32  Logical Fallacy and Assumptions

32.1 Introduction

In critical reasoning, arguments are often flawed because of fallacies (errors in reasoning) or unstated assumptions.
To answer questions correctly, you must identify these weak links and test the argument’s validity.

32.2 1) Assumptions in Arguments

An assumption is something the author takes for granted without proof.
It is the hidden bridge between premises (given statements) and the conclusion.

32.2.1 Types of Assumptions

  • Necessary Assumption: Without it, the argument collapses.
  • Unstated Premise: A belief that must be true for the conclusion to hold.
  • Value Judgment: Author’s belief about what is “good” or “bad”.

Example:
Premise: All MBA graduates get high-paying jobs.
Conclusion: Ravi will get a high-paying job.
Assumption: Ravi is an MBA graduate.

32.3 2) Logical Fallacies

A fallacy is a mistake in reasoning that weakens an argument.
Common fallacies tested in exams:

  1. Hasty Generalization
    • Making a broad conclusion from limited examples.
    • Example: “My two friends scored well in IPMAT. So, coaching is unnecessary.”
  2. False Cause (Post hoc fallacy)
    • Assuming correlation = causation.
    • Example: “Since online classes started, exam results improved. Therefore, online classes cause better results.”
  3. Circular Reasoning
    • Conclusion repeats the premise.
    • Example: “This book is popular because it is the best, and it is the best because it is popular.”
  4. Strawman Fallacy
    • Misrepresenting someone’s argument to attack it easily.
    • Example: “He said we should spend more on education. He must want to cut defense completely!”
  5. Either–Or Fallacy (False Dilemma)
    • Presenting only two options when more exist.
    • Example: “Either you join coaching or you will fail.”
  6. Ad Hominem
    • Attacking the person instead of the argument.
    • Example: “You cannot trust his opinion on economics because he is not rich.”
  7. Appeal to Authority
    • Assuming something is true because an expert said so.
    • Example: “A famous actor uses this product, so it must be the best.”
  8. Appeal to Popularity (Bandwagon)
    • Believing something is true because many people believe it.
    • Example: “Everyone invests in crypto, so it must be safe.”

32.4 3) Spotting Assumptions and Fallacies

When analyzing arguments:
- Ask: “What must be true for this conclusion to hold?” → assumption.
- Ask: “Is the conclusion logically valid or based on faulty logic?” → fallacy.
- Check if data and conclusion are mismatched.

32.5 4) Examples

32.5.1 Example 1

Argument: Online coaching is better than classroom learning because many students join online classes.
- Fallacy: Appeal to popularity.
- Assumption: Students join because of quality, not convenience.

32.5.2 Example 2

Argument: The company should reduce salaries because profits have fallen.
- Fallacy: False cause (profits may fall due to other reasons).
- Assumption: Salary reduction will improve profits.

32.5.3 Example 3

Argument: If a student studies hard, they will definitely succeed.
- Fallacy: Hasty generalization.
- Assumption: Success depends only on hard work.

32.5.4 Example 4

Argument: We must choose between better education or better healthcare.
- Fallacy: False dilemma.
- Assumption: Both cannot be improved together.

32.6 5) Practice Questions

  1. Argument: “Everyone in my family eats junk food and they are healthy. So, junk food is not harmful.”
    • Identify the fallacy and assumption.
  2. Argument: “Our school must be the best because it won the best school award.”
    • Identify the fallacy.
  3. Argument: “Ramesh failed the exam, so coaching classes are useless.”
    • Identify the fallacy.
  4. Argument: “If we don’t ban mobile phones in classrooms, students will never learn.”
    • Identify the fallacy.
  5. Argument: “The new app is great because a celebrity recommends it.”
    • Identify the fallacy.

32.7 6) Suggested Answers

  1. Fallacy: Hasty generalization. Assumption: Family’s health represents general truth.
  2. Fallacy: Circular reasoning.
  3. Fallacy: Hasty generalization (one case → whole system).
  4. Fallacy: False dilemma.
  5. Fallacy: Appeal to authority (celebrity endorsement).

32.8 Summary

  • Assumptions are hidden links between premises and conclusions.
  • Fallacies are logical flaws that weaken arguments.
  • Recognize common fallacies like generalization, false cause, false dilemma, ad hominem, popularity, and circular logic.
  • For exam questions: identify whether to spot the assumption or the fallacy.
  • Correct answers are always relevant, precise, and logical.