LESSON #3

Deduction And Induction

Reading Assignment: 1.3 (pp. 33-39)

Click here to skip the following discussion and go straight to the assignments.

Arguments can be divided into two groups:

deductive arguments- involve necessity

inductive arguments- involve probability

Remember that we are always concerned with the relation between the premise(s) and the conclusion.

In this lesson we are still only concerned with what an argument is intended to be. We are not yet concerned with evaluating arguments as good or bad.

In a deductive argument it is claimed that the premises provide necessary support for the conclusion. In logic we are dealing with a very specific meaning of necessity, absolute necessity. For example, although it is very, very likely that the sun will rise tomorrow, it is not necessary that it do so--only very probable. On the other hand, the proposition that 2+2=4 is a necessary proposition. There is absolutely no other way around it, it follows by the meaning of those terms.

Once you have internalized this sense of necessity it becomes hard to deal with "regular" people because you will have a hard time saying "certain," "sure," "always," "never" and related words that are part of your technical terminology in situations when other people expect you to use them easily (and inaccurately). I drive my kids crazy by (almost) never being sure of anything. Them: "Mom, will we get to school on time?" Me: "I think so." Them: "Well, are we almost late?" Me: "No, I think we'll get there with ten minutes to spare." Them: "So, then we'll certainly get there before the bell rings--right?--why didn't you say so?" Me: --gulp. . .

The easiest way to determine whether an argument is inductive or deductive is to recognize various specific types of arguments that fall under each of these categories. And, as in Lesson 1, there are sometimes certain indicator words that act as red flags and help us identify inductive and deductive arguments. But, note that it is risky to rely on indicator words because often an arguer will use deductive indicator words such as "certainly," "undoubtedly," etc. to embellish what is really an inductive argument. Once again, your common sense will be of great help.

Again, as with the first two lessons, When you feel confident that you have mastered these concepts, do the True/False exercise on p. 42 in the textbook. (section III) You can check your answers in the appendix.

Then do exercises 1.3 I 1-25 on your Logic Coach Software. If you need more practice, feel free to do more. 

When you are ready, complete the following assignment, using the book as little as possible. Hand in the following assignment together with a copy of your logic coach record screen. For more detailed instructions on doing this click here.

ASSIGNMENT:

Determine whether the following arguments are inductive or deductive. State the specific type of argument. If you believe that the argument does not fall under any of the eleven types listed above, explain in your own words the basis of your decision to classify it as inductive or deductive. (10 points each)

NOTE: An argument that is set up as a deductive argument but is invalid is still a deductive argument.

 

1. The sum of the interior angles of any triangle is 180°. In triangle #1, angle A is 30°, angle B is 90°. Therefore, angle C is 60°.

2. If I make an A, then I will pass this course. Odds are, I will make a B. So, I probably won't pass this course.

3. The situation in America today is much like that of ancient Rome before its fall, in that the U.S. is controlled by a small group of self-serving individuals. It is inevitable that within 50 years America will suffer the same fate as Rome.

4. The platypus is not a mammal because no mammal lays eggs and the female platypus does.

5. The last time I ate here, the shrimp dish I ordered was disgusting. It must be the case that this restaurant buys lousy seafood.

6. The sign on the candy machine reads "Out of Order." The candy machine must be broken.

7. All guitar players are musicians, and some guitar players are not astronauts. It follows that some musicians are not astronauts.

8. Irene likes either coffee or tea in the morning. But she doesn't like tea. Therefore Irene likes coffee in the morning.

9. These mushrooms have a very similar appearance to the ones growing in the garden. The ones in the garden are edible. The conclusion is therefore warranted that these mushrooms are edible.

10. My birthday is six days after my sister's birthday. My birthday is on the 12th. Therefore, my sister's birthday is on the 18th.

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