Logic Refined

Necessary-assumption practice

What does the argument need?

A necessary assumption is a condition the argument depends on. It need not establish the conclusion by itself.

Keep “needed” separate from “enough.”

A statement can provide powerful support without being required. Conversely, a modest condition can be indispensable. Ask what must hold for the author’s particular route from evidence to conclusion to work.

Track the connection the author relies on.

Look for a step that has not been defended: a comparison being treated as relevant, a measure being treated as informative, or an alternative being set aside. The correct answer should address a dependency in that reasoning rather than add a new argument.

Use negation as a check, with care.

Reverse the candidate statement accurately and ask what happens to the argument. A useful negation challenges the assumed connection. Do not replace “some” with “none” carelessly or transform a limited claim into an extreme one that the answer never made.

Read the scope of the sample closely

In the translation sample, keep the procedure, the judgment it is supposed to support and the limits of that judgment distinct. Compare each answer against the author’s actual inference. The explanations below show why a necessary condition differs from a tempting extra requirement.

A complete original example

Read it. Make a choice.
Then compare the reasoning.

This is the unchanged necessary assumption example from the public sample set. These are original practice questions, not official LSAT questions.

Editor: Our Dutch edition of an English card-game rulebook needs checking, but our Dutch-speaking expert can inspect only a few passages. Different wordings can prescribe the same moves, so checking for matching words would be unhelpful. We will have a translator who has not seen the English original render the Dutch text back into English. Our editors can recognize any difference in rules between a returned passage and the original. They will list every Dutch passage whose return translation prescribes different rules, and no others. A colleague objects that a second translation can introduce its own mistakes. Nevertheless, a passage selected at random from the list will be more likely to misstate the original rules than one selected at random from the entire Dutch text.

Which one of the following is an assumption on which the editor’s argument depends?

  1. The return translator would preserve the Dutch meaning of at least one passage that misstated the English rules.
  2. A Dutch passage that states the original rules correctly would not acquire different rules in the return translation.
  3. Not every Dutch passage that misstates the rules would be translated back into a version matching the original rules.
  4. The return translation would depart from the Dutch rules in fewer passages than the Dutch translation departs from the English rules.
  5. Any returned passage that matches the original rules would have come from a Dutch passage that also states them correctly.
Read the answer and all five explanations

Correct answer: C.

If every faulty Dutch passage returned with the original rules restored, none would enter the list. The list could then contain only correct Dutch passages, or be empty, defeating the claimed improvement in finding actual mistakes.

  1. A. The return translator would preserve the Dutch meaning of at least one passage that misstated the English rules.

    A faulty Dutch rule could become a different faulty rule on its return to English. That passage would still enter the review list. Faithfully preserving the Dutch mistake is therefore not required for the screening to help.

  2. B. A Dutch passage that states the original rules correctly would not acquire different rules in the return translation.

    This rules out false alarms, but the editor predicts an improved chance of finding an error, not a flawless list. Some correct Dutch passages could be listed while the list still contains a higher proportion of faulty ones.

  3. C. Not every Dutch passage that misstates the rules would be translated back into a version matching the original rules.

    If every faulty Dutch passage returned with the original rules restored, none would enter the list. The list could then contain only correct Dutch passages, or be empty, defeating the claimed improvement in finding actual mistakes.

  4. D. The return translation would depart from the Dutch rules in fewer passages than the Dutch translation departs from the English rules.

    The second translator need not make fewer departures from the source. A new mistake can replace an earlier mistake while leaving a genuinely faulty Dutch passage detectable. Even if the return translation creates more false alarms than the Dutch text has errors, the list can still hold a higher share of faulty passages than the whole text does.

  5. E. Any returned passage that matches the original rules would have come from a Dutch passage that also states them correctly.

    This would rule out every hidden Dutch error, but the proposed screening need not catch them all. One mistaken passage could return to the original rules and escape detection while other mistakes make the list more useful.

See it for yourself

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Read the arguments. Test your answer. Decide whether the explanations help you.

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