Necessary and Sufficient Assumption Drill from the Book

[cardsfan04]

If it's not OK to post things from the Manhattan books here, I apologize. Please delete.

On page 98-99 of the LR Strat Guide there is a N/S Assumption Drill. I struggled a little bit with it, but generally understood my mistakes after reading the explanations. However, I don't understand this one:

4. Maria must be a person who values faith because she attends church every Sunday.

(1) Which of the following is an assumption upon which the argument relies? (obviously a necessary assumption question)

Anyone who attends church every Sunday is a person who values faith, family and community.

If I negate the answer I get "No one who attends church every Sunday is a person who values faith, family, and community." If no one who goes to church each Sunday values faith, family and community, and she goes to church each Sunday, how can she value faith, family and community?

Yet, the solution says this is not a necessary assumption.

Where am I going wrong with this? Again, sorry if it's frowned upon to post directly from the book.

[chike_eze]

4. Maria must be a person who values faith because she attends church every Sunday.

(1) Which of the following is an assumption upon which the argument relies? (obviously a necessary assumption question)

Anyone who attends church every Sunday is a person who values faith, family and community.

If I negate the answer I get "No one who attends church every Sunday is a person who values faith, family, and community." If no one who goes to church each Sunday values faith, family and community, and she goes to church each Sunday, how can she value faith, family and community?

Not exactly.

The answer option above is equivalent to...

All people who attend church every Sunday value faith, family and community

Logical negation:

Not All people who attend church every Sunday value faith, family and community

Which is equivalent to:

Some people who attend church every Sunday do not value faith, family and community

Thus, the negation of "Any" (which is "some..not") does not destroy the argument that at least one (Mary) satisfies the rule.

[cardsfan04]

ohhhh. That makes sense. I wasn't negating it; I was going to the polar opposite.

Thanks!!!

[demetri.blaisdell]

Great question and an even better answer! This is a very common mistake and I think you did a great job of explaining it. Let me know if you have any further questions on this. The negation test is an important tool.

Demetri