Inductive and Deductive Logic

We are All Scientists:
Inductive and Deductive Logic

We have shamelessly lifted our title from an essay by Thomas Huxley (1825-1895). In this essay, Huxley, one of Darwin's foremost supporters, was trying to explain scientific thought to lay persons. Scientists, he said, employ processes of induction and deduction, arriving at natural laws through inference, and validating them by a deductive process of empirical investigation. All of which, thought Huxley, seems to cause the average person to shake his or her head, convinced that scientists engage in intellectual thought processes that are far out of reach of ordinary persons. But, he maintained, readers will be delighted to learn that we have been using induction and deduction all of our lives and that, indeed, "we are all scientists!" Of course, Huxley added, the scientist's use of these processes is far more refined, consistent, and precise. The basic processes, however, are the same.

Following are a few examples of inductive and deductive thinking in everyday life. The first is an updated version of one of Huxley's own examples.

Green apples are bitter. In your lunch you have included a beautiful green apple. You bite into it. "Blah!" you say, this apple is bitter." Being hungry, though, you eat the whole thing.

The next day you have another green apple for lunch, with the same result. And the next day, also.

At this point you calmly consider your experiences and you conclude, "So, green apples are bitter."

There you have it. You have just created a general law by the process of induction. As described in Chapter 2 of the text, you have made an inductive inference. Based on your experiences with specific green apples, you have inferred a general idea about all green apples.

Now, the next day at lunch your friend offers you a slice of a beautiful green apple. "No way!" you say, "That apple is bitter!"

You have, of course, just made a deductive inference. You have drawn a conclusion about a specific event (your friend's green apple) based on your previously developed general proposition concerning all green apples. From induction to deduction; from the specific to the general and back again from the general to the specific.

You could take the next step and test whether your deductive inference has merit. You might say, "Based on my general proposition about all green apples, I predict (a deduction) that this particular apple is bitter!" If you taste it and it is, indeed, bitter then you will have some empirical validation for your general proposition. Go ahead. Take a bite. You'll never know unless you do.

Never buy a Hachimura. Your brand-new Hachimura GSFX-II DVD player has stopped working. After banging on it for awhile and taking several other reasonable corrective steps, you return it to the dealer. "It didn't last two days!" You complain. The dealer, an agreeable sort, gives you a new one.

Two days later, the same thing happens. "Hachimura's are no good!" you tell the dealer, who exchanges it for a new Tokuri MLR-IV. He even gives you back $20.00 because the Tokuri is that much expensive.

That was three years ago, and your Tokuri MLR-IV is still playing perfectly.

"Never buy a Hachimura!" you tell your friends. "They are no good. But if you buy a Tokuri, you'll have a fine product." You have made inductive and deductive inferences, this time concerning Hachimuras and Tokuris.

Will Myron and Brenda Get Together? Brenda has been invited by Myron, whom she does not know very well, to go to a concert next week. The problem is that Myron wears big round eyeglasses, has four pens in his shirt pocket, and gets all A' s in his classes.

"Guys who look like that," Brenda confides to her friend, "are all totally nerdy bores (a general proposition). Myron looks like that. Therefore he's a totally nerdy bore (a deductive inference from the general proposition)."

"So?" her friend asks, "Are you going to go out with him, or not?"

Did Brenda choose to empirically test her deductive prediction about Myron? Tune in next week.

Study the Instructor. The second multiple choice exam is coming up in a course and you want to be prepared. Not only do you study the course material, but you decide to study the instructor, too, at least insofar as her exams are concerned. Going over the first exam you observe that nearly a third of the questions do not depend heavily on your specific knowledge of details in the course. Rather, if you understand the basic concepts and you very carefully read each question and the multiple answers, you can pick out the correct answer just by following the logic that is presented.

"Aha!" you conclude. This instructor tends to emphasize basic concepts when she makes up exams (your inductive inference). She also expects students to read her questions very carefully (another inductive inference) and to select the right answer logically from the clues she gives in the answer choices (again, induction). If this is the way she thinks about exams in general, then the next exam will have the same emphasis in about a third of the questions" (your deductive inference about the next exam).

Therefore, on the next exam, I will do well on a third of the items if I study general concepts, if I take my time and carefully read the questions and answers, and if I try to understand the logic of each (your deductive inferences about the outcome).

Good luck.

We are All Scientists: Inductive and Deductive Logic

We are All Scientists:
Inductive and Deductive Logic