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Inductive vs Deductive Reasoning Help (page 2)

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Updated on Sep 29, 2011

Deductive Reasoning

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:

  1. What is the conclusion?
  2. What evidence supports it?
  3. 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.

The Structure of Deductive Arguments

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.

The Structure of Deductive Arguments

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.

The Structure of Deductive Arguments

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.

Tip

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!

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