieces of evidence that support that conclusion are called premises. Keep in mind that an argument is not necessarily a fight. In talking about inductive and deductive reasoning, an argument refers to a claim that is supported by evidence. Whether or not that evidence is good is another matter!
Identifying the conclusion is often more diffi- cult than you might expect, because conclusions can sometimes seem like premises, and vice versa. Another difficulty is that you're used to thinking of conclusions as coming at the end of something. But in deductive arguments, the conclusion can appear anywhere. Thus, when someone presents you with a deductive argument, the first thing you should do is ask yourself: "What is the main claim, or overall idea, that the argument is trying to prove?"
In other words, just as a problem is often composed of many parts, the conclusion in a deductive argument is often composed of many premises. So it's important to keep in mind the "big picture."
Claim: assertion about the truth, existence, or value of something
argument: a claim supported by evidence
Conclusion: the main claim or point in an argument
premises: pieces of evidence that support the conclusion
The Structure of Deductive Arguments
The conclusion in a deductive argument can be supported by premises in two different ways. Say you have an argument with three premises supporting the conclusion. In one type of deductive argument, each premise provides its own individual support for the conclusion. That is, each premise alone is evidence for that main claim. In the other type of argument, the premises work together to support the conclusion. That is, they work like a chain of ideas to support the argument. These two types of arguments are represented as diagrams on page 56.
Here's how these two structures might look in a real argument:
Separate support: You shouldn't take that job. The pay is lousy, the hours are terrible, and there are no benefits.
Chain support: You shouldn't take that job. The pay is lousy, which will make it hard for you to pay your bills, and that will make you unhappy.
Notice how in the second version, the entire argument builds upon one idea, the lousy pay, whereas in the first, the argument is built upon three separate ideas. Both, however, are equally logical.
Of course, an argument can have both separate and chain support. We'll see an example of that shortly. What's important now is to understand that when premises depend upon each other, as they do in the chain support structure, what we really have is a chain of premises and conclusions. Look how the layers of a chain support argument work:
Conclusion: It will be hard to pay your bills.
Premise: The pay is lousy.
Conclusion: That will make you unhappy.
Premise: It will be hard to pay your bills.
Premise: That will make you unhappy.
Overall conclusion: You shouldn't take that job.
Because deductive arguments often work this way, it's very important to be able to distinguish the overall conclusion from the conclusions that may be used in the chain of support.
If the term conclusion rings a bell with you, it might be a leftover from English class. When you write an essay, the final paragraph is the conclusion. It is where you put everything else you wrote about together and write a summarizing statement of some kind. It's the same thing in an argument: put together all that you know and come up with asolid conclusion. Just make sure it makes sense!
Identifying the Overall Conclusion
Read the following sentences:
He's tall, so he must be a good basketball player. All tall people are good basketball players.
These two sentences represent a small deductive argument. It's not a particularly good argument, but it is a good example of deductive structure. If these two sentences are broken down into their parts, three different claims arise:
- He's tall.
- He must be a good basketball player.
- All tall people are good basketball players.
Now ask the key question: "What is this argument trying to prove?" In other words, what is the conclusion?
Two clues should help you come up with the right answer. First, look at which claims have support (evidence) in this example. Is there anything here to support the claim that "He is tall"? No. Is there anything to support the claim, "All tall people are good basketball players"? No. But there are premises to support the claim, "He must be a good basketball player." Why must he be a good basketball player? Because:
- He is tall.
- All tall people are good basketball players.
Therefore, the conclusion of this argument is: "He must be a good basketball player." That is what the writer is trying to prove. The premises that support this conclusion are "He is tall" and "All tall people are good basketball players."
A second clue in the conclusion that "He must be a good basketball player" is the word so. Several key words and phrases indicate that a conclusion will follow. Similarly, certain words and phrases indicate that a premise will follow:
Indicate a Conclusion:
- As a result
- It follows that
- That's why
- This shows/means/suggests that
Indicate a Premise:
- As indicated by
- As shown by
- Given that
- Inasmuch as
- The reason is that
Now, are the premises that support the conclusion, "He must be a good basketball player," separate support or chain support?
You should be able to see that these premises work together to support the conclusion. "He is tall" alone doesn't support the conclusion, and neither does "All tall people are good basketball players." But the two premises together provide support for the conclusion. Thus, the example is considered a chain of support argument.
The Position of the Conclusion
While you might be used to thinking of the conclusion as something that comes at the end, in a deductive argument, the conclusion can appear in different places. Here is the same argument rearranged in several different ways:
- He must be a good basketball player. After all, he's tall, and all tall people are good basketball players.
- All tall people are good basketball players. Since he's tall, he must be a good basketball player.
- He's tall, and all tall people are good basketball players. He must be a good basketball player.
- He must be a good basketball player. After all, all tall people are good basketball players, and he's tall.
- All tall people are good basketball players. He must be a good basketball player because he's tall.
In larger deductive arguments, especially the kind found in articles and essays, the conclusion will often be stated before any premises. But it's important to remember that the conclusion can appear anywhere in the argument. The key is to keep in mind what the argument as a whole is trying to prove.
One way to test that you've found the right conclusion is to use the "because" test. If you've chosen the right claim, you should be able to put because between it and all of the other premises. Thus:
He must be a good basketball player because he's tall and because all tall people are good basketball players.
Working with Arguments In Short
Unlike inductive arguments, which move from evidence to conclusion, deductive arguments move from the conclusion to evidence for that conclusion. The conclusion is the overall claim or main point of the argument, and the claims that support the conclusion are called premises. Deductive arguments can be supported by premises that work alone (separate support) or together (chain of support).
Skill Building until Next Time
- When you hear an argument, ask yourself whether it is an inductive or deductive argument. Did the person move from evidence to conclusion, or conclusion to evidence? If the argument is too complex to analyze this way, try choosing just one part of the argument and see whether it's inductive or deductive.
- When you come across deductive arguments today, try to separate the conclusion from the premises. Then consider whether the premises offer separate or chain support.
Exercises for this concept can be found at Inductive vs Deductive Reasoning Practice.
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