Critical Thinking
Critical thinking is the disciplined evaluation of statements, evidence, and arguments to decide what is reasonable to believe. The PMDC MDCAT 2026 syllabus expects you to identify cognitive biases and logical fallacies, recognise the structure of an argument (premises and conclusion), and evaluate statements as true or false against given premises. This is the highest-yield Logical Reasoning chapter and typically contributes 2-3 MCQs.
False Beliefs Identification
A logical fallacy is a flaw in reasoning that makes an argument unreliable. A cognitive bias is a systematic tendency of human thinking that pushes us toward error. The MDCAT especially loves the named fallacies below — learn them by name and example.
Attacking the person making the argument rather than the argument itself. Example: "You cannot trust her view on climate science — she failed maths in school." The speaker's history says nothing about whether her argument is sound.
Misrepresenting an opponent's position so it is easier to knock down. Example: "He says we should regulate sugar; he must want to ban all food." The actual position has been replaced with an exaggerated cartoon.
Claiming, without evidence, that a small first step will inevitably trigger a chain of disastrous consequences. Example: "If we let students retake one exam, soon they will demand to retake every exam and the system will collapse."
Presenting only two options when more exist. Example: "Either we ban mobile phones in schools or students will fail." There are obviously middle options — restricted use, lockers, supervised periods, etc.
Asserting a claim is true because an authority figure said so — especially when the authority is outside their field. Example: "A famous cricketer endorses this medicine, so it must be safe." Endorsement is not evidence.
Claiming a statement is true because it has not been proven false — or false because it has not been proven true. Example: "No one has proved aliens do not visit Earth, so they must." Absence of evidence is not evidence of absence.
Using the conclusion as one of the premises — the argument simply assumes what it should prove. Example: "The book is reliable because it says so itself."
Drawing a sweeping conclusion from too few examples. Example: "I met two rude tourists from country X; everyone from country X must be rude." A small sample cannot support a population claim.
Substituting an emotional reaction for evidence. Example: "Think of the children — therefore we must approve this policy without further debate." Emotion is not an argument.
Asserting that a claim is true because most people believe it. Example: "Millions of people use this remedy, so it must work." Popularity is not proof.
Concluding that A caused B simply because B followed A. Example: "I started taking vitamin pills last month and my mood improved; the pills must be working." Many other variables could explain the change.
Introducing an irrelevant topic to divert attention from the original issue. Example: Asked about exam cheating, a politician answers by talking about new sports facilities.
Fallacies at a glance — the master reference
| Fallacy | Tell-tale signal in the argument | What's wrong |
|---|---|---|
| Ad hominem | Attacks the person, not the claim | Speaker's character ≠ argument's truth |
| Straw man | Distorts opponent's view, then knocks it down | Refuting a position they never held |
| Slippery slope | "If A, then B, then C, then disaster" — chain unsupported | No evidence the chain actually follows |
| False dichotomy | "Either X or Y" — presents only two options | Ignores middle / other options |
| Appeal to authority | "X expert / celebrity says so" | Claim must stand on evidence, not endorsement |
| Appeal to ignorance | "Not proved false ⇒ true" (or vice versa) | Absence of evidence is not evidence of absence |
| Circular reasoning (begging the question) | Conclusion appears in the premises | Argument assumes what it should prove |
| Hasty generalisation | Sweeping claim from few examples | Sample too small to support population claim |
| Appeal to emotion | "Think of the children …" | Emotion replaces evidence |
| Bandwagon (appeal to popularity) | "Everyone is doing it / believes it" | Popularity ≠ truth |
| Post hoc (false cause) | "A happened, then B; therefore A caused B" | Sequence does not prove causation |
| Red herring | Introduces irrelevant side-topic | Diverts from the original issue |
| Tu quoque | "You did the same thing" | Hypocrisy doesn't refute the argument |
| Equivocation | Same word used in two different meanings | Argument relies on shifted meaning |
Cognitive biases (high-yield)
| Bias | What it does | Quick example |
|---|---|---|
| Confirmation | Seeks information confirming pre-existing beliefs | Reading only news sources you already agree with |
| Availability | Judges probability by how easily examples come to mind | Overestimating plane-crash risk after seeing a news story |
| Anchoring | Over-relies on the first number / fact encountered | Negotiating from the seller's first quoted price |
| Hindsight | "I knew it all along" after the event | Claiming you predicted the cricket match outcome afterwards |
| Survivorship | Considers only data that "survived" a selection | "All successful CEOs dropped out of college" — ignores all who dropped out and failed |
| Sunk cost | Continuing because of past investment, not future value | Watching a bad film to the end "because I paid for the ticket" |
Logical Arguments
An argument is one or more premises offered in support of a conclusion. To analyse an argument, identify each part and then test whether the structure is valid and whether the premises are true.
Premises and conclusion
Premises are usually signalled by words like "because", "since", "given that". Conclusions are signalled by "therefore", "thus", "so", "hence". Read the argument and underline these markers.
Valid vs invalid; sound vs unsound
- Valid: If the premises are true, the conclusion must be true. (Form-correct.)
- Invalid: The conclusion can be false even when the premises are true.
- Sound: Valid and the premises are actually true.
- Unsound: Either invalid, or has at least one false premise, or both.
Deductive vs inductive arguments
| Property | Deductive | Inductive |
|---|---|---|
| Direction | General → specific | Specific → general |
| Conclusion | Certain if premises are true | Probable, never certain |
| If premises true and form valid | Conclusion must be true | Conclusion is likely true |
| Strength judged by | Validity (form) + soundness (truth of premises) | Strength — how well evidence supports conclusion |
| Refuted by | One false premise / invalid form | One counter-example |
| Examples | Mathematical proofs, syllogisms (Aristotle), formal logic | Scientific generalisation, sample-to-population, "all swans observed are white" |
Valid argument forms (memorise these)
| Form | Pattern | Verdict |
|---|---|---|
| Modus ponens | If P then Q. P. ∴ Q. | Valid |
| Modus tollens | If P then Q. Not Q. ∴ Not P. | Valid |
| Hypothetical syllogism | If P then Q. If Q then R. ∴ If P then R. | Valid |
| Disjunctive syllogism | P or Q. Not P. ∴ Q. | Valid |
| Affirming the consequent | If P then Q. Q. ∴ P. | INVALID — common trap |
| Denying the antecedent | If P then Q. Not P. ∴ Not Q. | INVALID — common trap |
P1: All MDCAT candidates have studied biology. P2: Sara is an MDCAT candidate. C: Sara has studied biology. The conclusion follows necessarily from the premises — deductively valid.
P: Every swan I have seen is white. C: All swans are white. The conclusion is supported but not guaranteed — one black swan would refute it. This is an inductive generalisation.
True/False Evaluation
True/false items present a passage or set of premises and a list of statements; you must mark each statement as true, false, or undetermined based only on the given information. Outside knowledge is irrelevant.
The three-label rule
- True: The statement is logically forced by the passage.
- False: The passage logically forces the negation of the statement.
- Undetermined / Cannot be determined: The passage neither confirms nor contradicts the statement.
The single biggest error is bringing in real-world knowledge to "fill gaps" in the passage. If the passage does not mention something, you cannot use it — even if you know it personally.
"All", "some", "no", "most" change everything. "Some doctors are surgeons" does not imply "most doctors are surgeons" or "all doctors are surgeons." Read every quantifier as carefully as a number.
Worked MCQs
Five MCQs that capture the high-yield testing patterns for critical thinking. Read the explanation even when you get the answer right — it's where the deeper concept lives.
Q1. "You should not believe Dr. Khan's research on diabetes — he was once accused of plagiarism in his student days." This argument commits which fallacy?
The speaker attacks Dr. Khan's character rather than the content of his research. Whether his earlier conduct was good or bad has no bearing on whether his current data are sound — this is a textbook ad hominem.
Q2. "Either you support free education for all or you do not care about poor students." This argument is an example of:
The speaker presents only two options when many positions exist between them (subsidised education, scholarships, conditional support). False dichotomy traps the listener into accepting a forced choice.
Q3. Premise 1: All mammals breathe air. Premise 2: A whale is a mammal. Conclusion: A whale breathes air. This argument is:
The conclusion follows necessarily from the two premises (validity), and both premises are factually true (soundness). Note that "deductively valid" means the structure forces the conclusion if the premises hold.
Q4. "No one has ever proved that ghosts do not exist, so they must exist." This is an example of:
The argument treats the absence of proof against a claim as proof for it. This is the appeal-to-ignorance fallacy: lack of disproof is not the same as proof.
Q5. Passage: "All members of the chess club study mathematics. Some chess club members also study physics." Which of the following is necessarily true?
All chess club members study maths, and some chess club members also study physics. So those "some" members are studying both maths and physics — option C is forced. The other options either over-generalise or are unsupported.
Quick Recap
- Logical fallacies = predictable flaws in reasoning. Learn the named ones by example.
- Ad hominem attacks the person; straw man attacks a distorted version of the argument.
- Slippery slope claims a chain of dire consequences without evidence.
- False dichotomy forces only two options; appeal to authority or popularity is not evidence.
- Premises support a conclusion. Valid = structure forces it; sound = valid + premises true.
- Deductive arguments aim at certainty; inductive arguments aim at probability.
- For true/false items, use only the passage — never outside knowledge.