Module 2: Valid or Invalid Arguments

Question 1

Define proposition

Answer 1

Sentences with true/false value

Question 2

Validity

Answer 2

• Relationship the premises have to the conclusion
• may have false premises and false conclusions
• if assuming all premises are T, then have to accept conclusion even if it may be false
• can be ridiculous
• does not need to be strong or persuasive
• T of premises guarantees T of conclusion

Question 3

How to tell if an argument is valid or invalid?

Answer 3

“If the premises are true, would we be locked in to accepting the conclusion?” If the answer is “yes,” then the argument is valid. If the answer is “no,” then the argument is invalid.

Question 4

Medical school does not seem to be in Jamie’s future, for one needs an 80% average in science courses to be accepted into one, which is something Jamie hasn’t got.

Answer 4

Valid

Question 5

The gas in the chamber did not expand. From this we can infer that the temperature was not increase, for we know that, if the temperature pressure of any gas is increased while the pressure is kept constant, it will expand.

Answer 5

Invalid.

• Temperature of the gas may have increased, but perhaps the pressure was not kept constant.
• Since the premises may be true and the conclusion false, the argument cannot be valid

Question 6

Ski resorts will not make large profits this winter. For while they will make a fortune if it snows heavily, the long-range weather forecast predicts a dry season.

Answer 6

Invalid

1) the possibility of making huge profits without snow is still open
2) long-range weather forecasts may be wrong

Question 7

You will do well at golf, for if you can concentrate under pressure, as you can, you have got the game beat.

Answer 7

Valid.

Question 8

The theory of creation must be true. For scientists have found that there are inaccuracies in carbon dating, and that is just what we would expect if the theory of creation were true.

Answer 8

Invalid.

• the 1st premise does not rule out the possibility that inaccuracies in carbon dating are consistent with other theories of the origin of life besides the theory of creation
• Thus, it does not necessarily follow from the fact that carbon dating is inaccurate that the theory of creation is true

Question 9

The whale is sometimes described as a big fish. But this could not be right, for we know that all whales are mammals and fish are not.

Answer 9

Valid

Question 10

All fruit is a chair. Square is a chair. Therefore, square is a fruit.

Answer 10

Invalid.

• It is possible for squire to share with fruit the property of being a chair without being a fruit itself
• “all brilliant philosophers have lungs. Donald Trump has lungs so he’s a brilliant philosopher.” ABSURD!

Question 11

Either Russia will take Crimea or it won’t. If it does, Putin will be humiliated by the sanctions that the West will impose on his country. If Crimea is not taken, Putin will be humiliated in front of his own people by appearing weak and indecisive. So Putin will definitely be humiliated.

Answer 11

Valid

Question 12

A computer cannot cheat in a game, because cheating requires deliberately breaking rules in order to win. A computer cannot deliberately break rules, because it has no freedom of action.

Answer 12

Valid

Question 13

Ever since history has been recorded, the sun has risen everyday. Therefore, the sun will rise tomorrow.

Answer 13

Invalid

- Non-deductive argument so it is invalid