What is critical thinking? How to weigh a claim

Critical thinking is the habit of judging a claim by the evidence and reasoning behind it before deciding whether to believe it or act on it. It isn't being negative or doubting everything. It means asking a few steady questions, such as who says so, how they know and what else could explain it, and letting the answers, not the claim's appeal, set how confident you are.

In short

• Critical thinking weighs claims by their evidence and reasoning, and matches confidence to how strong they are.
• Four questions carry most of the work: who says so, how do they know, what's missing, and what else could explain it.
• It is a skill and a disposition: knowing the fallacies helps, but so does being willing to change your mind.

What does critical thinking mean?

Definitions differ in wording but agree in substance. The philosopher John Dewey, who wrote about it in 1910 under the name "reflective thinking," described it as "active, persistent and careful consideration of any belief or supposed form of knowledge in the light of the grounds that support it, and the further conclusions to which it tends" (Stanford Encyclopedia of Philosophy). In 1990 a panel of 46 experts convened by the American Philosophical Association called it "purposeful, self-regulatory judgment" that results in interpretation, analysis, evaluation and inference (Facione, 1990). A widely quoted 1987 definition by Michael Scriven and Richard Paul ends on the same point: it is a way of evaluating information "as a guide to belief and action" (Foundation for Critical Thinking).

Two things are in every good definition. There is a skill: being able to analyze an argument, judge evidence and spot a bad inference. And there is a disposition: actually doing it, especially when a claim is one you'd like to be true. The 1990 panel stressed the second as much as the first. Knowing the rules of good reasoning doesn't help if you only apply them to claims you already dislike.

What questions does a critical thinker ask?

Most of the work comes down to four questions.

1. Who says so? Is the source in a position to know? Do they have expertise in this specific area, a track record, or something to gain if you believe them? Expertise in one field doesn't carry over to another.
2. How do they know? Is it a measurement, an experiment, a survey, a single story, or an opinion? Can you find the original source, or only someone's summary of it?
3. What's missing? How many cases, compared with what, over what period? What would the claim look like if it were false, and has anyone looked for that? Missing cases are the root of survivorship bias.
4. What else could explain it? Could chance, a third factor, or cause and effect running the other way account for the same facts? When several explanations fit, prefer the one that needs fewest extra assumptions, which is the idea behind Occam's razor, and stay open to evidence that would pick one.

A fifth question tests you rather than the claim: what would change my mind? If no possible evidence would, the belief isn't being held for reasons.

How does it work on a real claim?

Here is a made-up headline of a familiar kind: "Students who sit in the front row score 10% higher on exams, study finds."

Who says so? Suppose it comes from a university education department, reported by a news site. That's a reasonable source, but the news summary may not match the study. Find the original.

How do they know? Suppose the study recorded where 400 students in one course chose to sit and compared their final grades. That is an observational study: it found an association, not a cause.

What's missing? One course, one university, one year. "10% higher" could mean 10 percentage points or 10% of the average grade; if the average was 70, that's 77 rather than 80. Were other courses checked and this one reported because it showed an effect?

What else could explain it? The obvious alternative is selection. Students who are already motivated may choose the front row, and they would score higher wherever they sat. The seat may be a sign of motivation, not its cause.

What would settle it? A study that assigned seats at random. If randomly seated front-row students still scored higher, the case for the seat itself would be much stronger.

The verdict isn't "false." It's "an interesting association, weak evidence that sitting in front causes better grades." That is often what critical thinking produces: not a yes or no, but a confidence level with reasons.

What are the common fallacies?

A fallacy is a pattern of reasoning that seems persuasive but doesn't support its conclusion (Internet Encyclopedia of Philosophy). These are the ones met most often:

Fallacy	What it does	Example
Ad hominem	Attacks the person instead of the argument	"He's not even an engineer, so ignore his point about the bridge."
Straw man	Answers a weaker version of the other side's claim	"You want fewer cars downtown? So you want to ban driving."
False dilemma	Presents two options when there are more	"Either we cancel the festival or the town goes bankrupt."
Appeal to authority	Treats an authority's word as proof outside their expertise	"A famous actor says this diet works."
Post hoc	Assumes that because B followed A, A caused B	"I wore my lucky socks and we won."
Hasty generalization	Draws a broad rule from a few cases	"Two people from that town were rude, so people there are rude."
Slippery slope	Claims one step will lead to an extreme without showing why	"If we let students redo one test, soon nobody will study."

Naming a fallacy isn't the same as refuting a claim. An argument can contain a fallacy and its conclusion can still be true; the fallacy only means that argument didn't show it.

What biases get in the way?

Fallacies are errors in arguments; biases are tilts in the person judging them. Confirmation bias, the tendency to notice and remember evidence that fits what you already believe, is among the most pervasive. Cognitive dissonance pushes people to adjust beliefs to fit what they've already done. And the Dunning-Kruger effect describes how people with little knowledge of a subject can overestimate how well they understand it.

Knowing about biases doesn't switch them off. What helps more is procedure: writing down your prediction before you look at results, deliberately seeking the strongest version of the opposing view, and asking someone who disagrees to check your reasoning.

Still argued

Researchers disagree on whether critical thinking is one general skill or many skills tied to particular subjects. One side argues it can be taught directly, through courses on argument and evidence. The other, associated with the philosopher John McPeck and, more recently, the psychologist Daniel Willingham, argues it depends heavily on knowing the subject: a chemist thinks critically about chemistry because they know chemistry. Many educators now combine both, teaching general habits inside real subject matter.

Critical thinking has deep roots in philosophy, from Socrates questioning what Athenians took for granted to modern logic. It is less a body of knowledge than a practice, and it gets better the way any practice does: by doing it on small, everyday claims.

Check yourself

• What are the four core questions, and which one tests you rather than the claim?
• In the front-row example, why is selection a better explanation than the seat itself?
• Why doesn't spotting a fallacy prove that a conclusion is false?

Sources

• Critical Thinking — Stanford Encyclopedia of Philosophy, David Hitchcock, 2022
• Critical Thinking: A Statement of Expert Consensus for Purposes of Educational Assessment and Instruction — Peter A. Facione, American Philosophical Association, 1990, via ERIC
• Defining Critical Thinking — The Foundation for Critical Thinking, Michael Scriven and Richard Paul, 1987
• Fallacies — Internet Encyclopedia of Philosophy