Common Logical Flaws
The LSAT is remarkably unoriginal about how its arguments fail — the same dozen flaws return on every exam wearing different topics. Confusing correlation with causation, generalising from a tiny sample, restating the conclusion as its own proof: once you can name the pattern, flaw questions become recognition rather than analysis, and the abstract answer-choice language ("takes for granted that…", "confuses a necessary condition for…") stops being intimidating.
Practice this set for free — no account needed. Loads 13 flashcards into the learner.
Practice in the free learnerHow to study this set
For every flaw, memorise the pattern AND one everyday example — abstractions stick when they have a face. When you review, practise translating each flaw into the test's answer-choice phrasing: circular reasoning becomes "presupposes what it sets out to prove". That translation step is exactly what the exam charges you time for.
All 13 flashcards
What is the correlation–causation flaw?
Concluding that A causes B just because they occur together. Two things can correlate by coincidence, or both be caused by something else.
An argument says "A causes B". What two rival explanations should you always check?
Reverse causation (B causes A) and a third factor causing both. Weaken answers love both moves.
What is an ad hominem flaw?
Attacking the person making the argument instead of the argument itself — "she is biased, so her claim is false".
What is circular reasoning?
Using the conclusion (in disguise) as its own support — the answer choices call it "presupposes what it sets out to prove".
What is a hasty generalisation?
Drawing a broad conclusion from a sample that is too small or unrepresentative — "both my neighbours oppose the law, so the town opposes it".
What is equivocation?
Letting a key word shift meaning mid-argument — "law" as legislation in the premise, "law" as scientific regularity in the conclusion.
What is a false dilemma?
Treating two options as the only options — "either we ban cars or the city chokes" — when middle paths exist.
What is an appeal to inappropriate authority?
Leaning on an expert outside their expertise — a celebrated physicist endorsing a diet does not make the diet sound.
What are the part-to-whole and whole-to-part flaws?
Assuming what is true of the parts is true of the whole (composition), or what is true of the whole is true of each part (division). Light bricks do not make a light wall.
What is a straw man?
Refuting a distorted, weaker version of an opponent's position instead of the position actually stated.
What is the percentage-vs-number flaw?
Sliding between rates and absolute counts — a rising SHARE of accidents involving cyclists does not mean MORE cyclist accidents if total accidents fell.
What does "confuses a necessary condition for a sufficient one" mean?
Treating a requirement as a guarantee — "passion is necessary for success, she is passionate, so she will succeed" ignores everything else success needs.
What is the temporal (past-to-future) flaw?
Assuming the future must resemble the past — "the strategy has always worked, so it will work next year" — with no support for the projection.
What to learn next
Flaws recognised on sight? Level 4, "Conditional Logic", formalises the sufficient/necessary machinery behind the subtlest of them — and behind the hardest questions on the test.
Continue to Level 4: Conditional Logic →