"If it rains, the game is postponed. It rained." What follows? The game was postponed — that reasoning is airtight. But flip it: "The game was postponed; therefore it rained." That one feels right yet is logically broken — the game could've been postponed for many reasons. Logical reasoning questions live in exactly this gap between what's guaranteed and what merely sounds plausible. Learning to tell them apart is a genuine superpower, protecting you from faulty arguments for life. This chapter builds that discipline.
Logical-reasoning questions present a short argument or set of statements and ask what can (or cannot) be validly concluded — or which statement is an assumption, or what strengthens or weakens the argument. The core discipline is distinguishing what the statements prove from what is merely consistent with them or plausible in the real world.
On the Civil Service Exam, logical reasoning sits in Analytical Reasoning, closely related to Syllogisms and Statement & Conclusion. It rewards careful, structural thinking over gut feeling — and the same skill sharpens your judgment of arguments everywhere.
The difficulty is intermediate to advanced. The keys are to separate "must be true" from "could be true," master the valid conditional patterns (modus ponens and modus tollens) and their look-alike fallacies, tell an assumption from a conclusion, and stay strictly inside the argument's own logic. This lesson builds each.
After completing this lesson you will be able to:
You should be comfortable with:
A quick refresher on the central distinction: a conclusion "must be true" only if it holds in every situation where the statements are true. A conclusion that "could be true" is merely consistent with the statements — possible, but not forced. The exam almost always wants "must be true." So for every proposed conclusion, ask: Can I imagine the statements all true while this conclusion is false? If yes, it does not follow. Keep that test ready.
Sound reasoning protects and empowers you:
The exam tests logical reasoning because valid inference is the backbone of clear, defensible thinking.
Let's build the tools for evaluating arguments rigorously.
The correct answer to a strict logic question is something the statements guarantee — not something merely consistent with them or likely in reality.
"All engineers passed the exam. Maria passed the exam." Can we conclude Maria is an engineer? No. The statement guarantees that engineers passed, not that only engineers passed. Maria might have passed for another reason. Her passing is consistent with being an engineer, but not forced.
Analogy: "Consistent with" is like a suspect who could have been at the scene; "must be true" is a suspect caught on camera committing the act. Courts (and logic) demand proof, not mere possibility. The exam wants the camera footage.
A conditional "If A, then B" supports exactly two valid inferences:
"If it rains, the game is postponed. It rained." → The game was postponed. ✓
"If it rains, the game is postponed. The game was NOT postponed." → It did not rain. ✓ (Because if it had rained, the game would have been postponed.)
These look valid but are not — they're favorite wrong answers:
"If it rains, the game is postponed. The game was postponed. Therefore it rained." ✗ (It might have been postponed for another reason.)
"If it rains, the game is postponed. It did not rain. Therefore the game was not postponed." ✗ (It could be postponed anyway.)
Memory hook: you may affirm the "if" part (A) or deny the "then" part (B) validly; affirming B or denying A is a fallacy.
An assumption is an unstated belief the argument depends on to work; a conclusion is what the argument is trying to prove. An "assumption" question wants the missing link that, if false, would break the argument — not just any related true-sounding fact.
Argument: "Ben studied hard, so he will pass." Assumption: "Studying hard leads to passing (for Ben)." If that link is false, the argument collapses.
Argument: "Sales rose after the ad campaign, so the ads worked." Weakener: "A competitor closed that same month" (an alternative cause). Strengthener: "Sales rose only in regions where the ads ran" (rules out other causes).
Evaluate strictly whether the given statements support the conclusion. Don't import outside knowledge or personal opinions about whether the conclusion seems reasonable in real life. A conclusion can be true in the world yet not follow from these statements — and that makes it wrong here.
With the toolkit built, let's picture, tabulate, and drill.
Logical reasoning uses inference rules, not formulas:
| Pattern | Valid? |
|---|---|
| If A then B; A → B | Valid (modus ponens) |
| If A then B; not B → not A | Valid (modus tollens) |
| If A then B; B → A | Invalid (affirming consequent) |
| If A then B; not A → not B | Invalid (denying antecedent) |
| Must be true | forced by the statements |
| Could be true | merely consistent (not enough) |
| Assumption | unstated link the argument needs |
| Strengthen / weaken | add support / add a gap or alternative |
| When the question asks… | Do this… |
|---|---|
| "what must be true / can be concluded" | require a forced conclusion |
| "If A then B" + "A" | modus ponens → B |
| "If A then B" + "not B" | modus tollens → not A |
| a conclusion that affirms B or denies A | flag the fallacy (invalid) |
| "the assumption is…" | find the unstated needed link |
| "which strengthens/weakens" | add support / add an alternative cause |
| a real-world-true but unsupported option | reject it (stay inside the argument) |
Step 1 — Identify the structure: premises, conclusion, and any conditional ("if… then"). ↓ Step 2 — For conditionals, check the pattern: is it affirming A (valid), denying B (valid), or a fallacy (affirming B / denying A)? ↓ Step 3 — For "must be true," test: can the premises be true while the conclusion is false? If yes, it doesn't follow. ↓ Step 4 — For assumptions, find the missing link; for strengthen/weaken, look for alternative explanations. ↓ Step 5 — Stay inside the argument — reject options relying on outside knowledge or mere plausibility.
Why Step 3 matters most: the counterexample test — imagining the premises true and the conclusion false — is the decisive tool. If you can build one, the conclusion is not forced, no matter how reasonable it sounds.
Example 1 (modus ponens). "If a number is divisible by 4, it is even. 12 is divisible by 4." Conclusion? Solution: 12 is even (modus ponens). Difficulty: ★☆☆☆☆
Example 2 (must vs. could). "All cats are animals. Rex is an animal." Is Rex a cat? Solution: Not necessarily — Rex could be another animal. "Could be true," not "must." Difficulty: ★★☆☆☆
Example 3 (modus tollens). "If the alarm rings, everyone leaves. Not everyone left." Conclusion? Solution: The alarm did not ring (modus tollens — deny B → deny A). Difficulty: ★★★☆☆
Example 4 (affirming the consequent). "If it snows, school is closed. School is closed. Therefore it snowed." Valid? Solution: Invalid — school could be closed for another reason (affirming the consequent). Difficulty: ★★★☆☆
Example 5 (denying the antecedent). "If you water the plant, it grows. You did not water it. Therefore it did not grow." Valid? Solution: Invalid — it could grow from rain (denying the antecedent). Difficulty: ★★★☆☆
Example 6 (assumption). "The new policy will reduce traffic, because it raises parking fees." What assumption does this rely on? Thinking: The argument needs higher fees to actually discourage driving. Solution: Assumption: higher parking fees will lead people to drive less (if false, the argument fails). Difficulty: ★★★★☆
Example 7 (weaken). "Since the tutoring program started, test scores rose. So the program raised scores." Which weakens this? Thinking: Find an alternative cause. Solution: A weakener: "A new, easier test was introduced the same year" (an alternative explanation for the rise). Difficulty: ★★★★☆
Example 8 (strengthen). Same argument. Which strengthens it? Solution: A strengthener: "Scores rose only for students who attended tutoring" (rules out other causes). Difficulty: ★★★★☆
Example 9 (must be true). "Every member who paid dues received a card. Ana received a card." Which must be true? Thinking: Receiving a card doesn't prove she paid — non-members might also get cards, or the statement only says payers get cards, not only payers. So "Ana paid dues" is not forced. Solution: We cannot conclude Ana paid dues (only "could be true"). What must be true is nothing beyond the given — beware the affirming-the-consequent trap. Difficulty: ★★★★★
Example 10 (chain of conditionals). "If A then B. If B then C. A is true." What must be true? Thinking: A → B (modus ponens), then B → C (modus ponens). Solution: C is true. Difficulty: ★★★★☆
Example 11 (outside-knowledge trap). "All the survey respondents preferred tea. Therefore most Filipinos prefer tea." Valid? Thinking: The survey group may not represent all Filipinos; the conclusion overreaches the given data (and importing real-world beliefs doesn't help). Solution: Invalid — the sample doesn't guarantee the general claim. Difficulty: ★★★★☆
Example 12 (identify the fallacy). "If a plant is healthy, it has green leaves. This plant has green leaves. Therefore it is healthy." Name the flaw. Solution: Affirming the consequent — green leaves don't guarantee health (a plant could have green leaves yet be diseased). Difficulty: ★★★★☆
Valid / Invalid, or answer the question.
Logical reasoning is about the gap between guaranteed and plausible. The exam wants conclusions that must be true — forced by the statements — not merely consistent with them; test by trying to make the premises true and the conclusion false. For conditionals, only two moves are valid: modus ponens (affirm A → B) and modus tollens (deny B → not A); their look-alikes, affirming the consequent and denying the antecedent, are fallacies. Distinguish an assumption (the unstated link the argument needs) from the conclusion, and analyze strengthen/weaken questions through alternative causes. Above all, stay inside the argument — reject options that rely on outside knowledge or real-world plausibility. Demand proof, check the conditional pattern, and judge only the given statements.
| Item | Key point |
|---|---|
| Must vs. could | require a forced conclusion |
| Modus ponens | If A→B, A ⟹ B |
| Modus tollens | If A→B, not B ⟹ not A |
| Affirming consequent | If A→B, B ⟹ A (invalid) |
| Denying antecedent | If A→B, not A ⟹ not B (invalid) |
| Assumption | unstated needed link (negate to test) |
| Weaken / strengthen | add / rule out an alternative cause |
| Scope | judge only within the argument |
What does "must be true" mean exactly? It means the conclusion holds in every case where the statements are true — it's forced. A conclusion that's merely possible or consistent with the statements ("could be true") is not enough.
Which conditional inferences are valid? Only two: modus ponens (given "If A then B" and A, conclude B) and modus tollens (given "If A then B" and not-B, conclude not-A). Affirming B or denying A are fallacies.
How do I find an argument's assumption? Look for the unstated link the argument needs to work. Test a candidate by negating it: if the argument falls apart, that's the assumption.
How do I weaken or strengthen an argument? Weaken it by introducing an alternative explanation or contradicting evidence; strengthen it by ruling out alternatives or adding support for the conclusion.
Can a true statement be the wrong answer? Yes. If a statement is true in the real world but isn't supported by the argument's premises, it doesn't "follow" and is wrong for a strict logic question. Judge only within the argument.
Tick them all and logical reasoning becomes a precise, dependable strength — and you'll spot faulty arguments in news, ads, and debate for the rest of your life.
Put it to the test with 191 practice questions on this topic.