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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).

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.

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).

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.

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.
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