Inductive and Inductive and Deductive ReasoningDeductive Reasoning

AP Language and Composition

Deductive ReasoningDeductive Reasoning

Deductive reasoning works from the more general to the more specific. Sometimes this is informally called a "top-down" approach. We might begin with thinking up a theory about our topic of interest. We then narrow that down into more specific hypotheses that we can test. We narrow down even further when we collect observations to address the hypotheses. This ultimately leads us to be able to test the hypotheses with specific data -- a confirmation (or not) of our original theories.

Deductive ReasoningDeductive Reasoning

Theory◦HypothesisObservation◦Confirmation

Inductive ReasoningInductive Reasoning

Inductive reasoning works the other way, moving from specific observations to broader generalizations and theories.

Informally, we sometimes call this a "bottom up" approach (please note that it's "bottom up" and not "bottoms up" which is the kind of thing the bartender says to customers when he's trying to close for the night!).

In inductive reasoning, we begin with specific observations and measures, begin to detect patterns and regularities, formulate some tentative hypotheses that we can explore, and finally end up developing some general conclusions or theories.

Inductive ReasoningInductive Reasoning

Induction vs. deductionInduction vs. deduction

Induction is usually described as moving from the specific to the general, while deduction begins with the general and ends with the specific

arguments based on experience or observation are best expressed inductively, while arguments based on laws, rules, or other widely accepted principles are best expressed deductively.

ExamplesExamples

Adham: I've noticed previously that every time I kick a ball up, it comes back down, so I guess this next time when I kick it up, it will come back down, too.

Rizik: That's Newton's Law. Everything that goes up must come down. And so, if you kick the ball up, it must come down. ◦ Adham is using inductive reasoning, arguing from

observation, while Rizik is using deductive reasoning, arguing from the law of gravity.

◦ Rizik's argument is clearly from the general (the law of gravity) to the specific (this kick) Adham's inductive argument, above, is supported by his previous

observations, while Rizik's deductive argument is supported by his reference to the law of gravity.

Thus, Adham could provide additional support by detailing those observations, without any recourse to books or theories of physics, while Rizik could provide additional support by discussing Newton's law, even if Rizik himself had never seen a ball kicked.

Exercises for Induction and Exercises for Induction and DeductionDeduction

1. Which of the following claims would be best expressed by inductive reasoning?

A. Your first quiz grade usually indicates how you will do in the course.

B. The final exam accounts for 30% of the course grade.

C. Late papers will not be accepted. D. Gravity's Rainbow

is required reading in your course.

2. Which of the following claims would be best expressed by deductive reasoning?

a. California's population growth rate slowed last year.b. California residents appreciate their good weather.c. California residents are residents of the United States.d. More cars are registered in California than in any other state.

3. Which of the following arguments would lead to a deductive conclusion?

a. There was a mild winter this year, and previously whenever there's been a mild winter the cherry crop suffers.

b. The cherry crop needs at least a week of freezing temperatures for best results, and this winter the temperature stayed several degress above freezing.

c. Primo noticed that whenever the skiing was good in the winter, the cherry crop was profitable, and this year the skiing was good.

d. Not since 1972 have I seen a good cherry crop after a mild winter, and this winter has been mild.

AnswersAnswers

1. AIt is much more likely that this claim stems from personal observation than from adherence to some general principle, and so it would be best expressed by an inductive argument.

2. CSince California residents are residents of

the U.S. by definition, and definition is one sort of general claim on which a deductive argument can be based, this claim would be best supported by a deductive argument.

3. DThough the premise about this year is based on observation, the general or "major" premise, "The cherry crop needs at least a week of freezing temperatures," seems to be an assertion of established principle, making this a deductive argument with the conclusion, "The results won't be best this year."

Induction vs. deductionInduction vs. deduction

The difference between inductive and deductive reasoning is mostly in the way the arguments are expressed.

Any inductive argument can also be expressed deductively, and any deductive argument can also be expressed inductively.

Induction and Deduction Induction and Deduction combinedcombined

Induction is the method of reaching a potentially useful generalization◦People attending meetings after lunch are

invariably less attentive than those at morning meetings

Deduction is the method of using such a generality, now accepted as a fact◦Because we need an attentive audience, we

had better schedule the meeting at 10:30 rather than 1:00

Induction and Deduction Induction and Deduction combinedcombined

A personnel manager may have discovered over the years that electrical engineering majors from Central College are well trained in their field.◦His induction may have been based on records,

observations and opinions of people at his company and perhaps he has made the usable generalization about the training of Central College electrical engineering majors.

Induction and Deduction Induction and Deduction combinedcombined

He has received an application from a graduate of Central college

His deductive process will probably work as follows:◦Central college turns out well trained engineers◦The applicant was trained at central college◦Therefore the applicant must be well trained.

Syllogism◦Major premise◦Minor premise◦Conclusion

SyllogismSyllogism

Create your own syllogism.

Induction vs. DeductionInduction vs. Deduction

Patterns of Exposition p. 551.

Logical FallaciesLogical Fallacies

Do not use flimsy evidence◦ Opinion, hearsay, or analogy that do not support a valid generalization

Don’t use too little evidence leading to a premature inductive leap Don’t use misdirected appeals

◦ Misdirected Appeals Appeal to Authority, or Appeal to Questionable Authority Appeal to Common Belief, or Appeal to Belief, Appeal to Popular Belief Appeal to Common Practice, or Appeal to Tradition Two Wrongs Make a Right Appeal to Indirect Consequences, or Slippery Slope, Domino Theory Appeal to Wishful Thinking

◦ Emotional Appeals Appeal to Fear, or Scare Tactics, Appeal to Force Appeal to Loyalty, or Peer Pressure, Bandwagon, Ad Populum Appeal to Pity, or Sob Story Appeal to Prejudice, or Appeal to Stereotypes Appeal to Spite, or Appeal to Hatred, Appeal to Indignation Appeal to Vanity, or Apple Polishing

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