You flip AND negate both sides. So A → B becomes ¬B → ¬A. That's always valid.
The trap is when you flip WITHOUT negating — turning A → B into B → A. That's the sufficiency/necessity swap the lesson is warning about.
So for Gryffindor (G → B): "no bravery, no Gryffindor" (¬B → ¬G) is legit, but "brave, so Gryffindor" (B → G) is the mistake. Just remember: swap + negate = good, swap only = trap.
"To distract you with something unimportant so you overlook something crucial" this is basically the whole LSAT lol. I think keeping this in mind is significant, realize that 4/5 questions are there to waste your time
It was taught that when unless shows up we pick a side, then negate it and make it the sufficient and other side necessary. Can someone help me under why it's not that in this case. Very confusing lessons.
@AliMerhi Ideas: chosen for Gryffindor and exhibit bravery. Let's pick exhibit bravery. Negate it and put it as sufficient. /B --> /G. Contrapositive: G --> B. Since the stimulus says "no student is chosen for Gryffindor", I negated that too. Idk if this made sense
@AliMerhi Here we have Group 4 (necessary) and Group 3 (sufficient) indicators. What I understood is that when no and unless appear in the same sentence, using unless rule is usually less complicated.
In the argument, we have two ideas: Gryffindor (G) and bravery (B). Using the unless rule, I usually choose the idea that comes after unless, which here is bravery. we negate it and get:
/B → /G
Since Gryffindor is already negated in the sentence, the translation becomes
/B → /G
Then we take the contrapositive:
G → B
That matches what he got in the video. However, the conclusion says B → G, which confuses the sufficient and necessary conditions. The original statement only gives us G → B, so we cannot conclude B → G. That reversal is why the argument is not valid. I hope this makes sense.
i am a little confused because based on the example he gave, the word "no" is group four, negate necessary condition and when I applied the rule I got:
Gryffindor->/Brave
Since the rule says you can negate either idea and make it the necessary condition but in the example he got:
Gryffindor->Brave
and I'm confused how he got that by applying the rule because he didn't negate either idea.
If someone can explain that would be great. Thank you.
@Tmitch243 I have this question too. But he is using "unless", which is group 3, instead of the group 4 "no". So my question is when there are two groups in the same sentence, how do you choose which one to use over the other?
If you do it either way, using "no" or "unless" indicators, you should be getting the same lawgic. But what I understood from previous exercises and skill builders, using "unless" is less complicated than say "No" or "Any".
Let's say if we use No, we know the rule is to chooce any of the concepts, make it necessary condition and slap a negation on it. We choose bravery. But it is already negated by the word unless. So the two negations cancel each other and what you get is the below:
Gryffindor --> Bravery
The equivalent of this original sentence,
No student is chosen for Gryffindor unless they exhibit bravery.
would be:
No student is chosen for Gryffindor if they do not exibit bravery.
@MarisolSanchez I don't recall the exact lesson, but I remember him mentioning that in a sentence, the sufficient condition is typically the subject, while the necessary condition usually serves as the predicate.
To play for an NFL football team, you must exhibit strength, speed, and athleticism. Therefore, any athlete exhibiting strength, speed, and athleticism will receive a contract offer from an NFL team.
how come we didn’t follow group 4 indicator rules for the first sentence. It has “no” so why wouldn’t we pick either idea and negate it to make it the necessary condition?
@BrockFreeman In the conditional logic module he said if there is also "unless" in the sentence you should follow the group 3 indicator rules. So negating the sufficient condition: chosen for gryffindor--> brave or /brave --> /chosen for gryffindor
@VanillaCat I don't know if I made this specific enough the original two concepts would be "/chosen and brave." We keep the "no" as a negation since we are using group 3 indicator rules
It is true that infatuation can distract humans from their obligations. As such, LSAT takers under the spell of romantic infatuation are distracted from their studying. Over lunch, Sarah's friend group shared their frustrations over being distracted from studying for the LSAT. It must be that Sarah's friend group is under the spell of romantic infatuation.
No student is chosen for Gryffindor unless they exhibit bravery. Therefore, if a student exhibits bravery, they will be sorted to Gryffindor.
Here is how I went about translating it to Lawgic, tell me if I did it wrong and if so where?
1. No is a G4 Necessary condtion negation term and Unless is a group 3 Sufficent conditon negation term.
2. I picked the Unless term since it was a suff. condtion and followed the translation rule. Pick one of the 2 ideas in this case "exibit bravery" or "choosen for Griff." I picked "Eb" negated it and kept the Necc. the same giving me in
Lawgic: /EB -> /G
Counterpositive: G -> EB
idk if I did it right because the tutor got it as G-> EB when he translated it into lawgic instead of /EB -> /G
@Gotham You haven't done anything wrong, the counterpositive is just a restatement of the original, and in the case of "unless" it's really no different. You got the same logic out of the statement in the end, so it's fine.
@msfro He skips that first "negate/sufficient" step ("Unless" = Group 3) and deriving its contrapositive, and jumps immediately to G --> B. I did those other prelim steps as well. We're not wrong, but I think the idea is to be able to recognize these patterns and jump to the later parts of the reasoning, just as a time-saving trick.
@KSki Necessary condition is the superset. Sufficient condition is the subset.
umm a simpler way to visualize it for me was to think of the sufficient condition as a trigger, if it is completed then the necessary condition must follow. However if the necessary condition doesnt happen (aka contrapositive) then the sufficient condition didn't happen.
Ex: if i press my finger on the trigger then the gun will explode.
Sufficient Condition: Pressing finger on the trigger
Necessary Condition: Gun will explode
If the gun didnt explode then I didnt press my finger on the trigger.
But on thing to remember is that pressing my finger on the trigger isnt the only thing that will make the gun explode. There can be a myriad of things that produce that result, all we can say for 100% certainty is that if the gun didnt explode then I didnt press my finger on the trigger
113 comments
You flip AND negate both sides. So A → B becomes ¬B → ¬A. That's always valid.
The trap is when you flip WITHOUT negating — turning A → B into B → A. That's the sufficiency/necessity swap the lesson is warning about.
So for Gryffindor (G → B): "no bravery, no Gryffindor" (¬B → ¬G) is legit, but "brave, so Gryffindor" (B → G) is the mistake. Just remember: swap + negate = good, swap only = trap.
"To distract you with something unimportant so you overlook something crucial" this is basically the whole LSAT lol. I think keeping this in mind is significant, realize that 4/5 questions are there to waste your time
I THOUGHT IT WAS VALID UP UNTIL I ACTUALLY NEGATED THE WORDS UP
OHHHHHHHHHH NOW I UNDERSTAND LOL.
the lsat test makers won't get me!!! I refuse to let their psychological traps win EEEEEEEEEEEEEEEEE
this helped me so much! thank youuu
It was taught that when unless shows up we pick a side, then negate it and make it the sufficient and other side necessary. Can someone help me under why it's not that in this case. Very confusing lessons.
@AliMerhi Ideas: chosen for Gryffindor and exhibit bravery. Let's pick exhibit bravery. Negate it and put it as sufficient. /B --> /G. Contrapositive: G --> B. Since the stimulus says "no student is chosen for Gryffindor", I negated that too. Idk if this made sense
@AliMerhi Same I was like what the heck
@AliMerhi Here we have Group 4 (necessary) and Group 3 (sufficient) indicators. What I understood is that when no and unless appear in the same sentence, using unless rule is usually less complicated.
In the argument, we have two ideas: Gryffindor (G) and bravery (B). Using the unless rule, I usually choose the idea that comes after unless, which here is bravery. we negate it and get:
/B → /G
Since Gryffindor is already negated in the sentence, the translation becomes
/B → /G
Then we take the contrapositive:
G → B
That matches what he got in the video. However, the conclusion says B → G, which confuses the sufficient and necessary conditions. The original statement only gives us G → B, so we cannot conclude B → G. That reversal is why the argument is not valid. I hope this makes sense.
i am a little confused because based on the example he gave, the word "no" is group four, negate necessary condition and when I applied the rule I got:
Gryffindor->/Brave
Since the rule says you can negate either idea and make it the necessary condition but in the example he got:
Gryffindor->Brave
and I'm confused how he got that by applying the rule because he didn't negate either idea.
If someone can explain that would be great. Thank you.
@Tmitch243 I have this question too. But he is using "unless", which is group 3, instead of the group 4 "no". So my question is when there are two groups in the same sentence, how do you choose which one to use over the other?
@Tmitch243
If you do it either way, using "no" or "unless" indicators, you should be getting the same lawgic. But what I understood from previous exercises and skill builders, using "unless" is less complicated than say "No" or "Any".
Let's say if we use No, we know the rule is to chooce any of the concepts, make it necessary condition and slap a negation on it. We choose bravery. But it is already negated by the word unless. So the two negations cancel each other and what you get is the below:
Gryffindor --> Bravery
The equivalent of this original sentence,
would be:
@RennyG when you see no and unless you use the group 3 indicator and because it says no already you negate it which reverses the no
The way I got it right because I was like "THERE ARE OTHER HOUSES TO BE SORTED INTO"
@Oblivion This made me lol because same :D
Thought I was doing well, until I tried to map this out without my notes... back to square one I guess
How can we always know which is the sufficient v necessary condition without indicator words?
@MarisolSanchez I don't recall the exact lesson, but I remember him mentioning that in a sentence, the sufficient condition is typically the subject, while the necessary condition usually serves as the predicate.
Doing the lawgic mentally now. No one can stop me
@MRod this is the way
I’ve been doing this so much that I’ve started thinking of my life in sufficient and necessary conditions. Guess that means it’s working!
Example of this flaw im just gonna write down
To play for an NFL football team, you must exhibit strength, speed, and athleticism. Therefore, any athlete exhibiting strength, speed, and athleticism will receive a contract offer from an NFL team.
Am I understanding this correctly?
@ThomasKruza Yes! That is incorrectly understanding the first set of conditions. Play for NFL --> strength + speed + athleticism.
The accurate contrapositive of this is ~strength or ~speed or ~athleticism --> ~play for NFL.
how come we didn’t follow group 4 indicator rules for the first sentence. It has “no” so why wouldn’t we pick either idea and negate it to make it the necessary condition?
@BrockFreeman In the conditional logic module he said if there is also "unless" in the sentence you should follow the group 3 indicator rules. So negating the sufficient condition: chosen for gryffindor--> brave or /brave --> /chosen for gryffindor
@VanillaCat I don't know if I made this specific enough the original two concepts would be "/chosen and brave." We keep the "no" as a negation since we are using group 3 indicator rules
Confusing Sufficiency for Necessity Example
It is true that infatuation can distract humans from their obligations. As such, LSAT takers under the spell of romantic infatuation are distracted from their studying. Over lunch, Sarah's friend group shared their frustrations over being distracted from studying for the LSAT. It must be that Sarah's friend group is under the spell of romantic infatuation.
aight imma diary my way to LSAT studying yall
No applicant will get into law school unless they take the LSAT. Therefore, an applicant who takes the LSAT, will get into law school.
I shall use this example bc I took the LSAT last year and I didn't get in :( lol
@DaisyVidana A very good example lol. We can all relate
Premise: If you're in NYC, you are in the USA. (NYC -> USA)
Conclusion: Does this mean then that, if you're in the USA, you're in NYC? (i.e. USA --> NYC)
Clearly not: because being in NYC is enough for you to know (sufficient condition) that you're in the USA.
And, by the same token, being in the USA is a requirement (necessary condition) to be in NYC (i.e. you can't be in NYC without being in the USA).
To buy an older car, you must be able to drive stick. Therefore, if you can drive stick you've driven older cars.
Could someone review this example and tell me if I'm understanding it right?
@mkittrell
To buy an older car, you must be able to drive stick.
Must indicates necessary, so the argument is:
To buy an older car --> Able to drive stick.
Therefore, if you can drive stick you've driven older cars.
Therefore indicates conclusion, this is the conclusion of the argument.
If indicates sufficient. The argument is as follows:
Able to drive stick --> Have driven an older car
Overall arguement:
To buy an older car --> Able to drive stick.
Able to drive stick --> Have driven an older car
If easier to understand
A --> B
B --> C
This is invalid, but not for the reasons I think you meant it to be.
I think you should have said in the conclusion
"Therefore, if you can drive stick, you've bought an older car.
Does this make sense?
#Help
No student is chosen for Gryffindor unless they exhibit bravery. Therefore, if a student exhibits bravery, they will be sorted to Gryffindor.
Here is how I went about translating it to Lawgic, tell me if I did it wrong and if so where?
1. No is a G4 Necessary condtion negation term and Unless is a group 3 Sufficent conditon negation term.
2. I picked the Unless term since it was a suff. condtion and followed the translation rule. Pick one of the 2 ideas in this case "exibit bravery" or "choosen for Griff." I picked "Eb" negated it and kept the Necc. the same giving me in
Lawgic: /EB -> /G
Counterpositive: G -> EB
idk if I did it right because the tutor got it as G-> EB when he translated it into lawgic instead of /EB -> /G
Where am I going wrong and how do I fix it?
@Gotham You haven't done anything wrong, the counterpositive is just a restatement of the original, and in the case of "unless" it's really no different. You got the same logic out of the statement in the end, so it's fine.
@Gotham i had this same exact thought and issue but i can't seem to get to bravery > gryffindor from the vid?
@msfro He skips that first "negate/sufficient" step ("Unless" = Group 3) and deriving its contrapositive, and jumps immediately to G --> B. I did those other prelim steps as well. We're not wrong, but I think the idea is to be able to recognize these patterns and jump to the later parts of the reasoning, just as a time-saving trick.
to play piano you must be able to read musical notes. Therefore all who can read musical notes play piano --not valid
Loving the Harry Potter examples!
Gryffindor ----> bravery
Snape exhibits bravery.
"Snape is a Gryffindor" is confusing sufficiency for necessity.
What is a necessary condition?
What is a Sufficient condition?
@KSki Necessary condition is the superset. Sufficient condition is the subset.
umm a simpler way to visualize it for me was to think of the sufficient condition as a trigger, if it is completed then the necessary condition must follow. However if the necessary condition doesnt happen (aka contrapositive) then the sufficient condition didn't happen.
Ex: if i press my finger on the trigger then the gun will explode.
Sufficient Condition: Pressing finger on the trigger
Necessary Condition: Gun will explode
If the gun didnt explode then I didnt press my finger on the trigger.
But on thing to remember is that pressing my finger on the trigger isnt the only thing that will make the gun explode. There can be a myriad of things that produce that result, all we can say for 100% certainty is that if the gun didnt explode then I didnt press my finger on the trigger