Weaken Questions in LR Bible (S-N) Confusion

[Mark Yerrid]

Sufficient Vs. Necessary Confusion in Weaken Questions



On page 115 of the Logical Reasoning Bible, it states that a Sufficient Condition indicates that a Necessary Conditition "MUST" occur.

S N.

The contrapositive is : If not N then not S. However, we aslo know that just because the N condition occurs, this does not mean that the S condition has to occur; it can, but does not have to.

This is how I have been studying and understanding these terms for months. I have had these locked down and doing well, but I was on page 185 in the LR Bible, reading Weaken questions and it states on pg 184..."WRP AR.

Then it states underneath that connection of WRP AR; that "As you read the question stem, you should have immediately prepharased an answer that would allow the SUFFICIENT condition to occur WITHOUT the Necessay condition .

Further, on page 182, it states "to weaken a conditional conclusion, attack the necessary condition by showing that the necessary condition DOES NOT NEED TO OCCUR in order for the Sufficient to occur.

Now I am totally confused, because if we were to use a simple example such as "In order to drive, one must have a license or D L. The contrapos. is If NO license, one cannot drive. Since the Necessary cannot happen (no license), then you or I, cannot drive. (We could, in the real world, but we might be arrested if we did not have a license).

As far as 'weaking' I am getting tripped up on the Conditional Weakening Reasoning because it seems to contradict the "rules" of conditional reasoning overall.

How can you have the SUFFICENT occur without have the NECESSARY (i.e. page 182)? Or have I totally misunderstood the bible and it's explanation? It just appears contradicting in terms of all other conditional reasoning rules.

Thank you. Mark

[Dave Killoran]

Hi Mark,

Stop for a moment and consider what has changed given where you are in the book. You are now in the Weaken chapter, and thus what you are doing is predicated upon trying to undermine the given nature of the statements. This changes the game entirely!

In a nutshell, to weaken a conditional statement, you do indeed try to "break the rules." For example, let's say an author claims that, "To get to the roof, you must take the stairs." We could diagram that: Roof Stairs.

How could we weaken that initial statement? Well, we'd want to show that the author was wrong, and that this conditional statement isn't representative of what can occur. There are many ways to do that, but they all revolve around showing that the conditional relationship is not valid. That means you need to show that the sufficient condition can occur without the necessary condition occurring as well (note that these "break the relationship" guidelines refer to answer choices). That occurrence breaks the "truth" of the initially stated conditional relationship. So, for example, if I replied back to the claim above by saying, "No, that's not true, you can get a helicopter to drop you off on the roof, or simply take the elevator," what I've effectively done is show that the conditional relationship isn't true, because I showed that the necessary condition is not, indeed, necessary.

When you first encountered Conditional Reasoning in the LRB, it was presented mostly in the Must Be True context, to show how it works when it is validly applied. Weaken questions--which is where you are now--try to subvert that reasoning and show that it is invalid. That's a game changing difference, and hence you see entirely different descriptions of how to proceed.

Please let me know if that helps. Thanks!

[Mark Yerrid]

Dave. Once again, your explaination of the issue makes complete sense. I did not know that the 'rules' for conditional reasoning could ever change. I assumed that in all questions types (12 types) that the CR rules "must" apply at all times.

Thank you for this explaination. It was very helpful.

As an aside, are there any other question-types I should be aware of whereby I may have to prove that the SC can happen w/o the NC being involed? Or, if I continue to read the entire Bible; will this issue be explained to me IF and WHEN it does occur?

Again, I assumed one could NEVER have a SC without the NC present in an answer choice. However, I see from your example that we "can" reach the roof from a source outside the "stairs" which indeed would 'weaken' the argument that "only" the 'stairs' can get us to the roof. Very interesting; very easy to prove when weaking the argument just by showing that the SC can still be present (getting to the roof) even if the NC (only by the stairs) is not there. We might climb the side of the building with ropes and "STILL" get to the roof;-).

Mark

[Dave Killoran]

Hi Mark,

Glad I could help! It's not that rules for conditional reasoning are changing; it's that the context of how the reasoning is considered changes based on the type of question (and thus activity) you are asked to perform on or with that reasoning.

Weaken questions are typically the area where you see this "S but not N" type of answer, but Flaw is closely associated with Weaken, and would be the next most likely area that occurs (literally, anywhere you'd see disagreement or logical errors is where this would occur). Here are the two closest combinations that you would see in that same area that are worth knowing:

• Conditionality and Flaw in the Reasoning question: In this type of combination, you'd typically see a speaker present a conditional relationship, and then make a reasoning error with it (such as an MN or MR). Or you could see one speaker present a conditional relationship, and then a second speaker make a mistake with that reasoning (again, likely an MR or MN).
  
  Conditionality and Point at Issue question: Here, you would see two speakers disagree about the conditional relationship. This could be one speaker stating a conditional relationship and then the second speaker disputing it (such as in our roof example).

Also, you're not done with conditional reasoning in the LRB. You'll see plenty more of it, including how to strengthen it, what assumptions are present, and so on. It's a pivotal concept, and I revisit it frequently.

Please let me know if that helps. Thanks!