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LOGIC. The process of proper reasoning & the study of good use of arguments. DEDUCTIVE REASONING. Deductive reasoning moves from the….. to the …… Deductive reasoning moves from the general to the specific . All dogs are mammals Chester is a dog. Therefore Chester is a mammal.
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LOGIC The process of proper reasoning & the study of good use of arguments.
DEDUCTIVE REASONING Deductive reasoning moves from the….. to the …… Deductive reasoning moves from thegeneralto the specific. All dogs are mammalsChester is a dog.Therefore Chester is a mammal. A deductive argument is VALID if it is logically constructed or INVALID if it’s not logical. Example: All Italians like spaghetti.Giovanni likes spaghetti.Therefore Giovanni is Italian.
How reliable is deductive reasoning? A deductive argument consists of premises/rules & a conclusion. A deductive argument is always true/soundif it’s valid and if its premises are true. All humans are mortal. (premise)Socrates is human. (premise)Therefore Socrates is mortal. But how do we acquire true premises? Obviously NOT through deductive reasoning, but through observation and induction.
Deductive reasoning in everyday use: In everyday use, we often don’t bother stating obvious premises, but we still use deductive arguments very commonly: Jenny goes to Stanford, so she must be very smart. What is the unstated premise? Only smart people go to Stanford. Since it’s natural to eat meat, there’s nothing morally wrong with it. What is the unstated premise? Whatever is natural is also moral.
Deductive Reasoning in groups – solve “The Liar Problem” & explain your deductive reasoning to the class.
INDUCTIVE REASONING Inductive reasoning moves from thespecific data to the general conclusion. To use the previous example… How would you come to the conclusion that humans are mortal? By collecting specific data that shows that humans die and also by lack of any contradictory evidence (lack of humans who have lived forever). Inductive reasoning relies on the coherence of data – all evidence fits together.
Inductive reasoning in everyday use: We use it constantly as a practical way of knowing: My neighbor’s dog has been friendly to me day after day. Therefore, I have accepted that it will never bite me. What is the problem with this? Generalizations can easily turn into stereotyping and prejudice. This is made worse by a phenomenon knows as confirmation bias: people tend to select and look for evidence that confirms their expectations and prejudices. Examples? Politics, personal looks, journalism, research, etc.
Other limitations of inductive reasoning It depends on observation and on our senses. Metal A expands when heated; metal B expands when heated; metal C expands when heated. Therefore all metals expand when heated. What percentage of the metal existing on our planet would you guess scientists have tested to see if it expands when heated? What does this suggest about the certainty or otherwise of scientific laws? Science typically formulates “laws” on the basis of a limited number of observation. Relying too much on the past to predict the future.
The dog gets excited when you get her leash out and seems to know that you are about to go for a walk. What kind of reasoning does she use?
Inductive or deductive reasoning? Your English teacher has asked you to find examples of metaphor in the text. In science, you need to conduct a series of experiments to prove a hypothesis. In history, you need to read a section in the textbook and write two paragraphs about the causes of the French Revolution.
Lateral/Creative Reasoning Also known as “thinking outside the box” Looking for creative solutions to problems. Connect the nine dots below by using only 4 straight lines and without taking your pen off the paper.
LOGICAL FALLACIES They are errors in reasoning. As a speaker, you need to avoid them. As a listener, you need to be alert to recognize them. People use them when they don’t have enough sound arguments. They are illogical, unethical and misleading. But they work…so they are commonly used.
Irrelevant Evidence (non sequitur) • When the information has nothing to do with the argument; the conclusion does not follow from the evidence. • Example: The merchandise at the Ultra Store is top quality. The manager has clothes shipped in from all over the world. • Example: My Baseball team spent more money on players than your baseball team. Therefore my team is better
Name Calling (ad hominem)Also known as “poisoning the well” • Attacking the person rather than dealing with the real issue. • Bob says XYZ Bob is a _____ (communist, felon…) Therefore not XYZ • Example: A speaker might ask you to vote against a candidate, not because of what he stands for, but because he is “divorced and his wife left him for a good reason.”
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Red Herring • Using an irrelevant issue in order to divert attention from the subject. • The name of this fallacy comes from an old trick used in England to keep fox hunters from galloping through the crops by dragging a red herring with a strong odor along the edge of the field to distract the hounds. • Example: Why should we worry about endangered animal species when thousands of people are killed in automobile accidents each year?
Card Stacking • Presenting only the facts that build the best case for your argument, ignoring completely any other facts that exist. • Example: “Mom, I did all the dishes, I made dinner, and I put all my clothes away, so I should be allowed to stay out until midnight with my friends.” This person is using card stacking, because he ignores the facts that he broke half of the dishes, burned the dinner, and flunked all of his classes.
Slippery Slope • Implying that taking a first step will lead to a second step and so on down the slope to disaster. • The name of this fallacy comes from the image of taking a step down a slippery hill and tumbling down to the bottom. • Example: If we pass laws to limit violence shown on TV, this will soon result in absolute government control of the media and total censorship. That’s communism! • Example: You can never give anyone a break. If you do, they'll walk all over you!
Either-Or(Black or White) • Forcing listeners to choose between only two alternatives, when many more exist in reality. Deliberately oversimplifying a complex issue to create fear. • Example: Either we build a new high school or the children in this community will never be able to go to college.
Anecdotal Evidence • Using a few personal experiences to prove that something is true or false. • Example: "A Volvo! You've got to be kidding. My brother-in-law had a Volvo. First, that fancy fuel injection computer thing went out. Had to replace it. Then the transmission and the clutch. Finally sold it in three years for junk."
False/Hasty Generalization • When generalizations are based on very limited data… often due to bias. • Example: asking one person what she thinks about gun control would clearly not provide an adequate sized sample for determining what Americans in general think about the issue.
Circular Reasoning • Also known as “vicious cycle” or “begging the question.” • X is true because of Y.Y is true because of X. • Example: The Bible is the Word of God, because God tells us so ... in the Bible.
Emotional appeal – Argumentum ad Miscordiam • My client is a good family man. • My client had a messed up childhood. • Therefore my client is not guilty.
Authority • Tiger Woods drives a Buick. • Tiger Woods is great at golf. • Therefore Buicks are great cars.
Bandwagon • Do something that others are doing simply because others are doing it. • Example: “Four out of five dentists preferred Crest toothpaste.” • Example: “It’s alright for me to cheat on my taxes because everyone else does it.”
Argument from ignorance • There is no proof unicorns do not exist.Therefore they exist… • You cannot prove that God does not exist, so He does.
