Reference

Glossary

Plain-language definitions of the terms used across the lessons and exercises — fallacies, biases, reasoning types, and the concepts that tie them together.

Critical thinking has a working vocabulary. Naming a move precisely — recognizing that an argument begs the question, that a statistic invites base-rate neglect, or that a rebuttal is really a straw man — is what lets you spot it under pressure and explain it to someone else. The 46 terms below are defined briefly and grouped by theme; each links to deeper practice in the relevant exercises and lessons.

Core Concepts

Critical Thinking: The disciplined process of actively analyzing, synthesizing, and evaluating information to decide what to believe or do, rather than accepting claims at face value.; Example: A friend forwards a shocking health headline; instead of resharing, you pause to ask who measured it and how.
Logical Fallacy: An error in reasoning that weakens an argument. Formal fallacies break the logical structure; informal fallacies fail because of their content or context.; Example: "My grandfather smoked and lived to 95, so smoking isn't harmful" — the facts may be true, but the conclusion doesn't follow.
Cognitive Bias: A systematic, predictable pattern of deviation from rational judgment, arising from mental shortcuts the brain uses to process information quickly.; Example: After deciding a coworker is careless, you notice every mistake they make and forget every job they do well.
Metacognition: Thinking about your own thinking — monitoring, evaluating, and regulating your reasoning processes as they happen.; Example: Mid-argument you catch yourself defending your view because you're embarrassed, not because you're right — and that noticing is the skill.
Epistemic Humility: Recognizing the limits of your own knowledge and holding beliefs with confidence proportional to the available evidence.; Example: A brilliant physicist offers you confident stock tips, and you remember that expertise in one field doesn't transfer to another.
Steelmanning: Engaging with the strongest, most charitable version of an opposing argument rather than a weakened caricature of it. The opposite of straw-manning.; Example: Before rebutting a proposal, you restate it in its strongest form — one its supporters would happily sign off on.

Reasoning & Logic

Deductive Reasoning: Reasoning from general premises to a conclusion that follows with logical certainty. If the premises are true and the form is valid, the conclusion must be true.; Example: All the club's members are over 18; Maria is a member; so Maria is over 18 — the conclusion is locked in.
Inductive Reasoning: Reasoning from specific observations to a broader generalization that is probable but not guaranteed, even when the premises are true.; Example: Every swan you've ever seen is white, so you conclude all swans are white — a strong bet, until someone spots a black one.
Abductive Reasoningalso: inference to the best explanation: Inferring the most plausible explanation for a set of observations. Common in diagnosis and detective work, it yields a likely cause rather than a certain one.; Example: You come home to a shredded pillow and a guilty-looking dog, and settle on the simplest explanation: the dog did it.
Validity: A property of an argument's structure: an argument is valid when its conclusion follows necessarily from its premises, regardless of whether those premises are actually true.; Example: "All cats can fly; Whiskers is a cat; so Whiskers can fly" is valid — the logic holds even though the first premise is false.
Soundness: A valid argument whose premises are also all true. Soundness guarantees a true conclusion; validity alone does not.; Example: "All humans are mortal; Socrates is human; so Socrates is mortal" is sound: valid in form and true in every premise.
Premise: A statement offered as a reason or evidence in support of a conclusion. An argument is built from one or more premises.; Example: In "it's raining, so take an umbrella," the premise is "it's raining" — the reason offered for the conclusion.
Syllogism: A deductive argument with two premises and a conclusion, e.g. 'All humans are mortal; Socrates is human; therefore Socrates is mortal.'; Example: "All doctors studied medicine; she's a doctor; so she studied medicine" — two premises leading to a forced conclusion.

Argumentation & Evidence

Argument: A set of premises put forward to support a conclusion. In critical thinking it means a reasoned case, not a quarrel.; Example: Not a shouting match: "we should leave now, because the roads ice over after dark" is an argument — a claim plus a reason.
Burden of Proof: The obligation to provide evidence for a claim. It rests with the person making the assertion, not with those asked to accept it.; Example: Someone insists a supplement cures colds; it's on them to show the evidence, not on you to prove it doesn't.
Falsifiability: The property of a claim that some possible observation could prove it false. Unfalsifiable claims cannot be tested and carry little evidential weight.; Example: "This diet works, and if it fails it's because you didn't believe hard enough" can never be proven wrong — which is the problem.
Correlation vs. Causation: A statistical association between two variables does not by itself establish that one causes the other; a third factor, coincidence, or reverse causation may explain it.; Example: Ice cream sales and drownings rise together, but cones don't cause drowning — hot weather drives both.
Necessary vs. Sufficient Condition: A necessary condition must be present for an outcome to occur; a sufficient condition guarantees the outcome on its own. A condition can be one, both, or neither.; Example: Oxygen is necessary for a fire but not sufficient; you also need fuel and a spark.
Occam's Razoralso: parsimony: A heuristic favoring the explanation that requires the fewest assumptions when competing explanations fit the evidence equally well.; Example: Your keys are missing: 'I left them in my coat' beats 'a burglar broke in, took only my keys, and left,' with no extra evidence.
Base Rate: The underlying prevalence of something in a population. Ignoring it in favor of vivid specific detail leads to base-rate neglect and miscalibrated probabilities.; Example: A test for a 1-in-10,000 disease flags you positive — but because the disease is so rare, you're still probably fine.
Bayesian Reasoning: Updating the probability assigned to a belief as new evidence arrives, in proportion to how strongly that evidence distinguishes between competing hypotheses.; Example: You think a coworker is upset; then you learn they're always quiet on Mondays, and you dial your confidence back accordingly.

Logical Fallacies

Ad Hominemalso: argumentum ad hominem: Attacking the person making an argument rather than the argument itself, as though their character settled the truth of their claim.; Example: "You can't trust her budget plan — she's been divorced twice" attacks the person, not the plan.
Straw Man: Misrepresenting an opponent's position as a weaker or distorted version, then refuting that easier target instead of the real argument.; Example: You suggest lighter regulations; they reply, "So you want zero rules and total chaos!" — refuting a claim you never made.
False Dilemmaalso: false dichotomy: Presenting only two options as if they were exhaustive when other alternatives exist.; Example: "Either we cut this program entirely or we go bankrupt" ignores every option in between.
Slippery Slope: Claiming that a relatively small first step will inevitably trigger a chain of significant (usually negative) consequences, without evidence for the chain.; Example: "If we let students retake one exam, soon they'll expect to retake everything and grades will mean nothing."
Circular Reasoningalso: begging the question: An argument in which the conclusion is assumed within its own premises, so it proves nothing beyond what it presupposes.; Example: "This book is trustworthy because it says so right here on the cover."
Hasty Generalization: Drawing a broad conclusion from a sample that is too small, biased, or unrepresentative to support it.; Example: You have one rude driver cut you off and conclude that everyone from that city drives terribly.
Post Hoc Fallacyalso: post hoc ergo propter hoc: Assuming that because one event followed another, the first caused the second. Sequence is mistaken for causation.; Example: You wore your lucky socks and your team won, so you decide the socks did it.
Appeal to Authorityalso: argumentum ad verecundiam: Treating a claim as true merely because an authority endorses it — especially when the authority is biased, anonymous, or speaking outside their expertise.; Example: "A famous actor says this cleanse detoxifies your liver, so it must work" — fame isn't medical expertise.
Appeal to Emotion: Manipulating feelings such as fear, pity, or pride to win acceptance of a conclusion in place of relevant evidence.; Example: A charity ad shows one heartbreaking photo instead of any evidence that your donation actually helps.
Bandwagon Fallacyalso: appeal to popularity: Arguing that a claim is true or an action right because many people believe or do it.; Example: "Millions of people can't be wrong about this stock" — popularity isn't proof.
Red Herring: Introducing an irrelevant point to divert attention away from the issue actually under discussion.; Example: Asked about missed deadlines, a manager suddenly starts talking about how hard everyone worked over the holidays.
Tu Quoquealso: appeal to hypocrisy: Deflecting criticism by charging the critic with the same fault, rather than addressing the substance of the criticism.; Example: A doctor tells you to quit smoking; you snap, "But you smoke too!" — which doesn't make the advice wrong.
Equivocation: Exploiting two different meanings of the same word within an argument to create a misleading impression of validity.; Example: "A feather is light; light things aren't dark; so a feather isn't dark" — 'light' quietly switches meaning.
No True Scotsman: Defending a generalization against a counterexample by arbitrarily redefining the category to exclude it.; Example: "No real fan would boo the team." "My brother did." "Well, no real fan would."
Appeal to Ignorancealso: argumentum ad ignorantiam: Claiming something is true because it has not been proven false, or false because it has not been proven true.; Example: "No one has proven ghosts aren't real, so they must exist" — absence of disproof isn't proof.

Cognitive Biases

Confirmation Bias: The tendency to seek, interpret, and remember information that confirms what you already believe while discounting evidence that contradicts it.; Example: Convinced a diet works, you remember the days you felt great and forget the days you felt awful.
Anchoring Bias: Relying too heavily on the first piece of information encountered (the 'anchor') when making subsequent estimates or decisions.; Example: A jacket marked down from $500 to $300 feels like a steal — only because $500 grabbed you first.
Availability Heuristic: Judging how likely something is by how easily examples come to mind, so vivid or recent events seem more probable than they are.; Example: After seeing a plane crash on the news, flying feels dangerous, even though the drive to the airport was riskier.
Dunning–Kruger Effect: A pattern in which people with limited skill in an area overestimate their competence, partly because the same gaps hide their mistakes from them.; Example: After one weekend course, someone starts correcting the seasoned expert — too new to see how much they're missing.
Hindsight Biasalso: the 'I-knew-it-all-along' effect: Perceiving past events as having been more predictable than they actually were once you know the outcome.; Example: Once the election result is in, everyone insists they 'knew it all along,' though they'd have bet the other way.
Sunk Cost Fallacy: Continuing a course of action because of resources already invested, rather than judging it by its future costs and benefits.; Example: You're an hour into a bad movie and stay to the end 'because you already paid for the ticket.'
Survivorship Bias: Drawing conclusions only from cases that made it past a selection filter while overlooking the failures that did not, distorting the picture.; Example: Studying only the founders who got rich, you copy their habits — and never see the many who did the same and failed.
Framing Effect: Reaching different conclusions from the same information depending on how it is worded or presented — for example, '90% survive' versus '10% die.'; Example: 'This surgery has a 90% survival rate' feels far safer than 'a 10% chance of death,' though they're identical.
Fundamental Attribution Error: Over-attributing others' behavior to their character while attributing your own behavior to the situation.; Example: When a stranger is late, they're irresponsible; when you're late, the traffic was terrible.
Motivated Reasoning: Reasoning aimed at reaching a preferred conclusion, where the desired answer shapes which evidence and arguments feel persuasive.; Example: You want the new job, so every scrap of good news about the company feels convincing and every red flag feels minor.

Definitions stick when you use them

Recognizing these patterns in the abstract is the easy part. The exercises put them in realistic scenarios where they are harder to catch.

Practice fallacies Practice biases