A book tour will be successful if it is well publicized and the author is an established writer. Julia is an establis...

Mehran March 22, 2018

Hi there, thanks for your post. This is an Errors in Reasoning question. So, first, we need to assess the stimulus.

Here, the stimulus presents an argument. The conclusion is "[Julia's] book tour must have been well publicized." The premises given in support are (1) A book tour will be successful if it is well publicized and the author is an established writer, and (b) Julia is an established writer, and her book tour was successful.

OK, this is sufficient & necessary reasoning. Let's diagram the first premise to be sure we are correctly understanding which conditions are sufficient and which are necessary.

The word "if" introduces sufficient conditions. So, to diagram "A book tour will be successful if it is well publicized and the author is an established writer," you would write:

WP and AEW ==> BTS
(if well publicized and the author is an established writer, then a book tour will be successful)

You are told that Julia is an established writer. So the AEW sufficient condition is satisfied.

But you are not told that the other sufficient condition has been fulfilled (that is, you are not told that Julia's book tour was well-publicized).

Rather, you are given the necessary condition - you are told that her book tour was successful.

This is flawed reasoning.

Answer choice (C) mimics this flaw:
P: If cacti are kept in the shade and watered more than twice weekly, they will die.
KS and W2+/week ==> D
(if kept in shade and watered more than twice/week, will die)

P: This cactus was kept in the shade, and it is now dead.
KS
D

C: Therefore, it must have been watered more than twice weekly.
W2+/week

As in the stimulus, this answer choice presents a flawed argument. Just because you have one of two sufficient conditions and a necessary condition satisfied DOES NOT MEAN the second sufficient condition is also established.

Answer choice (A) is entirely different, because it involves the phrase "only if." "Only if" designates a NECESSARY condition.

So: This recipe will turn out only if one follows it exactly and uses high-quality ingredients.
RTO ==> FE and HQI
(recipe will turn out ONLY IF follow exactly and use high-quality ingredients)

Arthur followed the recipe exactly and it turned out.
FE
RTO

Thus, Arthur must have used high-quality ingredients.
HQI

This, too, is a flawed argument - but it's not the SAME flaw as the stimulus. The stimulus and answer choice (C) are the same: you are told one of two sufficient conditions and a necessary condition have been satisfied, and then an erroneous conclusion is drawn that the second sufficient condition must also have been satisfied.

Answer choice (A) is different: you are told one of two necessary conditions and a sufficient condition have been satisfied, and then an erroneous conclusion is drawn that the second necessary condition must also have been satisfied.

Hope this helps!

jenbeer2 September 29, 2019

This was super helpful. I feel like I'm never going to grasp these small details though

lilmansour June 9, 2020

Wow... that was such a minor detail that made a difference! It would take me a few minutes to realize that on my own! Good question

Brett-Lindsay July 12, 2020

Hi @Mehran,

I read Naz's explanation first, and was amazed that I hadn't noticed that (according to Naz) A was a valid argument. Taken from the thread:

"P: RTO ==> FE & HQI
not FE or not HQI ==> not RTO
P: FE & RTO
C: HQI
This argument is not actually flawed. Because we know that the recipe turned out. That means that we can conclude HQI."

When I read her explanation, it made perfect sense. As we have the sufficient (RTO), we must have the necessary condition (HQI). The additional FE shouldn't make any difference to the argument.

I've already spent a week on this lesson - it's frying my brain. I'm not trying to be troublesome, I'm just trying to figure it all out. Is A flawed or valid?

Thanks.