The author uses the word "immediacy" (line 39) most likely in order to express

hales on June 25, 2019

Sufficient and Necessary - Example 8

On question eight I have a clarifying question. The rule used to correctly identify the flawed logic in the explanation was "don't just negate" - however, when I solved on my own, I used the contrapositive and saw it as invoking N to conclude S. Would this also be correct? What is the difference in these two? When doing this I also came to the correct answer. Will this always be the case, or was this just a fluke instance? Do you have any recommendations for which rules to use on these types of questions - is it ok to use the contrapositive?

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Ravi on June 25, 2019


Great question. The truth is that it doesn't really matter how you
diagram it as long as you understand what the symbols you're using are
referring to so you don't get confused.

I recommend using the shortest possible symbols/words/phrases for you
to capture what's going on. This way, you're being efficient, but
you're also covering everything.

Instead of writing "distinguish," you could even just write "D" to
connote the phrase you're identifying as the necessary premise.

Does that make sense? Let us know if you have any other questions!

Ravi on June 25, 2019

@hales, please ignore the above response (that was for one of your other questions). I'll post another reply on this thread for this one.

Ravi on June 25, 2019


Great question. When Mehran was saying, "Don't just negate," he was
referring to the error in the reasoning in the stimulus. The stimulus

PL - >O

and the argument concluded

/PL - >/O (this is what don't just negate referred to)

The contrapositive of the argument's conclusion is O - >PL, and this is
exactly what you picked up on (invoking N to conclude S). This is the
same mistake that Mehran is referring to, so you're on the right track
with your analysis.

It's always o.k. to use the contrapositive since it's a logically
equivalent statement, so you're good there. Regarding which rules to
use, there's not hard rule—it really depends on the question. You're
doing great with how you're currently looking at it.

Hope this helps. Let us know if you have any other questions!