Deductive and Inductive Reasoning

Deductive and Inductive Reasoning represent the two basic ways of presenting an argument.

Deductive reasoning begins with a generalization and progresses to a specific case.

Inductive reasoning begins with a specific case or observation and progresses toward a generalization.

Deductive Reasoning Example:

When it rains, John’s old car won’t start. It’s raining. Therefore, John’s old car won’t start. (Applies a broad generalization to a specific case.)

Inductive Reasoning Example:

John’s old car won’t start. It’s raining. Therefore, John’s old car won’t start when it’s raining. (Uses a specific case to reach a broad generalization.)

To better visualize the difference between deductive and inductive reasoning imagine each as a triangle. The deductive pyramid is upside down, while the inductive pyramid is right side up:

Broad Generalization

To specific

Deductive Reasoning

Specific generalization

To broad

Inductive Reasoning

Inductive Reasoning: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:

1. What have you observed? What evidence is available?

2. What can you conclude from that evidence?

3. Is that conclusion logical?

Deductive Reasoning:

In looking at deductive arguments, you should first 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!

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)