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Logic. What is Logic?. Logic is a system of Rules for proving something is true, assuming other things are true. Why Study Logic?. We can attack opponent’s arguments by pointing out their flawed logic. Good arguments = true premises + valid l ogic .
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What is Logic? Logic is a system of Rules for proving something is true, assuming other things are true
Why Study Logic? We can attack opponent’s arguments by pointing out their flawed logic. Good arguments = true premises + validlogic. Logic helps us determine what is true in our daily lives. Flawed logic is everywhere.
Three Most Common Valid Logical Patterns Modus ponens (Affirming the Antecendent) Modus tollens (Denying the Consequent) Transitive logic
Modus Ponens Syllogism Premise 1: If something is a basketball, then it is an orange ball. Premise 2: I have a basketball. Claim: Therefore, I have an orange ball.
Modus Ponens Abstract Form If X is True, then Y is True. X is True. Therefore, Y is True. This is valid logic.
Modus Tollens Syllogism Premise 1: If there is no oxygen in this room, we will start suffocating Premise 2: We have not started suffocating Claim: There is oxygen in this room.
Modus Tollens Abstract Form If X is False, then Y is True. Y is False. Therefore, X is True. This is good logic.
Transitive Logic Syllogism Premise 1: If a river has a very fast current, people swimming in the river will be injured. Premise 2: If people swimming in the river will be injured, then I should not swim in the river. Claim: If a river has a very fast current, I should not swim in the river.
Transitive Logic Abstract Form If X is True, then Y is True. If Y is True, then Z is True. Therefore, If X is True, then Z is True. This is good logic.
Fallacies Some “Logic” isn’t so logical. Homer: Lisa, would you like a doughnut? Lisa: No, thanks. Do you have any fruit? Homer: This has purple in it. Purple is a fruit.
14 Fallacies (1. Fallacy of Ignorance) Fallacy of Ignorance: Lack of Proof or Lack of Disproof becomes evidence that something is true. “You can’t prove that Leprechauns don’t exist. Therefore, Leprechauns exist.” “We can’t find any hard, evidence that President Barack Obama is a Reptilian humanoid bent on killing us all. Therefore, the government must be covering it up.
14 Fallacies (2. Fallacy of Complex Cause) Many factors contribute to causing something. This fallacy picks out one factor as the only important factor. Lawyer: “My client’s motorcycle helmet failed during the motorcycle crash, and the company that made the helmet should be punished.” Missing Factors: The client was driving at 100 miles per hour while drinking his 11th beer and talking on a cell phone down the wrong side of the road.
14 Fallacies (3. Circular Reasoning) • Also known as begging the Question • The Premise of Argument 1 is the conclusion of Argument 2, while the conclusion of Argument 2 is the Premise of Argument 1 • Argument 1: Cornelius is right about nuclear energy because this book says he is right. • Obvious Question: Why is the book right? • Argument 2: Cornelius wrote the book.
14 Fallacies (4. False Dichotomy) Giving only two choices, when there are actually more choices available. “Your either with me or against me” “You can either buy the new iPhone 5, or be ridiculed by all of your friends”
14 Fallacies (5. Straw Man) Explaining you opponent’s argument in a weak and unfair way, then attacking it. “Obama wants to teach sex education to kindergarteners (2008 John Mccain Ad)” “[Romney’s Plan] could take away middle-class deductions for child care, home mortgages and college tuition (2012 Barack Obama Ad)”
14 Fallacies (6. Slippery Slope Fallacy) If A, then B is likely. If B, then C is likely. If C, then D is likely….. If Y, then Z is likely. Z is bad, therefore, don’t do A. “First it’s just a look in your bag for a missing pen, then it’s an invasion of your home, then they look through all your personal records and before you can say 1984 the thought police are telling you to bend and spread.”
14 Fallacies (7. Ad Hominem) Attacking the arguer rather than their argument. Arguer 1: “I think we should raise taxes to reduce government debt.” Arguer 2: “Well I think you’re an idiot!” “You’re biased, therefore your argument is wrong” “The real terrorist the US government. (Osama bin Laden)”
14 Fallacies (8. No True Scotsman Fallacy) Defining a term in such a way that it can’t be disproven Claim 1: A Scotsman has never lost a war. Attack: McClelland lost a war. Response: McClelland wasn’t a true Scotsman.
14 Fallacies (9. Guilt by Association) Argument X was believed by Person Y. Person Y is a horrible human being. Therefore, Argument X is a bad argument. 50% of these involve Hitler (at least on the Internet) Parent to Child considering applying to art school: “Hitler applied to Art School, and we all know how that turned out. What do you think about art school now?”
14 Fallacies (10. Hasty Generalization) Making a broad judgment based on limited evidence. Someone with Ohio License plates cuts you off while you’re trying to change lanes. You conclude that everyone from Ohio is a bad Driver.
14 Fallacies (11. Is-Ought Fallacy) Assuming that because something is true, it is the way things ought to be. “People Skip class everyday. Therefore, it’s okay to skip class.”
14 Fallacies (12. Argumentum ad Nauseam) Something is true because it is repeated over and over again. “President Obama will install death panels to kill your Grandmother with Obamacare.” 24 Hour News Cycle: “DEATH PANELS!” Local Representative: “I have real courage. I am not afraid to say I’m against Death Panels.”
14 Fallacies (13. Affirming the Consequent) We see an effect, and know of a cause that might cause that effect, then wrongly conclude that cause was the cause of the effect. Premise 1: If hundreds of people jump on your roof at the same time, it will do severe roof damage. Premise 2: Your roof has severe roof damage. Claim: Hundreds of people jumped on your roof at the same time.
14 Fallacies (14. Argumentum Ad Populum) Something is true because lots of people believe it. “80% of Americans are against the Estate Tax. The time has come to repeal the tax.” “Hundreds of Millions of Smokers can’t be wrong”
Other Fallacies Equivocation Fallacy of Division Fallacy of Composition Red Herring Non Sequitor Tautology Shifting Goalposts Lots More
