Formal vs. Informal Logic

Most logical reasoning questions use soft and fuzzy informal logic (with observations, lots of unstated assumptions, etc). Formal Logic is what a computer would do: hard, logic-following rules, like what you’ve done in this chapter. Formal logic is common in the legal field.

  1. You were on the neighbor’s property. (Premise)
  2. It is against trespassing laws to be on the neighbor’s property. (Premise)
  3. You are therefore guilty of trespassing. (Conclusion)

Not much wiggle room there! That’s why the GMAT often tests formal logic with must be true or must be false questions. We are dealing with certainty.

Now that you know the basics of formal logic we can move on to the questions. Formal Logic issues are often tested by a type of GMAT question called Must be True.

CHALLENGE: Identify Inferences

The GMAT often tests formal logic through Must Be True (MBT) questions. They’re identifiable through their distinctive question stems:

  • Which of the following must be false?
  • If the statements above are true, which of the following must also be true?
  • Which of the following may be correctly inferred?
  • Which of the following inferences (inference means the same thing as “must be true” on the test) is best supported by the statement made above?
    Conclusions differ from inferences in that conclusions are the result of premises and inferences must be true if the premises are true. Don’t panic if that sounds complicated; we review this concept extensively in the following chapter.

How to solve

  1. Diagram out the statements of the argument (if you can’t get a quick grasp of it in your head).
  2. Make valid inferences from these statements (transitive property and contrapositive). Note any false inferences, if any, such as the Fallacy of the Converse and Fallacy of the Inverse.
  3. In Must be True questions you’ll find four choices that can sometimes be wrong and one that can never be wrong (that’s the correct choice!). Go through every answer choice systematically and check if it is ALWAYS true. These questions should always be tackled using PoE (process of elimination) method.  If you can find a reasonable situation when it is not always true, eliminate it. Gradually eliminate answer choices until you have one left.

B. Cannot Be True

The opposite of Must be True is Cannot be true. In Cannot be True questions you will find four choices that are correct and one choice that can never be correct. So, you just run through the choices with Process of Elimination to knock the four workable ones and select the one answer that can never work.

This video nicely summarizes the lessons in this chapter.

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