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The Sufficient/Necessary Flaw: an approach to maximize points

BinghamtonDaveBinghamtonDave Alum Member 🍌🍌
edited September 2018 in General 8673 karma

Hey everyone, This post is intended to be an overview of how the sufficient/necessary flaw appears on the LSAT and how we can develop a system of attack for the questions on which the sufficient/necessary flaw appears. I have collected my approach to these questions to 1 single thread.
I want to build on an analogy here that we all might not be entirely familiar with. It is rooted in my understand of the philosophical principle of Occam’s Razor. Suppose your car won’t start and you need to get to work. In an effort to solve this problem, you start deducing what is wrong with the car on the simplest grounds first: you check to make sure the car is not out of gas. We start here prior to checking more elaborate/expensive and time consuming problems: like replacing the transmission, because why potentially waste time with the more elaborate problems if the issue can be solved by appeal to something much simpler?
We do this all the time in our daily lives and I will submit, we should do this on various aspects of the LSAT: start with the simplest explanation of the problem on flaw questions that contain a sufficient/necessary flaw in an effort to save time. If that simplest explanation does not appear, move on to the next description. I might not be the first to apply this system to a problem, but this is the process I use:

1.Identify the “form” in which the argument has taken. This step is a convergence of several skills outlined in 7Sage’s core curriculum. From recognizing conclusion/premise indicators, to properly translating conditional statements, here is where our drills pay off. Extract from the argument both the conditional statement and what the author of that argument does with that conditional statement, here is where the flaw usually occurs.
Drill tip: any time we have a flaw question and our author is using conditional language, alarm bells should be ringing, for our chances of landing in sufficient/necessary territory have increased (although this is not a “silver bullet” it is a decent place to start.)
Drill tip 2: I want to place here a skill I have been thinking about a ton lately for my retake. My tutor calls this skill: following the thread of the argument. Following the thread is granting the argument a certain amount of leeway when we read it. Constantly taking the sentence we just read and trying to see how it relates to the sentence we are reading is following the thread of the argument. Among other things, we are looking for possible negations of certain terms that might not be readily apparent. Take for instance PT 43-2-12, “center of the universe” is essentially negated by the concept of “revolves around a star in the outskirts of the galaxy.” The concept of “center of the universe” implies that it is not in the “outskirts.” Negations can be hidden in the argument in this manner. Note here that there is some debate whether this question is really a sufficient/necessary flaw but I believe it illustrates the point I am trying to communicate fairly well, look out for innovative and clever ways the argument negates terms in the conditional statement, this can happen on several fronts, but a great way to be on top of these possible ways is to “follow the thread” of the argument sentence by sentence.

The sufficient/necessary flaw takes two general forms that encompass the vast majority of sufficient/necessary flaws:
Form 1:
We are given:
We are told we have a specific ~A
We conclude we have a specific ~B
Note here: that we are given the ~A in the premise of the author’s argument and are given the ~B in the conclusion of the author’s argument. With this structure, the LSAT writers are telling us that the premise functions like the sufficient condition and that the conclusion functions like the necessary condition. this is an insight that I found helpful when I first noticed it.

Pt 45-1-13
Pt 37-4-22
Pt 40-1-14
Pt 23-3-17
Pt 27-4-7
Pt 11-2-9

Form 2:
We are given A---->B
We are told we have a specific B
We conclude we have a specific A

Pt 24-2-23
Pt 13-2-26
Pt 22-2-25
Pt 39-4-15

Note here that these are not exhaustive lists. So this is our first step, get a grasp on the form of the argument.

Drill tip: note here that the only question I have been able to find in which we are asked to tell the difference between these two forms is: Pt 51-1-20, in which answer choices (B) and (E) contain the sufficient/necessary flaw, but in different forms. Here we are choosing the form that mirrors our stimulus. Know these forms! Knowing the form in front of us is the first step to describing that form with an answer choice.

2.Once we have the entire “form” in front of us, we move on to the question of how a description of the flaw might appear. In the easiest way available: we scan the answer choices for the words sufficient/necessary. This is the analog to checking if our car is out of gas, if we have found a description of our problem that uses the words sufficient/necessary correctly: choose it and move on, congrats, you’ve answered this question in 40 seconds.
Drill tip: the LSAT writers are aware of this surface level approach to questions, occasionally, they will plant a misdescription of the sufficient/necessary flaw. For an example of this, please see: PT 75-1-12 answer choice (B),in which is using our words that we need, but is describing valid logic! Yes, the test writers are this petty. This is not the only question ever to contain a trap like this, so it begs the question of knowing how to describe a sufficient/necessary flaw beyond the buzz words sufficient/necessary. At this juncture, with some back of the envelope calculations, I believe with the first two steps, we are looking at getting 30-35% of flaw questions that contain a sufficient/necessary conflation correct. What I mean by this is that many flaw questions that contain the sufficient/necessary conflation that we have understood due to our application of step 1 will describe it using those words. This is merely noticing the reappearing elements to the question and describing it.

3.So we have reached step three. We have a handle on the form of the argument and we know that the author has committed the sufficient/necessary flaw. We also don’t see the words sufficient/necessary used in the answer choices as a way of describing the flaw in front of us. So at this point, we are going to need and are going to have to deploy an understanding of what makes the sufficient/necessary flaw bad reasoning. In service of this point I recommend three things: Mr. Ping’s individual explanations of questions contained in the various packages available for purchase on 7Sage, Douglas Walton’s book: “Informal Logic, A Pragmatic Approach” and an analysis of the answer choices the LSAT provides us with as credited responses to these questions. Lets take a look at a problem that contains the sufficient/necessary flaw in which we have to describe the flaw by reference to precisely how the reasoning is flawed rather than the use of buzzwords.

PT 37-4-22
Here we are given:
Irish stone--->very old
Premise: we have a particular ~Irish stone
Therefore we conclude that have a particular ~very old stone
Note here that we have to know that Scotland is not Ireland in order to draw this implied negation of our A term. I think we can rely on the fact that the terms are not literally the same here, rather than geographical knowledge. This is an example of ”following the thread” of the argument.

So this is the sufficient/necessary flaw, form 1. We are given A-->B, we say we don’t have an A, therefore we conclude we don’t have a B.
Here we are going to see two hurdles that the test writers throw our way: referential phrasing and what I call the fourth wall. Now, if you have read this far, bear with me for a moment! I promise this will be of value to you.

Quite often with sufficient/necessary flaws, our author is going to take the actual conditional statement that we translated correctly, reverse it and then run some sort of operation on that reversed (flawed) interpretation of the conditional statement.
So we are told if Irish stone then very old. This is the correct interaptation.
Our author takes that conditional statement and reads it as:
If very old then Irish stone
Our author then takes that wrong conditional statement and runs the contrapostive on it.
~Irish stone
~Very old
This is what answer choice (E) is describing:
“Takes the fact that all members of a group [Irish stones] have a certain property [very old] to mean that the only thing with that property [very old] are members of that group [Irish stones].”
This answer choice says:
takes Irish stones---->very old
to mean
Very old---->Irish stones
This is where the flaw occurs, when the author has mistranslated the initial conditional statement. At that point the author runs the contrapostive on this wrong conditional statement. Now this is weird, we are essentially engaging with what the author has erroneously translated and ran away with.

So this “fourth wall” of analysis is where we will move to if the words sufficient/necessary are not present in the answer choices. Lets take a look at another example:
Pt 23-3-17
Here we are given:
If punishment deters then punishment justified
We say that we have evidence that punishment does not deter
Therefore we conclude that punishment is never justified
Again, take a look at answer choice (C)
The author has mistaken
“Being sufficient to justify punishment [deters] to be necessary to justify punishment.”
The author has taken:
If punishment deters then punishment justified
To mean
If punishment justified then punishment deters
The author then runs the contrapositive on that mistaken translation of the conditional statement.
Step 3 is about finding how the author might have messed up the conditional statement we were given and that we correctly translated. In the above example the author has taken
To mean
At this juncture, it doesn't matter what operation the author of the argument takes, it will still be wrong, because the statement is translated wrong.

Here is the heart of what makes a sufficient/necessary flaw bad reasoning: sufficient conditions do not operate the same way as necessary conditions. Lets look at another example to engrain this intuitively:
If something is a dog then it is a mammal
My neighbor has a cat (not dog)
Therefore my neighbor doesn’t have a mammal
Depriving a necessary condition of its sufficient element does not allow us to draw any conclusions about the necessary element [mammal]. This is covered in the core curriculum. Depriving “mammal” above of something that is sufficient to produce it doesn’t mean we don’t have a mammal in front of us. Something else could be sufficient to produce mammal: cat, gerbil etc.
Now, saying my neighbor does not have a mammal, allows us to draw the conclusion that my neighbor does not have a dog, for (as we all know) depriving a sufficient element (dog) of its necessary condition (mammal) allows us to say we don’t have the sufficient condition, because we don’t have the thing necessary for having that sufficient condition.
Operationally, sufficient and necessary conditions hold different powers for our conditional statement. On sufficient and necessary flaws, the author of the argument (usually through a mistranslation of the initial conditional statement) has granted the sufficient element the powers of the necessary element.

So, if we are given:
and the argument says we have a particular ~A
therefore we conclude we have a particular ~B
The argument might be erroneously taking A---->B to mean B---->A

Similarly, if we are given:
And the argument says we have a B
Therefore the argument says we have an A
The argument might be erroneously taking A----->B to mean B----->A

There are a finite number of ways in which the LSAT describes how this flaw has taken place and I have found it helpful to look at the flaw the way we do in step 3 as my main method of cutting through the haze of referential phrasing on many of these answer choices, because quite often the heavily referential phrasing answer choices to sufficient necessary flaws will be saying just this: that our author has taken A---->B to mean B---->A. If we know that the author has misinterpreted the conditional statement to mean something that it doesn’t, we can get a grasp on a majority of the ways in which it is described not using the words sufficient/necessary. It has been my experience, that with some back of the envelope calculations, an application of steps 1-3 will net us the form and description of 85-90% of sufficient/necessary flaw questions.

I hope this approach helps:
I will monitor this thread for additional questions and will be adding a part two in the coming weeks.

*full disclosure, I'm not sure if there is anyone else out there that approaches these questions like this. Any similarity is purely coincidental. I am also open for any correction you might see fit, we can make our approach stronger together as a community.

Edit 1: more specific elaboration in paragraph starting "There are a finite..."


  • Logic GainzLogic Gainz Alum Member
    700 karma

    Great read! The best thing about this post is the use of examples depicting each of the steps you were talking about. I liked that. That must've taken a lifetime to compile. My personal method for moving through these types of flaws is most like step 3. I tend to think in informal terms

  • Lucas CarterLucas Carter Alum Member
    2793 karma

    This is an awesome post.Thank You!

  • BamboosproutBamboosprout Alum Member
    1694 karma

    Wow, this is an amazing breakdown, especially step 3 for the harder questions. The referential language can be really distracting, especially when a question has more than one of those answer choices and you have to actually break each one down. =(
    When you say you use step 3 for cutting down the haze, does that mean you typically already pre-phrased your own answer choice in referential wording before you go to the answer choice, or are you saying that we should get use to step 3 so that we can easily interpret the referential language when reading the answer choices?

  • AngusMcGillisAngusMcGillis Legacy Member
    403 karma


  • nmmizokaminmmizokami Alum Member
    edited September 2018 128 karma

    Wow this great! Thanks!

    In case anyone is wondering, forms 1 and 2 have names:
    Form 1 - denying the sufficient/antecedent
    Form 2 - affirming the necessary/consequent

  • keets993keets993 Alum Member 🍌
    6045 karma

    I've noticed as well that the writers are inclined to use words other than sufficient and necessary, bummer for us. Particularly in the recent tests, they seem to rely on descriptions such as, 'something that guarantees/ensures the truth of the conclusion' and 'a statement whose truth is required to be true.' I think the latter one would be easier to identify as necessary because when something is required it's necessary. The former, of course, would be a sufficient condition because when you have a conditional (A ->B) and I say 'A' then that guarantees 'B'.

  • gabes900-1gabes900-1 Alum Member
    855 karma

    Thanks for this wonderful post! This definitely helped solidify my understanding of these questions and I will continue to reference back to this as I think it was excellent dive on the topic.

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