LSAT (Law School Admission Test) Logical Reasoning: Argument Fundamentals Syllabus

What is a defender assumption?

An assumption that protects the argument by ruling out a potential objection or alternative explanation. It does not add a bridge; it 'defends' the conclusion against something that would weaken it.

How do supplier and defender assumptions typically differ in wording?

Supplier assumptions tend to be positive bridges connecting concepts. Defender assumptions are often phrased negatively—e.g., 'X is not the case,' 'there is no other cause'—closing off attacks on the argument.

In everyday/LSAT arguments, what is the assumption underlying an argument that treats a survey or poll result as fact?

That the respondents answered honestly and accurately, and that the sample is representative of the larger group the conclusion is about.

What common assumption underlies an argument that recommends an action because of its benefits?

That the action will not produce countervailing harms or costs that outweigh the benefits, and that no better alternative achieves the same goal.

What is the correlation-to-causation flaw?

Concluding that one thing causes another merely because the two are correlated (occur together), without ruling out coincidence, reverse causation, or a third common cause.

List the three main alternatives that a correlation-causation argument fails to rule out.

(1) Reverse causation (B causes A instead of A causes B), (2) a third factor causing both, and (3) coincidence/chance correlation.

What is the post hoc ergo propter hoc fallacy?

'After this, therefore because of this'—concluding that because event B followed event A in time, A must have caused B. Mere temporal sequence is treated as proof of causation.

How can a causal argument be weakened on the LSAT?

Provide an alternative cause, show the correlation could be reversed, present a counterexample where the cause occurs without the effect (or vice versa), or show the correlation is coincidental.

In conditional logic, what is the fallacy of denying the antecedent?

From 'If A then B' and 'not A,' wrongly concluding 'not B.' The failure of the sufficient condition does not establish that B is absent—B could still occur for other reasons.

In conditional logic, what is the fallacy of affirming the consequent?

From 'If A then B' and 'B,' wrongly concluding 'A.' B being true does not prove A, since other things besides A could lead to B.

What is the Mistaken Reversal (illegal conversion) of a conditional?

Treating 'If A then B' as if it also means 'If B then A.' The converse is not logically equivalent to the original statement; only the contrapositive is.

What is the Mistaken Negation of a conditional?

Treating 'If A then B' as if it means 'If not A then not B.' Negating both terms without reversing them is invalid; only the contrapositive (reverse AND negate) is valid.

What is a sampling flaw (unrepresentative sample) on the LSAT?

Drawing a conclusion about a whole population from a sample that is too small, biased, or self-selected, and therefore not representative of the group the conclusion describes.

What is selection bias / self-selection in a sample?

When the people or items in a sample are chosen (or choose themselves) in a way correlated with the trait being studied, so the sample systematically differs from the population—e.g., only volunteers or only customers who complain respond.

What three conditions make a sample reliable for a generalization?

It must be (1) large enough, (2) randomly/representatively drawn, and (3) free of bias in how members were selected, so it mirrors the target population.

What is a flawed analogy (false analogy) argument?

An argument that assumes two things alike in some respects are alike in the relevant respect at issue, when the differences between them undermine the comparison.

How is a comparison/analogy argument typically weakened?

By pointing out a relevant dissimilarity between the two compared cases that breaks the inference, showing the cases differ on the very feature the conclusion depends on.

What is an ad hominem fallacy?

Attacking the person making an argument—their character, motives, or circumstances—rather than addressing the argument's actual reasoning or evidence.

On the LSAT, how is a 'source attack' (genetic fallacy) flaw described?

Rejecting a claim or argument based on its source (who said it or where it came from) instead of evaluating the claim's merits—e.g., dismissing a study because of who funded it without addressing its content.

What is the tu quoque ('you too' / hypocrisy) variant of ad hominem?

Dismissing someone's argument by charging that they fail to follow their own advice or are hypocritical, rather than addressing whether the argument itself is sound.

What is circular reasoning (begging the question)?

An argument in which the conclusion is assumed in (or restated by) the premises, so the 'support' merely repeats the claim it is supposed to prove rather than providing independent evidence.

How do you recognize circular reasoning on the LSAT?

The premise and conclusion say essentially the same thing in different words; the argument provides no evidence independent of the conclusion. The reasoning assumes what it sets out to prove.

What is the part-to-whole (composition) fallacy?

Assuming that what is true of the parts (individual members) must be true of the whole (the group)—e.g., each player is excellent, therefore the team is excellent.

What is the whole-to-part (division) fallacy?

Assuming that what is true of the whole must be true of its parts—e.g., the company is profitable, therefore every department is profitable.

In LSAT notation, how is the conditional 'If A then B' written, and what are A and B called?

Written A → B. A is the sufficient condition (triggers B) and B is the necessary condition (required by A). A guarantees B; B is needed for A.

State the rule for forming the contrapositive of A → B.

Reverse and negate both terms: the contrapositive is not-B → not-A (¬B → ¬A). It is logically equivalent to the original statement.

How do you diagram an 'only if' statement, e.g., 'A only if B'?

'Only if' introduces the necessary condition: 'A only if B' becomes A → B. The term after 'only if' is necessary, so it goes on the right (arrow points to it).

How do you diagram 'No A are B' and 'All A are B' in conditional notation?

'No A are B' becomes A → not-B (and contrapositive B → not-A). 'All A are B' becomes A → B.

In LSAT logic, what does 'unless' introduce, and how do you diagram 'A unless B'?

'Unless' (like 'except,' 'until,' 'without') introduces a necessary condition. 'A unless B' is diagrammed by negating the other term as sufficient: not-B → A (equivalently ¬A → B).

Which words signal sufficient conditions vs. necessary conditions in conditional reasoning?

Sufficient indicators: 'if,' 'when,' 'all,' 'every,' 'any,' 'people who.' Necessary indicators: 'then,' 'only,' 'only if,' 'must,' 'requires,' 'unless,' 'except,' 'until.'

How do you chain conditional statements, and what can you validly infer from A → B and B → C?

Link statements sharing a common term: from A → B and B → C, validly infer A → C (transitivity). The contrapositive chain is ¬C → ¬B → ¬A.

State modus ponens and confirm it is valid.

From 'A → B' and 'A,' validly conclude 'B.' Affirming the sufficient condition guarantees the necessary condition. This is a valid inference pattern.

State modus tollens and confirm it is valid.

From 'A → B' and 'not B,' validly conclude 'not A.' Denying the necessary condition forces denial of the sufficient condition (it is the contrapositive). This is valid.