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Logical Reasoning: The Assumption Family

The missing block: what an argument needs to be true, what would make it certain, and why confusing those two is the costliest error in the section.

9 topics · 36 questions

Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.

What an Unstated Assumption Is

  1. How it worksHow does an unstated assumption show up as the missing block holding a conclusion off the ground?
  2. Worked exampleFind the unstated assumption in 'Jo studies every single night, so Jo will pass the bar exam.'
  3. Common mistakeWhy isn't an assumption just anything the arguer probably happens to believe?
  4. Going furtherWhat happens to the whole structure the moment an argument's assumption turns out to be false?
Your turneasy1 of 3
Naturalist: The pond at Alder Farm has held breeding frogs in each of the past 12 years, and this spring the count reached 65 egg masses. The pond will hold breeding frogs again next spring. The naturalist's argument depends on assuming which one of the following?

Necessary Assumptions

  1. How it worksHow does a necessary assumption behave like a load-bearing wall the argument cannot stand without?
  2. Worked exampleFind what must be true for 'The lamp will light because the bulb is brand new' to hold up.
  3. Going furtherWhat makes weak, hedged wording so common in correct necessary-assumption answers?
  4. Common mistakeWhy isn't the strongest-sounding choice usually the right necessary assumption?
Your turnmedium1 of 3
Dietitian: Residents of Kalso province live on average 7 years longer than residents of the province next to it, and they eat 61 percent more fish. Higher fish consumption is what accounts for the difference in lifespan. The dietitian's argument requires which one of the following assumptions?

The Negation Test

  1. How it worksWalk me through negating an answer choice and watching whether the conclusion collapses.
  2. Worked exampleShow me the negation test run on 'The survey respondents answered honestly' for an argument built on survey results.
  3. Common mistakeWhy isn't the negation of 'all bears hibernate' the claim that no bears hibernate?
  4. Worked exampleWalk me through negating 'Most voters read the bill' and seeing what becomes of a conclusion about voter intent.
  5. Going furtherWhat goes wrong when you run the negation test on a sufficient-assumption question?
Your turnmedium1 of 3
Agronomist: Fields treated with the new soil bacterium yielded 23 percent more wheat than untreated fields across trials at 18 sites. Farmers who adopt the treatment will raise their yields. Which one of the following, when negated, most weakens the agronomist's argument?

Sufficient Assumptions

  1. How it worksHow does a sufficient assumption bridge a premise term to a conclusion term in one single arrow?
  2. Worked exampleFind the claim that makes 'Ana is a pianist, so Ana is disciplined' completely airtight.
  3. How it worksWalk me through matching the leftover term in the premises to the leftover term in the conclusion.
  4. Common mistakeWhy isn't a sufficient assumption required to be something the arguer actually believes?
Your turnmedium1 of 3
Sommelier: This vintage took the gold medal at the 2021 regional fair. It will therefore sell for 29 percent above its release price within a year. Which one of the following, if assumed, allows the sommelier's conclusion to be properly drawn?

Necessary Versus Sufficient Assumption

  1. How it worksHow does a necessary assumption differ from a sufficient one when both of them fill the same gap?
  2. Worked exampleShow me the necessary and the sufficient assumption for 'Max jogs daily, so Max is healthy' side by side.
  3. How it worksWalk me through reading a question stem to decide which of the two an item is really asking for.
  4. Common mistakeWhy doesn't a claim that would guarantee a conclusion also have to be required by it?
  5. Ties togetherHow does this split mirror the same necessary-versus-sufficient split inside a conditional statement?
Your turnhard1 of 3
Critic: Every film that wins the Delmar Prize is later shown at the national archive. 'Harrow Lane' won the Delmar Prize in 2011, so it will be studied in film schools within 9 years. Which one of the following, if assumed, is sufficient to make the critic's conclusion follow?

The Term That Appears Only in the Conclusion

  1. How it worksHow do you circle the brand-new concept in 'The bridge is old, so the bridge is unsafe' and then build across?
  2. Worked exampleFind the unlinked term in 'This wine won a gold medal, so it will sell well' and connect it up.
  3. Going furtherWhat changes when the premises and the conclusion each carry a term the other one lacks?
Your turnmedium1 of 3
Engineer: The Calder bridge was built in 1958 and has carried 29 thousand vehicles a day since the ring road opened. The bridge is unsafe. Which one of the following identifies the term in the conclusion that the premises leave unconnected?

Assumptions About What a Survey Measures

  1. How it worksHow does the distance between reported behavior and actual behavior become an assumption you can attack?
  2. Worked exampleFind the assumption in '80 percent of diners said they want salads, so the restaurant should add salads.'
  3. Common mistakeWhy isn't a large sample enough to make a self-reported survey trustworthy?
  4. Going furtherWhat happens to the argument when the survey question was itself worded to suggest an answer?
Your turnmedium1 of 3
Restaurant consultant: In our survey, 81 percent of the 572 diners polled said they want more salads on the menu. The restaurant should add salads, since they will sell. The argument depends on assuming which one of the following?

Assumptions About What a Comparison Holds Constant

  1. How it worksHow do you find the variable an arguer forgot to hold steady while comparing two groups?
  2. Worked exampleFind the assumption in 'Town A reports more crime than Town B, so Town A is more dangerous.'
  3. Common mistakeWhy isn't a comparison drawn across two different years automatically comparing like with like?
Your turnmedium1 of 3
Town A recorded 1387 reported crimes last year and Town B recorded 560, though the two have populations within a few hundred of each other. Town A is therefore the more dangerous place to live. The argument depends on assuming which one of the following?

Assumptions Behind a Plan or Proposal

  1. How it worksHow does a plan argument assume no side effect cancels out the benefit it is aiming for?
  2. Worked exampleFind the assumption in 'To cut traffic the city will double bus service, so congestion will drop.'
  3. Going furtherWhat changes about a plan's assumptions when the goal is stated but the method stays vague?
  4. Common mistakeWhy isn't showing that a plan carries a cost the same as showing the plan will fail?
Your turneasy1 of 3
City planner: Congestion downtown has risen for 8 straight years. We will double bus service on the 11 busiest routes, and congestion will fall. The planner's argument depends on assuming which one of the following?

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