OG蓝书中的一道逻辑题，求好心人解惑！

sdcar2010

This is a typical puzzle of correlation and causation.  If event A and event B happend together, there are several possible scenarios: A causes B; B causes A; a third event C causes both A and B.

In the question your posted, it is a necessary assumtion.  If you use negation method on answer D, you will get:
Mental illness DOES cause people's immune-system activity to decrease.  If that is the case, then the conclusion that the immune system protects against mental illness does not hold. Hence, answer D is required for the original conclusion.

sdcar2010

楼上讲的很清晰，但我不明白为什么，Mental illness DOES cause people's immune-system activity to decrease，会weaken the conclusion that the immune system protects against mental illness

-- by 会员 zhijingya (2011/2/22 22:43:57)

Simple.

If mental illness causes people's immune-system activity to decrease，then the conclusion that the immune system protects against mental illness is wrong, isn't it?