Induction vs. Deduction

Induction From a set of specific observation to a general conclusion. Uses no distinct form and conclusions are less definitive. Conclusions must be reasonable based on all of the evidence and must fit the evidence better than all other conclusions (results vary).

Induction People follow a particular pattern when using induction. – 1. decide on question to be answered or a tentative answer to a question (hypothesis – think science) – 2. gather evidence relevant to your hypothesis or question. – 3. move from evidence to conclusion (called an inductive leap) using inference (a statement about the unknown based on the known)

Induction In Chicago last month, a nine-year-old boy died of an asthma attack while waiting for emergency aid. After their ambulance was pelted by rocks in an earlier incident, city paramedics wouldn’t risk entering the Dearborn Homes Project (where the boy lived) without a police escort. Write a conclusion on your paper.

Induction Notice what happens if you put that conclusion as the first sentence of the paragraph. What does it serve as. Again, we’re looking for reasonableness

Induction Yesterday, I watch a person with green hair try to pull open a door with a big “push to open” sign on it. Today, a green haired person came to the help desk and asked where the “any” key was on the computer. People with green hair sure are dumb.

Induction Is the conclusion reasonable? Determine the following – 1. is the evidence sufficient? – 2. is the sample from the entire population? – 3. is the sample large enough to show a pattern? – 4. is the sample representative (typical) of the entire population?

Induction If not, you have committed a logical fallacy – the fallacy of Hasty Generalization: a fallacy based on too little evidence that is not representative. Stereotypes are examples of hasty generalizations.

Induction Observations – 1000 Martians have 3 eyes. Conclusion: All Martians have 3 eyes. Is this a reasonable inductive leap?

Induction A nice little Martian baby comes in with only 2 eyes. Is our conclusion still reasonable? What do we do?

Deduction Moves from a general statement to a specific conclusion. Strict form of logic If all premises are true then the conclusion must be true. Deductive reasoning deals with validity and truth of the conclusion (versus inductive reasoning which deals with reasonableness)

Deduction Basic form of a deductive argument is called a syllogism Syllogism consists of: – Major premise (general statement usually begins with “all”) – Minor premise (related but more specific) – Conclusion (drawn from those premises)

Deduction Major Premise: All Olympic runners are fast Minor Premise: Jesse Owens was an Olympic runner Conclusion: Therefore, Jesse Owens was fast. If we grant the premises as true then we must grant the conclusion as true. It’s our only choice.

Deduction Major Premise: All Olympic runners are fast Minor Premise: Jesse Owens was an Olympic runner Conclusion: Therefore, Jesse Owens was slow. -What’s wrong with this syllogism?

Deduction The conclusion is not true (actually contradicts the premises) Major Premise: All Olympic runners are fast Minor Premise: Jesse Owens was an Olympic runner Conclusion: Therefore, Jesse Owens was slow.

Deduction Major Premise: All Olympic runners are fast Minor Premise: Jesse Owens was an Olympic runner Conclusion: Therefore, Jesse Owens was tall. What’s wrong with this syllogism?

Conclusion may be true, but it doesn’t follow the premises so it’s not valid (and is a logical fallacy called non-sequitur)

Deduction Validity: Conclusion must follow logically from the major and minor premises. If the syllogism is not logical, the argument is not valid, and the conclusion is not sound.

Deduction Major Premise: All dogs are animals Minor Premise: All cats are animals Conclusion: Therefore, all dogs are cats What’s wrong with this syllogism?

Deduction Two major premises. Both start with all. The syllogism is defective so the argument is invalid Major Premise: All dogs are animals Minor Premise: All cats are animals Conclusion: Therefore, all dogs are cats

Deduction All dogs are animals Ralph is an animal Therefore, Ralph is a dog. Is this a good argument?

Deduction All dogs are animals Ralph is an animal Therefore, Ralph is a dog. This is not a good argument. The problem is that the minor premise refers to a term in the major premise that is undistributed (covers only some of the items in the class it denotes; In other words, it is not distributed over the entire class of things). To be valid the minor premise must refer to the term in the major premise that’s distributed – covers all the items in the class it denotes)

All dogs are animals Ralph is an animal Therefore, Ralph is a dog. In the major premise, dogs is distributed (look for the all), but minor premise takes the term animal which is undistributed.

Deductive All dogs are animals Ralph is an animal Therefore, Ralph is a dog. All animals are dogs Ralph is an animal Therefore, Ralph is a dog All dogs are animals Ralph is a dog Therefore, Ralph is an animal.

Deductive Validity of an argument doesn’t always = truth. All dogs are brown My poodle Toby is a dog Therefore, Toby is brown This is a valid argument. However, it is not true because the Major premise is not true.

Deductive To be sound a deductive argument must be valid and true. Major and Minor premises should be as close to self-evident as possible. In other words, the premises should be ones that most reasonable people believe are true. If not, use inductive reasoning.

Deductive Racism should be opposed The Anna Todd Jennings scholarship is racist Therefore, the Anna Todd Jennings scholarship should be opposed.

Deductive In order to accept the conclusion of this argument as true, what would the author have to do to avoid begging the question? (Begging the question refers to putting forth a premise as true that still needs to be proven true)

Logical Fallacies