Deductive & Inductive Forms of Reasoning

Deduction: reasoning from general premises, which are known or presumed to be known, to more specific, certain conclusions. Induction: reasoning from specific cases to more general, but often uncertain, conclusions Both deductive and inductive arguments occur frequently and naturally Both forms of reasoning can be equally compelling and persuasive, and neither form is preferred over the other

Deduction vs. Induction commonly associated with “formal logic” involves reasoning from known premises, or premises presumed to be true, to a certain conclusion the conclusions reached are certain, inevitable, inescapable Induction commonly known as “informal logic,” or “everyday argument” involves drawing uncertain inferences, based on probability rather than certainty the conclusions reached are probable, reasonable, plausible, believable

Deductive Reasoning Starts with a general principle or universal truth (a major premise) and applies it to a specific case (a minor premise) The fundamental property of a valid, deductive argument is that if the premises are true, then the conclusion necessarily follows The conclusion is said to be contained in the premises Often structured as a syllogism—a logical structure that uses the major premise and minor premise to reach a necessary conclusion

Inductive Reasoning Arranges an argument so that it leads from particulars/specifics to universals or generalizations; uses specific cases to draw a conclusion Arguments developed inductively can never be said to be true or false, right or wrong; instead, they can be considered strong or weak based on vulnerabilities that might be present (flawed logic or anomalies) The process of induction involves collecting evidence and then drawing an inference based on that evidence in order to reach a conclusion

Example Deductive and Inductive Arguments Example of Deduction Major premise: All tortoises are vegetarians Minor premise: Bessie is a tortoise Conclusion: Therefore, Bessie is a vegetarian Example of Induction Specific: Exercise promotes weight loss Specific: Exercise lowers stress levels Specific: Exercise improves mood and attitude Generalization: Exercise contributes to better health (115)

Example deductive argument Suppose every place in the world that people live is represented by the blue space inside the rectangle. Suppose the long pink oval represents all the wooden houses in the world. And, suppose the green circle represents Canada. The most logical conclusion one can draw from the figure is: C. All wooden houses are found in Canada Everyone lives in a wooden house Some Canadians live in wooden houses No one lives in Canada

Another example of deductive argument Person L is taller than J Person X is taller than J Person J is taller than L Person J is taller than M Person M is taller than Y Suppose the following statements are all true: Person L is shorter than person X Person Y is shorter than person L Person M is shorter than person Y What additional piece of information would be required to conclude that “Person Y is shorter than Person J”? Solution: C M < Y < L < X So, if J is taller than L, Y must be shorter than J

Example of Induction From Outliers by Malcolm Gladwell (pp.116-118) Example of an argument structured largely by induction Gladwell uses various types of evidence to support his conclusion Gladwell presents and explores many examples and specific cases to reach a general conclusion

Example inductive arguments There are varying degrees of strength and weakness in inductive reasoning; the stronger the examples, the stronger the argument. Consider the following examples: The chair in the living room is red. The chair in the dining room is red. The chair in the bedroom is red. All chairs in the house are red. Every time you eat peanuts, your throat swells up and you can't breath. So, you are allergic to peanuts. In Animal Farm, Boxer proves to be gullible in specific situations; therefore, Boxer is gullible. In Nickle & Dimed, Ehrenreich provides multiple examples and descriptions of her life as a low-wage worker, reaching the conclusion that these workers are struggling and being taken advantage of by managers, business owners, and corporations.

Deduction v. Induction Deductive reasoning is either “valid” or “invalid.” A deductive argument can’t be “sort of” valid If the reasoning employed in an argument is valid and the argument’s premises are true, then the argument is said to be sound valid reasoning + true premises = sound argument Inductive reasoning enjoys a wide range of probability; it can be plausible, possible, reasonable, credible The inferences drawn may be placed on a continuum ranging from cogent at one end to fallacious at the other fallacious cogent

Deduction v. Induction --overview Inductive reasoning is found in the courtroom, the boardroom, the classroom, and throughout the media Most, but not all everyday arguments are based on induction Examples: The “reasonable person” standard in civil law, and the “beyond a reasonable doubt” standard in criminal law Deductive reasoning is commonly found in the natural sciences or “hard” sciences, less so in everyday arguments Occasionally, everyday arguments do involve deductive reasoning Example: “Two or more persons are required to drive in the carpool lane. You don’t have two or more persons. Therefore, you may not drive in the carpool lane”

Activity: Review The Declaration of Independence (pp Activity: Review The Declaration of Independence (pp. 120-123) & “The Declaration of Sentiments” on pp. 123-125 / Explain, analyze, &/or evaluate the use of both induction and deduction in each text—please be prepared to share your ideas with the class!