LSAT Logic Games | Conditional Logic | Only if, Unless

Learn how to handle the conditional indicators "only if" and "unless" in LSAT logic games.
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[@zenaidarojas5689]

To be honest, for the "Unless" part, what I usually do is just automatically cancel out "not" and read it as is. And if there was no "not", then I would add one. That's a formula I essentially use. I also appreciate that you said "unless" introduces a necessary condition. I feel like that was super helpful!

[@r.p.8906]

In short, "Only if" is a straight arrow ---->. Unless is: "If Not" and goes on the left. Crystal clear :)

[@ajbronson516]

If “if and only if” signals a biconditional statement, doesn’t that mean it actually is true both ways (positive, converse, inverse, and contrapositive?)

[@LuminateLSAT]

Yes, that's exactly right.

[@theresan8880]

I’m so confused with the “only if” example. “Rakesh is ranked fifth, only if Eustace is ranked first.” So to me that means the same thing as saying……If Eustace is ranked first, then we know Rakesh is ranked fifth. Why is that not so?? How are “if” and “only if” different? I get the umbrella and US president example. What doesn’t make sense is the diagram on this example. If I read that rule, I would diagram it - if E1-->(then) R5. In the video while you diagram you actually say, “If Rakesh is 5th then Eustice is 1st.” That seems like the opposite of the rule.

[@LuminateLSAT]

That's because your understanding of "only if" is a bit flawed (as is most people's!). In real life, we often use "only if" interchangeably with "if". But they actually do mean different things, as I describe in the video. Rakesh is 5th only if Eustace is 1st. That really does mean IF R is 5, then E must be 1. But it does *not* mean that if E is 1, R must be 5th. R is free to go anywhere even if E is in 1. If you understand the umbrella and US president example, then see if you can import that understanding into any other use of "only if". It always means the same thing. You can win the competition only if you try your best. If you win the competition -> You tried your best But that does not mean that if you try your best, you'll win the competition. Trying your best is just 1 requirement for winning; there could be many other requirements that you also must satisfy in order to win.

[@jelenajovanovic1286]

I still don't understand chow can " R is ranked 5th only if E is ranked first." Produce the same diagram as "Only if E is first can R be 5th."

[@LuminateLSAT]

It has to do with how we can change the order of the parts of the sentence without changing the meaning. Consider the following: 1. If it's raining, I'll bring an umbrella. 2. I'll bring an umbrella if it's raining. You probably wouldn't think of those two sentences as expressing different ideas, right? If you can see that they mean the same thing, then think of the "only if" example as similar. 3. You can become president of the US only if you are a US citizen. 4. Only if you are a US citizen can you become president of the US. Those are both saying the exact same thing - being a US citizen is necessary in order to be the president. The order of the sentence doesn't change the logical structure being expressed.

[@iStorm-my5fp]

I don't understand how a2 ===>e1??? Also why eat veg ===> dessert?

[@r.p.8906]

@@iStorm-my5fp if no veggies---> no dessert. CP: dessert--->veggies.

[@r.p.8906]

@@iStorm-my5fp If Not ants popular---> Not elephants popular. CP: Elephants are popular----> Ants are popular. Real simple. All you need to remember is that unless is if not and simply goes to the left as is. No need to negate anything...

[@sanish9d]

This is really insightful….so that means this link uses unless incorrectly? In this kzbin.info/www/bejne/ooPTlamPl79_isk and also kzbin.info/www/bejne/eYeZaaRvp7V1iqs Also it would be great if you could let me know why interpreting ‘if B then not A’ from ‘A unless B’ is not logically correct as it sounds valid For example ‘I will go to school unless it rains’ doesn’t it imply that the exception of me going to school is when it rains I.e if it rains then I will not go to school This is pretty confusing…I would appreciate it if you could help

[@LuminateLSAT]

This is actually a really interesting issue that is beyond what's important for the LSAT. (See an example paper for more on this: web.mit.edu/fintel/fintel-1991-unless.pdf) Practically speaking, I do think "unless" can take on slightly different meanings and, in everyday contexts, is not always reducible to the "if not..." formula, depending on phrasing. However, on the LSAT, I cannot recall every going wrong with applying the formulas I describe in the video. So if I saw the statement "I will go to school unless it rains" as "If not rain -> Go to school". Does this also mean "If rain -> NOT go to school"? On the LSAT, I would not be so quick to accept that, even though I would accept that meaning in regular everyday conversation. Instead, I think I'd say "If it rains, then there's no longer a guarantee that you'll go to school. However, perhaps you could still go to school." I think what I'm doing is inserting a "definitely" to the word "will": "I will definitely go to school unless it rains". That construction makes it a little bit easier to interpret, I think. If it rains...you won't necessarily "definitely" go to school. But it's not guaranteeing that you won't go to school. Does this make sense?

[@beefchalupa]

@sanish9d interesting discussion here, I'll help chime in. The real confusion here is that it depends on some subtext in the exact wording you use. Firstly, unless absolutely does NOT mean you can assume "if B then not A". The reason it works in some cases, such as "I will go to school unless it rains" is because the wording of "I WILL go to school unless" essentially implies that "I will ALWAYS go to school unless it rains." Thus, if it doesn't rain, then you must be going to school. However, if we said something like "I will not go to school unless it rains" then we cannot assume "if B then not A" because if it rains, we dont logically have to go to school. We said we won't go to school unless it rains, but if it rains we could still choose to not go to school. All we said is that if we go to school it must mean it's raining. I think the insertion of the word "not" actually changes things. For example: "I am happy unless I am sick." This does seem to logically imply that if I am sick, I am not happy. I reason that this is because "I am happy" sets happiness as the default state of being, such that you are always happy unless the exception occurs. But take "I am not happy unless I am sick." This doesn't imply the same thing. You might intuit that I could be sick and still be unhappy. All I've said is that happiness requires sickness. This is because "I am not happy unless" doesn't imply that not happy is the default state of being. If I change it simply to "I am unhappy unless I am sick" strangely enough it now does imply that if I am sick I cannot be unhappy.