LSAT Lesson 1

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LSAT Lesson 1
2013-08-26 11:27:03
Logical Reasoning Logic Games

Lesson 1
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  1. Components of a Logical Reasoning Question
    • 1. Stimulus
    • 2. Question Stem
    • 3. Answer Choices
  2. Types of Stimulus
    1. Argument: Conclusion and Premises ISO conclusion

    *70% of questions are arguments*

    2. Set of Facts: group of statements without conclusion

    *Must consciously decide if a question is an argument or set of facts*
  3. 3 Steps to Evaluate an Argument-Type Stimulus
    • 1. Determine premises
    • 2. Determine conclusion
    • 3. Evaluate conclusion (i.e., determine validity of conclusion)
  4. Valid Conclusion
    A statement that must be true  according to the premises.

    *80% of LSAT arguments are invalid*

    *Soundness (factual accuracy) does not affect answer choices*

  5. Sufficient and Necessary Conditions
    Sufficient Condition: what is enough to make something true

    Necessary Condition: what is required to make something true

    *A sufficient condition is by definition necessary, but not the other way around*

    Negating a necessary condition always negates the sufficient, but not the other way around*
  6. Assumption
    Unstated Premise

    *Assumptions strengthen arguments

    Type II Question: Strengthen/Assumption/Premise
  7. Structural Indicator
    Words that create relationships between other words or statements

    Arguments do not require structural indicators:

    "That movie sucked [C]. The acting was bad [P]."

    "(Since) the acting was bad [P], (it follows that the movie sucked [C]."
  8. Structural Indicators: Premises
    • *Because
    • *Since

    • -For
    • -After all
    • -It can be proven by the fact that
  9. Structural Indicators: Conclusions
    • Therefore
    • Consequently
    • Thus
    • Hence
    • So
    • It follows that
    • It can be concluded that
  10. Invalid Forms of Argumentation: Incorrect Negation
    If it's an apple, then it's a fruit.


    If it's not an apple, then it's not a fruit.

    A ---> F: WRONG
  11. Invalid Forms of Argumentation: Incorrect Reversal
    If it's an apple, then it's a fruit.


    If it's a fruit, then it's an apple.

    F ---> A: WRONG
  12. Contrapositive
    Correct form of argumentation.

    Reverse and negate.

    If it's an apple, then it's a fruit.

    A ---> F

    If it's not a fruit, then it's not an apple.

    F ---> A
  13. And/Or Rule
    To convert to the contrapositive in statements with multiple sufficient or necessary conditions, change all and's to or's and all or's to and's
  14. Breaking Down And/Or Statements
    • A ---> B and C
    • A ---> B
    • A ---> C

    • A ---> B or C 
    • A ---> B
    • A ---> C
    • A or B ---> C
    • A ---> C
    • A ---> C

    • A and B ---> C
    • A ---> B
    • A ---> C
  15. Type I Logical Reasoning Question
    Must be true/Conclusion/Inference

    Stimulus (Premises) lead to Answer choice (Conclusion).

    *Ask: Did they prove it?

    *13.7% of LR Questions*
  16. Type II Logical Reasoning Question

    Answer Choice (Premise) supports Stimulus (Conclusion)

    *27% of LR Questions*
  17. Type III Logical Reasoning Question

    Answer Choice (Statement) weakens Stimulus (Conclusion)

    *10.5% of LR Questions*
  18. If Formula
    "If" introduces the sufficient condition.

    If it's an apple, then it's a fruit.

    A ---> F

    I won't play again if he keeps hogging the ball.

    HB ---> P

    • *When, Whenever, As long as
    • *Where, Wherever
  19. All Formula
    "All" introduces the sufficient condition.

    All dogs bark.

    D ---> B

    *Each, every*

    Any at the beginning of a sentence has the same meaning.

    "Any bird has feathers" = B ---> F

    "Would any of you like cake?" NOT SAME
  20. No Formula
    Introduces sufficient condition. Negate other part of statement for the necessary condition.

    No cat barks.

    C ---> B

  21. Only If Formula
    "Only if" introduces the necessary condition. 

    "You can enter the club only if you have a membership."

    EC ---> HM

    "Only if you drive will I come to the movies."

    CM ---> YD

    *Only by itself refers to the necessary condition, but does not necessarily introduce it. 

    *Only if/when/where introduce the necessary.

    *"The only" introduces the sufficient condition.

    "The only fruits are apples."

    F ---> A

    "Only fruits are apples."

    A ---> F
  22. Not Both Formula
    One variable is the sufficent. Negate the other for the necessary.

    She cannot run and walk at the same time.

    • R ---> W
    • W ---> R

    *Implies that at least one of two variables must be absent"*

    *Both variables can be absent*

  23. Unless Formula
    "Unless" introduces the necessary condition. Negate the other part for the sufficient."

    We will lose unless we play like a team.

    L ---> PT

    Unless we get air support, this mission will fail.

    F ---> GAS

    *"Not...unless" = "Only if"*

    Only if we get air support can the mission succeed.

    MS ---> GAS

    The mission cannot succeed unless we get air support.

    MS ---> GAS
  24. Either/Or Rule
    Negate one part for the sufficient. Other part is necessary.

    Either she eats, or she dies.

    • E ---> D
    • D ---> E

    *Implies at least one variable must be present.*

    *Can have both.*