PT40.S1.14 - Suff/Nec and Arg from Ignorance Flaw

Slow is Fast

I was reviewing a question I have seen many, many... many times over. PT40.S1.14 for anyone interested.

Today as I was analyzing the stimulus, my pre-phrase for the flaw was that it was the classic argument from ignorance fallacy (an assertion that a claim is either true or false because of a lack of evidence to the contrary; aka "you didn't disprove/prove this, so you're wrong/I'm right"). This is an informal logical fallacy.

I went to the answer choices and liked C the best. It reads, "The argument takes for granted that if the truth of one claim implies the truth of a second claim, then the falsity of the first claim proves that falsity of the second claim." I thought this wasn't written exactly as I had in mind, but it described the flaw in formal logic. The even more classic sufficient/necessary mix-up/incorrect negation/incorrect reversal (all of these describe the same formal error). Oldest trick in the book. And then the lightbulb in my brain went off.

Answer choice C is saying the argument assumed that if A --> B, then /A proves /B. Isn't this very similar to that argument from ignorance fallacy? You didn't prove A to me (you presented /A), so your claim about B is wrong (/B).

Basically, the sufficient/necessary flaw is the formal logic mirror to the informal argument from ignorance fallacy. The former requires a tight argument structure, while the latter is more flexible with looser language. In essence, however, they are the same error. Until now, I had kept formal and informal logic as separate and unrelated, but they actually overlap. And this is exactly how the test writers will disguise/describe the same flaws in different ways-- not just in the stimuli but in answer choices as well. 🤯

Thinking about the contrapositives is also interesting. The formal logic error of an incorrect reversal is pretty obvious: B --> A. But if we consider the "expressed contrapositive" (in quotes because I just made up this term and don't think there is such a thing in informal logic), then it's something like, "Your conclusion about B is not wrong, so you proved A." Your conclusion exists, so your premises are proven? Lol, wtf? No wonder these are logical fallacies.

Anyway, maybe I'm slow and everyone already knew this, lol. It was an a-ha moment for me so I wanted to write it out. If I have made a mistake, someone please correct me. If you have other realizations about the flaws/fallacies/question types, please share as well!

Admin Note: https://7sage.com/lsat_explanations/lsat-40-section-1-question-14/

Burden.of.Floof

This is an interesting observation! I feel like I need to sit with this and think about it some more, but when I did this question I observed two flaws in the stimulus:

1) Appeal to ignorance
2) Mistaken negation

It's really smart to think of them as two sides of the same coin. Like you said, the formal flaw needs that formal language (and you have the conditional statement in the stimulus on this one), but, it can help to bring informal reasoning into focus by trying to think about it in more formal terms. I know for me, appeal to ignorance always gets me... I can recognize it when I see it, but the reasoning has always felt a bit convoluted to me. For that reason I have struggled to find the abstract description of that in the answer choices. Thinking of it in these terms could definitely make that language clearer when I see it!

Slow is Fast

@"Burden.of.Floof" ooh, I only saw one of those two flaws so you're ahead of the game. 😊 But I think if you make up an example of an appeal to ignorance ("I don't believe other people's claim about A, so they're wrong"), the same mistaken negation of /A --> /B might be a little clearer? It seems to me if we start to recognize these two as the same/similar flaws, we won't need to keep two different ideas in mind-- just one! Haha, I'm all about efficient thought processes.