The Toolkit You Should Memorize
For these questions, always identify:
Facts vs. conditionals — Don't treat an "if" statement as a fact.
Trigger → Result — Mark the antecedent and consequent.
Universal vs. existential — Words like all, no, each versus some, at least one.
Contradiction pairs:
All A are B ↔ Some A are not B
No A are B ↔ Some A are B
Modus Tollens:
If A → B
Not B
Therefore, not A.
Negating universals:
Not(all A are B) = Some A are not B.
Detailed form
Inference CR - One-Page Cheat Sheet
- Facts vs Conditionals
A fact is a statement known to be true. Treat it as given.
A conditional states only a relationship, not that the trigger actually occurred.
Form: If A -> B
Never assume A happened just because the conditional exists.
- Identify the Structure
Every conditional has two parts.
Trigger (Antecedent) -> Result (Consequent)
Always identify the trigger first.
- English to Logic
If, Whenever, Every, Each, All -> A -> B
Only if -> A -> B
Only -> B -> A
Unless A -> Not A -> B (equivalently, Not B -> A)
- Quantifiers
Universal quantifiers apply to the entire group: All, Every, Each, No, None, Always.
Existential quantifiers require only one example: Some, At least one, Exists, Several, Many, Often, Usually.
Remember: Some means >= 1.
- Set Translation
All A are B -> A is a subset of B.
No A are B -> A and B have no overlap.
Some A are B -> A and B overlap.
Some A are not B -> Part of A lies outside B.
Not all A are B -> Some A are not B.
- Contradiction Patterns
All A are B contradicts Some A are not B.
No A are B contradicts Some A are B.
Every A is B contradicts At least one A is not B.
One counterexample destroys a universal statement.
- Valid Conditional Rules
Modus Ponens:
If A -> B
A
Therefore B.
Modus Tollens:
If A -> B
Not B
Therefore Not A.
Contrapositive:
A -> B is logically equivalent to Not B -> Not A.
Chain Rule:
A -> B
B -> C
Therefore A -> C.
- Invalid Conditional Rules
Affirming the Consequent:
If A -> B
B
Does NOT imply A.
Denying the Antecedent:
If A -> B
Not A
Does NOT imply Not B.
- Negation Rules
All A are B -> Some A are not B.
No A are B -> Some A are B.
Some A are B -> No A are B.
Some A are not B -> All A are B.
Every -> Not every (Some do not).
Always -> Not always (Sometimes not).
- Master Conditional Pattern
If A -> B.
Evidence shows Not B.
Apply Modus Tollens.
Conclusion: Not A.
- Universal Failure Rule
If "All A are B" is false, conclude only "Some A are not B."
Do NOT conclude "No A are B."
- 30-Second Attack Plan
Step 1: Separate facts from conditionals.
Step 2: Identify Trigger -> Result.
Step 3: Identify quantifiers (All, Every, No, Some, At least one, None).
Step 4: Convert statements into set relationships (inside, overlap, outside, disjoint).
Step 5: Check whether later evidence contradicts the conditional result.
Step 6: If yes, apply Modus Tollens.
Step 7: Negate universal statements correctly.
- Most Common Elite Pattern
If:
All P are Q.
No Q are R.
But:
Some Q are R.
Therefore:
"No Q are R" is false.
Apply Modus Tollens.
Therefore:
Not(All P are Q).
Final inference:
Some P are not Q.
Golden Rules
Some means >= 1.
One counterexample destroys "All" and "No."
Never assume the trigger of a conditional occurred.
Translate English -> Logic -> Sets before evaluating answer choices.
For elite Inference CR, think in this order:
Facts -> Conditionals -> Quantifiers -> Sets -> Contradictions -> Modus Tollens -> Inference.