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LGBW #18 under Rule Origin Drills

Stephanie Turaj
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We recently received the following question from a student. An instructor will respond below. Thanks!


I’m using the PowerScore books to study for the LSAT and am currently working my way through the LG workbook.

I have a question about #18 under Rule Origin Drills.

The explanation states that “unless” introduces the necessary condition, however, I also took a BluePrint prepcourse and the materials they gave out states that “unless” introduces the sufficient, but negated.

The way the question is written creates a double negative sufficient. condition for Brad using BP method (If Brad does not NOT attend), thus creating the same diagram as PowerScore presents, and leading to the same answer.

In this instance, does it not really matter which is the sufficient condition and which is the necessary?

Hoping you can offer some clarification.

Thanks so much,
Jonathan Evans
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Location: DFW, Texas

Hi, Regina,

Thanks for the great question! The PowerScore approach is to use our Unless Equation™, which is what the explanation describes. Here is a link to a blog post about how the Unless Equation works and why it is in many respects superior to the "if not" approach taught elsewhere:

Essentially, it is true that employed correctly both approaches will lead to the same pair of conditional statements (a conditional and a contrapositive). It is also true the necessary condition of a conditional statement, when negated, will be the sufficient condition of a contrapositive. For example:

    James won't buy dinner unless everyone else chips in.
    James buys dinner :arrow: Everyone chips in
    Not everyone chips in :arrow: James doesn't buy dinner

"Everyone chips in" is the necessary condition of the first conditional. "Not everyone chips in" is the sufficient condition of the contrapositive.

Does it matter which is sufficient and which is necessary? Yes it does because whether something is sufficient or necessary tells you where it appears in a conditional statement. Consider the example above.

"Everyone chips in" and "Not everyone chips in" are different statements. One is the negated form of the other. We have to pay attention to which one of these statements functions as a sufficient condition and which one of them functions as a necessary condition. Given our original sentence, "James won't buy dinner unless everyone else chips in," we use the Unless Equation to determine that "Everyone chips in" is the necessary condition. This means the arrow points towards it. Negate "James won't buy dinner" to get "James buys dinner" as the sufficient condition; the arrow points away from it.

    James buys dinner :arrow: Everyone chips in

Using the "if not" approach, you're basically engaging in the same process. You might not talk about which statement is necessary and which is sufficient, but that's what you're doing.

    "James won't buy dinner unless everyone else chips in"
    "unless" :dbl: "if not"
    "James won't buy dinner [if] not everyone else chips in"

Now "not everyone else chips in" is the sufficient condition. The arrow points away from it. "James won't buy dinner" is the necessary condition. The arrow points towards it.

As you continue to master conditional reasoning, do keep in mind which statements are necessary and which are sufficient. Do also track conditionals and their contrapositives. By being precise and ordered in your reasoning, you will both achieve a greater fluency and understanding of these topics and consequently be more confident and accurate on the LSAT.

I hope this helps!