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How to improve mapping out logic?

Alum Member
in General 149 karma
What lessons are the best to review for mapping out logic? Perhaps maybe a couple rule of thumbs to always go back to would be great! I use to do very well on SA questions and now I see myself getting more than half of them wrong, I know these type of questions usually play leave on mapping out the logic.
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• Free Trial Member
393 karma
I came into the LSAT with a very strong formal logic background. I have a BS in compsci, with a focus in theory (AI, discrete math, etc) so I have a somewhat unusual perspective on the test. But I think the logic is pretty simple on the LSAT. It's just the applications and the reading that is difficult.

All the LSAT logic that you need is contained in only a few rules. If you full understand these, then you should have no trouble at all (provided you understand the question/passage) Here's what you should know:

1) Implies: A->B (written with an arrow) means IF A is true THEN B is true ELSE the logical statement is itself false

The above has a logically equivalent truth table to the statement NOT A OR B. This is because a->b is a true statement any time that B is true or any time that a is not true. (consider it, A->B is only a false statement when A is true and B is not true. The same is true of NOT A OR B because both conditions (NOT A, are FALSE in this case.

Whew. That's how you have to read implies.

2) Contrapositive is tested extensively. The contrapositive is the law that says A->B <=> NOT B -> NOT A That is, if A->B is a logically TRUE statement, then NOT B -> NOT A must be a logically TRUE statement. Else you could have A TRUE and B FALSE in the former statement.

3) Transitive. This is pretty simple and also tested extensively. A->B and B->C means that A->C. You can also put together the transitive with the contrapositive to say that IF A->B AND B->C THEN NOT C -> NOT A

So, I honestly think that this covers 90% of the logic tested on the test. Not 100% sure that this is what you were asking for, but if you have this down, it should cover mapping for everything. In order to translate it do what you do anyway (identify premise, conclusion) and then translate them into A, B, C etc and draw conclusions.

Incidentally, the powerscore blog says that formal logic has been seriously de-emphasized on the LR section. I think I can vouch for that, given that I don't recall much formal logic on any of my three tests (Feb, June, Oct)
• Alum Member
149 karma
THIS WAS AMAZING! Thank you!!!!!
• Free Trial Inactive Sage Inactive ⭐
2398 karma
@josephellengar's explanation is a good one, but in general you don't have to worry too much about the truth and falsity of conditional statements for the test (in terms of truth tables). The only time this really comes up as a test item is when you're trying to disprove a conditional statement by showing it is false, which you would do by showing that A is true while B being false. Incidentally, this is how you show a counterexample to a general claim, so you could also file that information under that umbrella as well and pay no attention to the truth tables if they end up confusing you.

Good luck!
• Inactive Sage Inactive ⭐
7965 karma
@c.janson35 said:
but in general you don't have to worry too much about the truth and falsity of conditional statements for the test (in terms of truth tables).
+1
• Inactive Sage Inactive ⭐
7965 karma
@josephellengar said:
But I think the logic is pretty simple on the LSAT. It's just the applications and the reading that is difficult.
Well put!