Critical Thinking Foundations: Claims, Evidence & Assumptions

Most of what you read, hear, and scroll past is trying to convince you of something. This chapter gives you a simple, repeatable way to take any statement apart and ask: is this actually true, and does it actually follow? When you can do that, you stop being a passive reader who just nods along — and you start spotting gaps, generating your own ideas, and speaking with more confidence because you know why you believe what you believe.

What critical thinking really is (and isn't)

Let's clear up the biggest misunderstanding first.

Critical thinking

Carefully checking what is true and what reasonably follows from it, before you accept it. The word "critical" here means careful judgment, like a film critic studying a movie — not "negative" or "harsh".

Critical thinking is not arguing with everyone. It is not assuming everything is a lie. It is a calm, neutral habit: you slow down, look at the parts, and decide what's solid. You apply it to people you agree with just as much as people you disagree with — that's the hard part.

The anatomy of an argument

In everyday life an "argument" sounds like a fight. In thinking, an argument just means: a claim plus reasons given to support it. Almost everything persuasive has the same skeleton. Learn the four parts and you can dissect anything.

Claim (also called the conclusion)

The main thing the author wants you to believe. The point.

Reasons (also called premises)

The "because" statements offered to support the claim.

Evidence

The facts, data, examples, or sources that back up the reasons.

Assumptions

The unstated beliefs that must be true for the reasons to actually support the claim. These are usually invisible — and that's where most weak arguments hide.

   ASSUMPTIONS  (hidden, unstated)
        |  must hold for the link to work
        v
  EVIDENCE  -->  REASONS  -->  CLAIM
  (facts)        (because…)     (the point)

The four questions that crack open anything

You do not need a philosophy degree. You need four questions, in order. Memorize them.

1. What is being claimed? State the main point in one plain sentence. If you can't, the author was vague — already a warning sign.
2. What's the evidence? What facts or data support it? Is there any, or just confident tone?
3. What's being assumed? What must be true, but went unstated, for the evidence to lead to the claim?
4. What could make this wrong? What missing fact, alternative explanation, or counter-example would break it?

That last question is the one beginners skip. Our brains love information that confirms what we already think — psychologists call this confirmation bias. Asking "what would prove me wrong?" is the deliberate antidote.

Facts vs opinions vs inferences

People mix these three together constantly, often inside a single sentence, and it muddies clear thinking. Pull them apart.

An inference is the sneaky one. It sounds like a fact but it's a guess built on facts. "Sales dropped, so customers hate the new design" — the drop is a fact; "customers hate the design" is an inference. Maybe a competitor ran a sale. Maybe it was a slow season. Labeling inferences as inferences keeps you honest and opens your mind to other explanations — which, by the way, is exactly how you generate fresh ideas from data instead of jumping to the first story.

Intellectual humility and "strong opinions, loosely held"

Intellectual humility means accepting, in advance, that you might be wrong — and being genuinely willing to update when better evidence shows up. It is the engine of all real thinking. Without it, you just defend your first guess forever.

A useful phrase from Silicon Valley (often credited to forecaster Paul Saffo) is "strong opinions, loosely held." It means: form a clear view so you actually have something to test — but hold it loosely enough to drop it the moment the facts turn against it. You commit to a position and to changing your mind. Both at once.

A reusable checklist for any article or report

Tape this near your desk. Run it on anything that's trying to persuade you — a news story, a sales pitch, a report your boss forwarded.

• Claim: What's the one-sentence point?
• Source: Who's saying it, and what do they gain if I believe it?
• Evidence: Real data, or just vibes and adjectives?
• Sample: "Most people…" — how many, chosen how? (A survey of 8 friends isn't "most people".)
• Assumptions: What unstated belief is this leaning on?
• Other explanations: Could the same facts point to a different conclusion?
• What's missing: What did they conveniently leave out?
• Falsify it: What single fact, if true, would break this argument?

This connects straight to your bigger goals. When you read this way, you're not just judging the author — you're building schemas, the mental frameworks cognitive scientists say turn scattered facts into connected understanding. Spotting a hidden assumption in someone's argument is often the exact seed of your own original idea: "If they assumed X, what happens if X is false? That's an opportunity nobody's looking at."

Practice

1. Dissect three claims. Take three statements you read today and, for each, write the claim, the evidence, and one hidden assumption in three short lines.
2. Label the sentence. Find a paragraph (a review, a news lead) and tag each sentence Fact / Opinion / Inference. Notice how often inferences are dressed up as facts.
3. Argue against yourself. Pick one belief you hold strongly. Write the single best piece of evidence that would prove you wrong. Sit with it for two minutes.
4. Run the checklist. Apply the 8-point checklist above to one full article and write your verdict in two sentences: do you buy it, and why or why not?