Logical Reasoning: Deduction, Assumptions, and Inference
Logical reasoning measures whether you can draw only the conclusions the evidence supports. Most items fall into a few families: syllogisms, conditional reasoning, assumption identification, and inference from a short passage.
Concepts to understand
- Validity vs. truth: an argument can be valid (conclusion follows from premises) even if a premise is false. Judge structure, not whether you agree.
- Conditionals: "If P then Q" means P guarantees Q, but Q does not guarantee P. The valid moves are affirming the antecedent (P, therefore Q) and denying the consequent (not Q, therefore not P). The fallacies are affirming the consequent and denying the antecedent.
- Necessary vs. sufficient: a necessary condition must hold; a sufficient condition guarantees the result. Confusing them is the most common trap.
- Quantifiers: "all," "some," and "none" have precise meanings. "Some" means at least one, not most.
- Assumptions: an unstated premise the argument needs. Negate a candidate assumption; if the argument collapses, it was necessary.
Traps
- Answer choices that are true but irrelevant to the conclusion.
- Choices that overreach with words like "always," "never," or "only."
- Reversing a conditional (treating Q as proof of P).
- Attacking the speaker instead of the reasoning.
Rule of thumb
Restate the argument in your own words, then ask: What must be true for this conclusion to follow? Eliminate any choice that goes beyond the premises.
Worked example: "All analysts who passed the exam studied nightly. Rivera passed. Did Rivera study nightly?" Yes-this affirms the antecedent. But "Mendez studied nightly; did Mendez pass?" No-that affirms the consequent and is invalid.