18  Types of Reasoning

Reasoning is the process of drawing conclusions from premises. Different types of reasoning differ in (a) the direction — from general to particular or particular to general — and (b) the certainty — whether the conclusion follows necessarily or only probably.

TipFive Major Types of Reasoning
  1. Deductive — General → Particular; conclusion certain if premises true.
  2. Inductive — Particular → General; conclusion probable, never certain.
  3. Abductive — Observation → Best explanation; “inference to the best explanation”.
  4. Analogical — A is like B; therefore what is true of A may be true of B.
  5. Critical — Evaluating arguments for soundness; not a separate “direction” but an evaluative stance.

18.1 Deductive Reasoning

Deductive reasoning moves from general principles to specific conclusions. If the premises are true and the form is valid, the conclusion is necessarily true.

TipClassic Syllogism — the Standard Form
Step Statement
Major premise All humans are mortal.
Minor premise Socrates is a human.
Conclusion Therefore, Socrates is mortal.

Two evaluative concepts apply to deductive arguments:

  • Validity — the conclusion follows from the premises (a property of the argument’s form).
  • Soundness — the argument is valid and the premises are actually true.
TipValidity vs Soundness — Quick Examples
  • “All birds can fly. Penguins are birds. Therefore, penguins can fly.” → Valid but not sound (premise about all birds is false).
  • “All metals conduct electricity. Copper is a metal. Therefore, copper conducts electricity.” → Valid and sound.

flowchart LR
  G[General principle] --> S[Specific case] --> C[Specific conclusion]
    classDef default fill:#003366,color:#ffffff,stroke:#ffcc00,stroke-width:3px,rx:10px,ry:10px;

18.2 Inductive Reasoning

Inductive reasoning moves from particular observations to general conclusions. The conclusion is probable but never certain — even strong induction can be overturned by a single counter-example.

TipInductive Argument — Example
Step Statement
Observation 1 Swan A is white.
Observation 2 Swan B is white.
Observation 3 Swan C is white.
Conclusion All swans are white. (False — black swans exist in Australia.)
TipStrong vs Weak Induction
  • Strong induction — large, representative sample; conclusion is highly probable.
  • Weak induction — small or biased sample; conclusion is unreliable.
  • All inductive conclusions are defeasible — open to revision in light of new evidence.

flowchart LR
  O1[Observation 1] --> P[Pattern]
  O2[Observation 2] --> P
  O3[Observation 3] --> P
  P --> G[General conclusion]
    classDef default fill:#003366,color:#ffffff,stroke:#ffcc00,stroke-width:3px,rx:10px,ry:10px;

18.3 Abductive Reasoning

Abduction starts from an observation and seeks the most plausible explanation. Unlike deduction (certain) or induction (generalisation), abduction is inference to the best explanation — the conclusion is one possibility among several, chosen for plausibility, simplicity, and explanatory power.

TipAbductive Reasoning — Example
  • Observation: The grass is wet this morning.
  • Possible explanations: It rained overnight; the sprinkler ran; someone watered the lawn; heavy dew formed.
  • Best explanation (given context): Last night’s weather forecast predicted rain → It rained.

Abduction is the engine of medical diagnosis, scientific hypothesis-formation, and detective work. The American philosopher Charles Sanders Peirce coined the term in the 1870s.

18.4 Analogical Reasoning

Analogical reasoning argues from similarity. If A and B are similar in known respects, they may be similar in further respects.

TipAnalogical Argument Example
  • Earth has water, atmosphere, and supports life.
  • Mars has (some) water and (thin) atmosphere.
  • Therefore, Mars may support life.

The strength of an analogical argument depends on:

  • Number of relevant similarities — more relevant shared features = stronger.
  • Relevance of the similarities — features must be relevant to the conclusion.
  • Number of disanalogies — significant differences weaken the argument.
TipVerbal Analogies — NTA Format

NTA Paper-I commonly uses verbal analogies of the form A : B :: C : ?.

  • Doctor : Hospital :: Teacher : ? → School (relation: workplace).
  • Pen : Write :: Knife : ? → Cut (relation: function).
  • Ophthalmologist : Eye :: Cardiologist : ? → Heart (relation: specialist organ).

The candidate’s task is to identify the relation in the first pair, then apply it to the second.

18.5 Critical Reasoning

Critical reasoning is the evaluative stance toward arguments. The critical reasoner asks:

TipFive Critical-Reasoning Questions
Question What it checks
Are the premises true? Empirical and conceptual accuracy
Does the conclusion follow? Logical validity
Are key terms defined? Conceptual clarity
Are assumptions identified? Hidden premises that must hold
Are alternative explanations considered? Open-mindedness; avoiding cherry-picking

18.5.1 Common Logical Fallacies

A fallacy is a flaw in reasoning that makes an argument unsound or invalid. NTA papers test recognition of fallacies frequently.

TipTwelve Common Fallacies
Fallacy What it does Example
Ad hominem Attacks the person, not the argument “You can’t trust her view on policy — she dropped out of college.”
Straw man Misrepresents the opponent’s view “He says we should reform; he must want to abolish everything.”
Appeal to authority (without expertise) Cites authority outside their expertise “A movie star says vaccines cause harm; therefore, they do.”
Appeal to popularity (ad populum) “Everyone believes it, so it must be true.”
Appeal to ignorance “It hasn’t been disproven, so it’s true.”
False cause (post hoc) Treats correlation as causation “I sneezed and it rained; my sneeze caused rain.”
False dichotomy Two options when more exist “You’re with us or against us.”
Hasty generalisation Generalising from too few cases “I met two rude people from city X; everyone there is rude.”
Slippery slope Treats a small step as leading inevitably to a disaster “If we allow X, soon Y, and finally Z.”
Circular reasoning (petitio principii) Conclusion is assumed in the premises “The Bible is true because it says so.”
Red herring Distracts with irrelevant information “We were debating taxes — but look at unemployment!”
Equivocation Uses a word with two meanings as if it has one “All trees have bark. My dog has bark. So my dog is a tree.” 

flowchart TB
  R[Reasoning] --> D[Deductive<br/>General → Particular<br/>Certainty]
  R --> I[Inductive<br/>Particular → General<br/>Probability]
  R --> A[Abductive<br/>Observation → Best Explanation]
  R --> AN[Analogical<br/>A is like B]
  R --> C[Critical<br/>Evaluative]
    classDef default fill:#003366,color:#ffffff,stroke:#ffcc00,stroke-width:3px,rx:10px,ry:10px;

18.6 Reasoning vs Argument vs Inference

TipThree Closely Related Terms
Term Meaning
Reasoning The mental process of drawing conclusions
Argument A set of premises offered in support of a conclusion
Inference The step from premises to conclusion

18.7 Practice Questions

Q 01 Deductive Reasoning Easy

Which of the following best describes deductive reasoning?

  • AMoves from particular observations to a general conclusion
  • BMoves from a general principle to a specific conclusion that necessarily follows
  • CSelects the best explanation among several
  • DArgues from similarity between two cases
View solution
Correct Option: B
Deduction = General → Particular; conclusion is necessarily true if premises are true.
Q 02 Validity vs Soundness Medium

"All birds can fly. Penguins are birds. Therefore, penguins can fly." This argument is:

  • AValid and sound
  • BValid but not sound
  • CInvalid but sound
  • DNeither valid nor sound
View solution
Correct Option: B
The form is valid (conclusion follows from premises), but the major premise is false (penguins cannot fly), so the argument is not sound.
Q 03 Inductive Reasoning Easy

A scientist observes that 1,000 swans she has seen are white and concludes "All swans are white." This is an example of:

  • ADeductive reasoning
  • BInductive reasoning
  • CAbductive reasoning
  • DCritical reasoning
View solution
Correct Option: B
Moving from particular observations to a general conclusion = inductive reasoning. The conclusion is defeasible — black swans in Australia falsified it.
Q 04 Verbal Analogy Easy

Doctor : Hospital :: Teacher : ?

  • AStudent
  • BSchool
  • CSubject
  • DLibrary
View solution
Correct Option: B
The relation is professional : workplace. Doctor works in Hospital; Teacher works in School.
Q 05 Abduction Medium

A doctor sees a patient with fever, cough and shortness of breath, and concludes "most likely COVID-19" while keeping other possibilities open. This is an example of:

  • ADeductive reasoning
  • BInductive reasoning
  • CAbductive reasoning (inference to the best explanation)
  • DAnalogical reasoning
View solution
Correct Option: C
Medical diagnosis from symptoms to most plausible diagnosis is the classic example of abduction — inference to the best explanation (Peirce).
Q 06 Logical Fallacies Medium

"You cannot trust her economic argument — she failed mathematics in school." This commits which fallacy?

  • AStraw man
  • BAd hominem
  • CSlippery slope
  • DRed herring
View solution
Correct Option: B
Ad hominem — attacks the person, not the argument.
Q 07 False Cause Medium

"I wore my lucky shirt on the day of the exam, and I passed. The shirt caused me to pass." This commits which fallacy?

  • AFalse cause (post hoc)
  • BHasty generalisation
  • CEquivocation
  • DCircular reasoning
View solution
Correct Option: A
False cause (post hoc ergo propter hoc) — treating mere correlation as causation.
Q 08 Reasoning vs Argument Easy

In logic, the *step* from premises to conclusion is called:

  • AArgument
  • BInference
  • CPremise
  • DConclusion
View solution
Correct Option: B
An inference is the step from premises to conclusion. An argument is the whole package; reasoning is the mental process.
ImportantQuick recall
  • Five reasoning types: Deductive · Inductive · Abductive · Analogical · Critical.
  • Deduction = General → Particular; Validity (form) vs Soundness (form + true premises).
  • Induction = Particular → General; conclusion probable, defeasible.
  • Abduction = Observation → best explanation (Peirce); used in diagnosis and detective work.
  • Analogy: A is like B in known ways → may be alike in further ways. NTA: A : B :: C : ?
  • Common fallacies: Ad hominem, Straw man, Appeal to authority/popularity/ignorance, False cause (post hoc), False dichotomy, Hasty generalisation, Slippery slope, Circular reasoning, Red herring, Equivocation.
  • Reasoning (process) vs Argument (package) vs Inference (step).