Logical Reasoning: Syllogism Rules & Tricks for Competitive Exams

Introduction

Syllogism is a form of logical reasoning where a conclusion is drawn from two given statements (premises). It is one of the most common topics in competitive exams like TNPSC, UPSC, Bank PO, and SSC.

Basic Structure

A syllogism consists of three parts:

• Major Premise: A general statement
• Minor Premise: A specific statement
• Conclusion: Derived from the two premises

Example:

• All dogs are animals. (Major Premise)
• All animals are living beings. (Minor Premise)
• Therefore, all dogs are living beings. (Conclusion)

Rules of Syllogism

Rule 1: Three Terms Only

A valid syllogism must have exactly three terms: the major term, the minor term, and the middle term.

Rule 2: Middle Term Must Be Distributed

The middle term (the term that appears in both premises) must be distributed (referring to all members of the class) in at least one premise.

Rule 3: If a Term is Distributed in Conclusion, It Must Be Distributed in Premise

If a term is distributed (referring to all members) in the conclusion, it must also be distributed in the premise where it appears.

Venn Diagram Method

The easiest way to solve syllogism questions is using Venn diagrams. Here's how:

For "All A are B"

Draw circle A completely inside circle B.

For "Some A are B"

Draw two overlapping circles A and B.

For "No A are B"

Draw two separate circles A and B (no overlap).

For "Some A are not B"

Draw circle A with a portion outside circle B.

Example with Venn Diagram

Statements:

• All cats are animals.
• All animals are living beings.

Conclusions:

• All cats are living beings. (TRUE - cat circle is inside living being circle)
• Some living beings are cats. (TRUE - living being circle contains cat circle)

Shortcut Techniques

Combination Method

When you have two positive statements, combine them:

• All A are B + All B are C = All A are C
• Some A are B + All B are C = Some A are C

Conversion Rules

• "All A are B" converts to "Some B are A"
• "Some A are B" converts to "Some B are A"
• "No A are B" converts to "No B are A"
• "Some A are not B" does NOT convert directly

Practice Questions

Q1: All students are hardworking. All hardworking people succeed.

Conclusions:

1. All students succeed. (TRUE)
2. Some who succeed are students. (TRUE)

Q2: Some books are novels. All novels are fiction.

Conclusions:

1. All books are fiction. (FALSE - only some books are novels)
2. Some fiction are books. (TRUE)

Key Tips for Exams

• Always draw Venn diagrams for complex questions
• Check if the conclusion follows from BOTH premises together
• Be careful with "Possibility" type questions
• Practice at least 10 questions daily
• Focus on understanding, not just memorizing patterns