Common Logical Flaws
The LSAT is remarkably unoriginal about how its arguments fail โ 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 thatโฆ", "confuses a necessary condition forโฆ") stops being intimidating.
Practice this set for free โ no account needed. Loads 13 flashcards into the learner. The learner asks it as shown here: phrase first, meaning second.
Practice in the free learnerHow to study this set
For every flaw, memorise the pattern AND one everyday example โ 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
Swap question and answerConcluding that A causes B just because they occur together. Two things can correlate by coincidence, or both be caused by something else.
What is the correlationโcausation flaw?
Reverse causation (B causes A) and a third factor causing both. Weaken answers love both moves.
An argument says "A causes B". What two rival explanations should you always check?
Attacking the person making the argument instead of the argument itself โ "she is biased, so her claim is false".
What is an ad hominem flaw?
Using the conclusion (in disguise) as its own support โ the answer choices call it "presupposes what it sets out to prove".
What is circular reasoning?
Drawing a broad conclusion from a sample that is too small or unrepresentative โ "both my neighbours oppose the law, so the town opposes it".
What is a hasty generalisation?
Letting a key word shift meaning mid-argument โ "law" as legislation in the premise, "law" as scientific regularity in the conclusion.
What is equivocation?
Treating two options as the only options โ "either we ban cars or the city chokes" โ when middle paths exist.
What is a false dilemma?
Leaning on an expert outside their expertise โ a celebrated physicist endorsing a diet does not make the diet sound.
What is an appeal to inappropriate authority?
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 are the part-to-whole and whole-to-part flaws?
Refuting a distorted, weaker version of an opponent's position instead of the position actually stated.
What is a straw man?
Sliding between rates and absolute counts โ a rising SHARE of accidents involving cyclists does not mean MORE cyclist accidents if total accidents fell.
What is the percentage-vs-number flaw?
Treating a requirement as a guarantee โ "passion is necessary for success, she is passionate, so she will succeed" ignores everything else success needs.
What does "confuses a necessary condition for a sufficient one" mean?
Assuming the future must resemble the past โ "the strategy has always worked, so it will work next year" โ with no support for the projection.
What is the temporal (past-to-future) flaw?
What to learn next
Flaws recognised on sight? Level 4, "Conditional Logic", formalises the sufficient/necessary machinery behind the subtlest of them โ and behind the hardest questions on the test.
Continue to Level 4: Conditional Logic โ