Common Logical Reasoning Flaws (LSAT)

Vocabulary flashcards covering major LSAT logical fallacies mentioned in the lecture notes.

Confusing necessary and sufficient conditions

Mistaking a condition that guarantees an outcome for one that is required, or vice-versa.

Example: Being able to breathe is necessary to live, but it's not a sufficient condition; you also need food and water.

Assuming correlation proves causation

Concluding that X causes Y merely because X and Y are observed together.

.Example: Ice cream sales and drowning incidents both increase in summer. This doesn't mean ice cream causes drowning; both are correlated with hot weather.

Overlooking costs/benefits

Evaluating a proposal by citing only its costs or only its benefits while ignoring the other side.

Example: A new highway will reduce commute times, so we should build it, ignoring the high construction costs and environmental impact.

Confusing part vs. whole (composition/division)

Assuming that what is true of the parts must be true of the whole, or that what is true of the whole must be true of the parts.

Example: Each player on the basketball team is excellent, so the whole team must be excellent (Composition). Or: The company is profitable, so every department must be profitable (Division).

Unrepresentative samples

Drawing conclusions about a group from a sample that is not typical of that group.

.Example: Surveying only people leaving a luxury car dealership to understand the average income of the city.

Hasty generalization

Forming a broad rule or conclusion from too little evidence.

Example: I met two rude New Yorkers, so all New Yorkers must be rude.

Analogies that aren’t analogous enough

Inferring that two things are alike in one respect because they share a superficial similarity in another.

Example- inferring that two things are alike in one respect because they share a superficial similarity in another.Example: Just like a car needs an engine to run, a country needs a dictator to function efficiently.

False dichotomy

Presenting only two options as though they are the only possibilities or are mutually exclusive.

Example: Either you're with us or you're against us.

Attacking the source (ad hominem)

Criticizing the person or group making an argument instead of addressing the argument itself.

Example: You can't trust Jane's argument for cleaner energy; she's just a tree-hugger.

Percentages vs. amounts

Assuming that a higher percentage proves a larger number (or vice versa) without considering absolute quantities.

Example: Company A increased its profits by 50%, while Company B increased by 10%. Company A had profits of 100to100to150, while Company B increased from 1,000,000to1,000,000to1,100,000. So Company A gained much more money than Company B.

This flaw involves mistaking the relationship between percentage increases and actual numerical changes. A higher percentage does not necessarily indicate a greater real-world effect unless the absolute bases of those percentages are considered.

Lack of support vs. false conclusion

Assuming a claim is false because the argument for it is weak, or true because it has not been disproven.

Example: No one has proven ghosts don't exist, so they must be real. Or: Scientists haven't definitively proven cold fusion, so it must be impossible.

Relative vs. absolute

Inferring an absolute quality from a comparative statement, or a comparative fact from an absolute statement.

Example: This phone is smaller than the old one, therefore it is small (inferring absolute from comparative). Or: He is a fast runner, therefore he is faster than everyone else (inferring comparative from absolute).

Equivocation

Using the same word or phrase in two different senses within an argument without acknowledging the shift.

.Example: Feathers are light. What is light cannot be dark. Therefore, feathers cannot be dark. ('light' is used in terms of weight and luminosity).

Appealing to authority outside their expertise

Relying on an expert’s opinion in a field where that expert lacks relevant credentials.

Example: My dentist said that global warming is a hoax, so it must be true.

Circular reasoning

Restating the conclusion as a premise, the argument’s proof depends on what it is trying to prove.

example- water is wet because it is water

Confusing "is" for "ought"

Deriving a value judgment or prescription solely from descriptive or historical facts.

Example- People have always eaten meat, so it is morally acceptable to continue doing so.

Belief vs. facts

Assuming that widespread belief makes a claim true, or that truth guarantees belief.

Example: Millions of people believe in astrology, so there must be something to it.

Confusing possibility, probability, and certainty

Treating what is merely possible as probable or certain, or conflating probability with certainty.