Inductive vs. Deductive Reasoning

Deductive Reasoning Starts with a general rule (a premise) which we know to be true. Then, from that rule, we make a true conclusion about something specific.

Deductive Reasoning The process of reasoning from known facts to conclusions. When you reason deductively, you can say “therefore” with certainty. If your facts were firm to begin with, then your conclusions will also be firm.

Deductive Reasoning

Deductive Reasoning From vague To specific

A deductive argument is one in which it is impossible for the premises to be true but the conclusion false. It is supposed to be a definitive proof of the truth of the claim (conclusion). Premise  All men are mortal. Premise  Socrates was a man. Conclusion  Socrates was mortal. If the premises are true (and they are), then it simply isn't possible for the conclusion to be false. If you have a deductive argument and you accept the truth of the premises, then you must also accept the truth of the conclusion.

A deductive argument True Premise Conclusion

Deductive Reasoning Example: Smith owns only blue pants and brown pants. Smith is wearing a pair of pants today. So, Smith is wearing either blue or brown pants today.

Inductive Reasoning The process of going from observations to conclusions. This type of conclusion is sometimes called an inference.

Inductive Reasoning Observing that something is true many times, then concluding that it will be true in all instances Using the data to make a prediction

Inductive Reasoning

Inductive Reasoning From specific To vague

An inductive argument is one in which the premises are supposed to support the conclusion. If the premises are true, it is unlikely that the conclusion is false. The conclusion probably follows from the premises. Premise  Socrates was Greek. Premise  Most Greeks eat fish. Conclusion  Socrates ate fish. Even if both premises are true, it is still possible for the conclusion to be false (maybe Socrates was allergic to fish). Words which tend to mark an argument as inductive include probably, likely, possibly and reasonably.

A inductive argument True Premise True Premise True Probably Conclusion

Inductive Reasoning Example: January has been cold here in Siberia. Today is January 14, so it is going to be another cold day in Siberia.

Which conclusion below shows evidence of deductive reasoning? One time an employee used store computers to download a harmful virus, so employee computer use is a security threat. Many customers complained about employees using computers rather than working, so computer misuse was harming the company’s sales. An employee on a break distracted other employees by using a computer in a public area for private business, so that employee was given a warning. Several employees left customer sales information visible on computer screens, so those employees were instructed to always close windows with personal information.

The word wall is Inductive The windows are Deductive Let’s Practice – The word wall is Inductive The windows are Deductive

#1 Conclusion: This wire will conduct electricity. Premise: Everything made of copper conducts electricity. Premise: This wire is made of copper. Conclusion: This wire will conduct electricity.

Premise: Socrates was Greek. #2 Premise: Socrates was Greek. Premise: Most Greeks eat fish. Conclusion: Socrates probably ate fish.

#3 The local branch of Wachovia Bank was robbed yesterday. Jenny needed money to pay off her gambling debts. She just bought a gun two days ago, and I saw her hanging around the local Wachovia Bank yesterday morning. Today the bookie’s goons stopped looking for Jenny. So Jenny robbed Wachovia Bank yesterday.

#4 Premise: All turtles have shells. Premise: The animal I have captured is a turtle. Conclusion: I conclude that the animal in my bag has a shell.

#5 Sherlock Holmes and Watson were on a camping trip. They had gone to bed and were lying there looking up at the sky. Holmes said, “Watson, look up. What do you see?” “I see thousand of stars.” “And what does that mean to you?” “I guess it means we will have another nice day tomorrow. What does it mean to you, Holmes?” “To me, it means someone has stolen our tent.”

#6 Conclusion: Tonya has bought the set of golf clubs for Jack. Premise: Tonya is seen walking from her car to her home with a set of golf clubs. Premise: Tonya’s husband Jeff loves golf and tomorrow is his birthday. Conclusion: Tonya has bought the set of golf clubs for Jack.