It would be wrong to conclude that a person has a Streptococcus infection if there is no other evidence than the fact...

Sangwook on August 30, 2014

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Please explain what logical structure in the stimulus is parallel with the choice (b)'s? Thanks,

2 Replies

Melody on September 9, 2014

Let's break down the argument in the stimulus.

We know that if there is no other evidence than the fact that Streptococcus bacilli are present in one's throat, then we cannot conclude that that person has a Streptococcus infection, since we also know that the infection does not occur unless the host is physically run down.

So, having a Streptococcus infection requires that Streptococcus bacilli are present in one's throat AND that the host is physically run down.

P: I ==> SBP & HPRD
not SBP or not HPRD ==> not I

Thus, if we do not know whether the host is physically run down, or in other words: if the only evidence we have is that Streptococcus bacilli are present in one's throat, i.e. we do not know that both requirements are present, then we cannot accurately conclude that that person has an infection.

C: not SBP or not HPRD ==> not I
I ==> SBP & HPRD

As you can see, our conclusion is the contrapositive of the principle rule.

Now, let's look at answer choice (B): "Even if a healthy lavender plant receives six or more hours of direct sunlight each day, one cannot predict on that basis alone that the plant will bloom, because lavender requires both six or more hours of sunlight per day and slightly alkaline soil to bloom."

Thus, for healthy lavender to bloom, it is necessary that the plant receives six or more hours of direct sunlight and that it has slightly alkaline soil.

P: LB ==> 6+ & SAS
not 6+ or not SAS ==> not LB

Thus, if we do not know whether there is slightly alkaline soil, i.e. if we only have one of the two requirements, we cannot predict that the lavender will bloom.

C: not 6+ or not SAS ==> not LB
LB ==> 6+ & SAS

Thus, both answer choice (B) and the stimulus have the following patterns of reasoning: A & B are required for the presence of C. Thus, C cannot be predicted on the sole basis that only one of A or B is known.

Hope that clears things up! Please let us know if you have any other questions.

Sangwook on September 9, 2014

I failed to connect two different statements with "AND" on the stimulus. Thanks a lot, Naz!!!