MCAT® ExamCritical Analysis and Reasoning SkillsEasy
Passage: While often conflated, the concepts of validity and soundness in logical arguments are distinct. A valid argument is one where, if all its premises are true, then its conclusion must also be true. This is a property of the argument's structure. A sound argument, however, is a valid argument that also has all true premises. Thus, an argument can be valid but not sound if one or more of its premises are false. Which of the following arguments is valid but not sound?
- AAll birds can fly. A penguin is a bird. Therefore, a penguin can fly.
- BAll mammals lay eggs. A platypus is a mammal. Therefore, a platypus lays eggs.
- CAll dogs have fur. My pet is a dog. Therefore, my pet has fur.
- DAll fish live in water. A shark is a fish. Therefore, a shark lives in water.
Show answer & explanationAnswer & explanation
Correct answer: B. All mammals lay eggs. A platypus is a mammal. Therefore, a platypus lays eggs.
Option B presents an argument where the structure is valid (follows the 'All X are Y, Z is X, therefore Z is Y' form), but the first premise ('All mammals lay eggs') is false. Thus, it is valid but not sound.
Why the other options are wrong
- A. This argument is valid, but the premise 'All birds can fly' is false, making the conclusion false. This is a common misconception, but penguins do not fly. Therefore, it is valid but not sound.
- C. This argument is both valid and sound, as all its premises are true and the conclusion necessarily follows.
- D. This argument is both valid and sound, as all its premises are true and the conclusion necessarily follows.
Validity vs. Soundness
Validity refers to an argument's structure: if premises are true, conclusion must be true. Soundness means an argument is valid AND all its premises are actually true.
- Validity is about logical structure.
- Soundness requires both validity and true premises.
- An argument can be valid but not sound if premises are false.
Memory trick: Validate the structure, then sound out the truth.