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 LSAT question called Must be True.

CHALLENGE: Identify Inferences

The LSAT 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).

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.

Next LSAT: January 26

Most of the formal logic LSAT questions are in the Logic Games section. We’ll return to this topic when we get to our Logic Games course. For now, we’ll move on to conclusions in argument structure.

Next LSAT: January 26