Module 2: Deductive/Non-deductive & Valid/Invalid or Strong/Weak

Question 1

Rugby is our national game. Our national game is also the Welsh national game. So Rugby is the Welsh national game.

Answer 1

Deductive & Valid

Question 2

If you love me, then you will buy me a diamond ring. You won’t buy me a diamond ring. Therefore, you don’t love me.

Answer 2

Deductive & Valid

• premises not leaving room for any error
• All arguments that have this form are valid

Question 3

Rugby is our national game. Our national game is also the Chinese national game. So Rugby is the Chinese national game.

Answer 3

• Deductive & Valid

- premises provide enough support for the conclusion

Question 4

Most Kiwis love their national game: rugby. Carol is a Kiwi, so it’s quite likely that she loves rugby.

Answer 4

• Non-deductive & strong
• non-deductive: “most Kiwis” and “it’s quite likely that”
• premises not intended to provide conclusive support for the conclusion
• strong: quite likely cus “most Kiwis” so it is a safe bet

Question 5

Every time I have pressed the light switch, the light has come on. I am pressing the light switch now. Wouldn’t you be surprised if the light didn’t come on now?

Answer 5

• Non-deductive & strong

- rhetorical question conclusion

Question 6

The face on the Sphinx at the Giza Plateau does not resemble any portraits of the Egyptian Pharaohs. Because of this and the lack of any true inscriptions in the Great Pyramid we can safely say that, in fact, aliens built the pyramids.

Answer 6

• Apply principle of charity

- Non-deductive (betting aliens built it) & weak

Question 7

The salaries of non-executive directors of the company should be raised from $28,000 to $45,000 per year. They have had no salary increase since 1995, and anyway, higher salaries are needed to attract people of the right calibre.

Answer 7

• Non-deductive & weak
• why specifically $45,000? no premise to support that
• Had it been phrased as something like “The salaries of non-executive directors of the company should be raised.” or “The salaries of non-executive directors of the company should be raised by at least 5%. “ then the premises would have provided stronger support for the conclusion

Question 8

Every proposition made by John Banks when he was mayor of Auckland last time is true. When he was mayor of Auckland last time, John Banks said that Britomart will completely revolutionise public transport in Auckland, given enough time. Without a doubt, then, Britomart will revolutionise public transport in Auckland, given enough time.

Answer 8

• Deductive & Valid
• “without a doubt” and if all premises r T and valid then conclusion = T
• when testing validity, dont worry if premise T or not.
• what matters is IF it IS true, then conclusion is true

Question 9

If Patrick is a cat, then he necessarily has a brain. Patrick is not a cat. Therefore, Patrick doesn’t have a brain.

Answer 9

• Deductive & invalid
• “necessarily” = no room for error so deductive
• Is not a cat and does have a brain so invalid.

Question 10

The human race has managed to land somebody on the Moon and split the atom. It’s thus quite reasonable to think that we should also be able to redistribute the world’s substantial food supplies so that the poor get plenty.

Answer 10

• Non-deductive & strong
• “it’s quite reasonable to think that” = provide probably support to conclusion
• reasonable support to conclusion cus since we have made such an incredible trip to the Moon, it seems like we could join forces and find a way to provide food to the poor.