Recitation Meeting 3

1. Recitation Meeting 3 Arguments, validity, soundness, persuasiveness

2. Exercise • Identify the premises and conclusion of each argument. Some of these can be done in more than one way. • 1. John has brown hair. John is not blond. • 2. Mary had a little lamb. Mary had five lambs. • 3. I still have five tin cans. I received three cans when I was younger. I received two cans when I was older. I did not throw any cans away. • 4. Mary is shorter than John. John is the tallest person in the world. • 5. Susan is taller than John. Susan is taller than Mary. John is taller than Mary. • 6. John likes broccoli or cabbage, but not both. John does not like broccoli. John likes cabbage. • 7. Mary is not at home. If she were home, she would answer her door. She isn't answering her door. • 8. My dog is better than your cat. Any dog is better than any cat. • 9. All the ravens I’ve seen so far are black. All ravens are black.

3. Deductive and Inductive • There are two ways in which the premises of an argument might support the conclusion. • Inductively: The premises provide reliable evidence for the likelihood of the conclusion. • Example: • 1. The postman has always come by at 2pm before. • 2. The postman will come by at 2pm today. • Deductively: The premises guarantee the truth of the conclusion. • Example: • 1. Judy either is at the store or at home. • 2. Judy isn’t at home. • 3. Judy is at the store.

4. Exercise • State whether each of the following arguments is deductive or inductive. • 1. The plaque on the leaning tower of Pisa says that Galileo performed experiments there. • 2. Galileo did indeed perform experiments there. • 1. There were pry marks around the lock on the rear door. • 2. There were deposits of mud near the rear threshold. • 3. The burglar entered through the rear door. • 1. Either global warming will be arrested, or hurricanes will become more intense. • 2. Global warming will not be arrested. • 3. Hurricanes will become more intense.

5. Validity • Deductive arguments are valid in the following sense. • An argument is invalid = there is a conceivable scenario in which the premises are true and the conclusion is false. • If an argument is not invalid, then it is valid.

6. Valid and Invalid Arguments • Examples: • 1. If Socrates was a philosopher, then he wasn’t a historian. • 2. Socrates wasn’t a historian. • 3. Socrates was a philosopher. • 1. All men are mortal. • 2. Socrates is a man. • 3. Socrates is mortal. • 1. If Col. Mustard is the murderer, then the murder weapon is the wrench. • 2. The murder weapon is not the wrench. • 3. Col. Mustard is not the murderer. • 1. Every plant has nine toes. • 2. President Obama is a plant. • 3. President Obama has nine toes.

7. Soundness • As the Obama-is-a-plant example shows, an argument can be valid without being successful. • In order to be successful, an argument must be sound. • And argument is sound = its premises are true and it is valid.

8. Exercise • State whether each of the following arguments is or isn’t valid and/or sound. • 1. If she studies hard, she will do well. She studied hard. Therefore she will do well. • 2. If I leave home early enough, I will get to school on time. I got to school on time. Therefore, I must have left home early enough. • 3. People that smoke raise their chances of getting cancer. Alice died from cancer. She must have been smoking too much. • 4. Bob has five kids. So, he has three kids. • 5. All dogs are cats. All cats are terrible. Therefore, all dogs are terrible. • 6. Mammals nurse their young with milk. Tigers are mammals. So, tigers nurse their young with milk.

9. Exercise 3 • 1. Come up with 2 valid arguments. • 2. Come up with 2 invalid arguments. • 3. Come up with 2 sound arguments. • 4. come up with 2 unsound arguments. • Answers to 3 and 4 should be different from answers to 1 and 2.