Syllogism (Mains & Prelims Level)
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Expert Answer & Key Takeaways
Mastering Syllogism (Mains & Prelims Level) is essential for high-fidelity technical architecture and senior engineering roles in 2026.
1. Core Concepts & Propositions
Syllogism deductions rely on 4 standard propositions:
• A (All): Universal Positive (e.g., All A are B).
• E (No): Universal Negative (e.g., No A is B).
• I (Some): Particular Positive (e.g., Some A are B).
• O (Some...Not): Particular Negative (e.g., Some A are not B).
• A (All): Universal Positive (e.g., All A are B).
• E (No): Universal Negative (e.g., No A is B).
• I (Some): Particular Positive (e.g., Some A are B).
• O (Some...Not): Particular Negative (e.g., Some A are not B).
2. Venn Diagram Method & Conversions
When using Venn Diagrams, always draw the Minimum Overlapping Diagram. A conclusion is 'Definite' only if it holds true in ALL possible diagrams.
Standard Conversions:
• All A are B converts to Some B are A.
• No A is B converts to No B is A and Some B are not A.
• Some A are B converts to Some B are A.
• Some A are not B CANNOT be strictly converted.
Standard Conversions:
• All A are B converts to Some B are A.
• No A is B converts to No B is A and Some B are not A.
• Some A are B converts to Some B are A.
• Some A are not B CANNOT be strictly converted.
3. The '100-50' Tick-Cross Method (Exam Attack)
For speed, assign values to Subject and Predicate:
• A (All): Sub=100, Pred=50 (+)
• E (No): Sub=100, Pred=100 (-)
• I (Some): Sub=50, Pred=50 (+)
• O (Some..Not): Sub=50, Pred=100 (-)
Rules:
1. Positive statements give Positive conclusions. Negative gives Negative.
2. If linking two statements, the common term MUST have at least one 100.
3. Value Check: A value of 50 in the statement cannot become 100 in the conclusion (You can't spend 100 if you only earn 50).
• A (All): Sub=100, Pred=50 (+)
• E (No): Sub=100, Pred=100 (-)
• I (Some): Sub=50, Pred=50 (+)
• O (Some..Not): Sub=50, Pred=100 (-)
Rules:
1. Positive statements give Positive conclusions. Negative gives Negative.
2. If linking two statements, the common term MUST have at least one 100.
3. Value Check: A value of 50 in the statement cannot become 100 in the conclusion (You can't spend 100 if you only earn 50).
4. 'Only' & 'Only a Few' (Mains Specific)
• Only A is B: Means 'All B is A' AND B cannot overlap with anything else. Think of B as a restricted entity.
• Only a few A are B: This means 'Some A are B' AND 'Some A are NOT B'. It is a combination statement. Conclusion 'All A are B is a possibility' is strictly FALSE. But 'All B are A is a possibility' is TRUE.
• Only a few A are B: This means 'Some A are B' AND 'Some A are NOT B'. It is a combination statement. Conclusion 'All A are B is a possibility' is strictly FALSE. But 'All B are A is a possibility' is TRUE.
5. Either/Or & Possibility Rules
Either/Or Conditions:
1. Both conclusions must be individually false (doubtful).
2. Subjects and Predicates must be exactly the same.
3. They must form a complementary pair: (Some + No) or (All + Some...Not) or (Some + Some...Not).
Possibility: If the relation between two elements is 'unknown' or 'doubtful', ANY possibility between them is TRUE.
1. Both conclusions must be individually false (doubtful).
2. Subjects and Predicates must be exactly the same.
3. They must form a complementary pair: (Some + No) or (All + Some...Not) or (Some + Some...Not).
Possibility: If the relation between two elements is 'unknown' or 'doubtful', ANY possibility between them is TRUE.
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