How We KnowAnswer
Are the colors in space photos real?
A space image can be faithful to measured light without matching human eyesight. Here is how to read the translation.
THE QUESTION LIBRARY HOW WE KNOW
How much evidence should it take to change your mind?
How to change your confidence with evidence, understand Bayesian thinking, and use Occam’s razor without assuming the simplest story must be true.
THE SHORT ANSWER
There is no fixed amount. Change your confidence when evidence favors one explanation over its alternatives. A clue may justify a small shift; a strong test may justify a larger one. Bayesian thinking helps you make that comparison. Occam’s razor discourages unnecessary assumptions, but cannot guarantee the simplest answer is right.
- Confidence can change gradually.
- Compare evidence under competing explanations.
- Simplicity cannot replace a good fit to observations.
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WHAT WE DON’T KNOW YET
There is no agreed starting probability or decision threshold for every everyday claim. Missing alternatives, uncertain observations and assumptions can change the conclusion.
WHAT WOULD CHANGE THIS ANSWER
For a particular belief, a checked observation that fits a competing explanation better, or a failed assumption behind the earlier judgment, would justify revision.
Changing your mind need not mean jumping from “definitely true” to “definitely false.” You can become less certain, prefer a different explanation, or say that you do not know yet. A useful question is: what would I expect to find if this claim were wrong?
Imagine your phone will not charge. A faulty cable and a damaged phone are two possibilities. If it charges with another cable but repeatedly fails with the original under the same conditions, the cable explanation gains support. One failed attempt alone tells you less.
THE LONG ANSWER
Free long answer 1 of 3 this month.
What is Bayesian thinking, in plain words?
Start with how plausible an explanation seemed before the new information. That starting view is called a prior. Then ask how likely the evidence would be under that explanation, compared with its alternatives. Evidence expected equally under competing explanations does little to distinguish them. Evidence much more expected under one shifts the balance toward it. Bayes’ theorem describes this relationship mathematically.
This is not permission to dress a hunch in impressive percentages. A calculation needs stated starting probabilities and assumptions about the evidence. Without defensible numbers, describe the direction of the change and your uncertainty. Starting views also deserve examination: familiarity alone is not evidence that something is true.
What is Occam’s razor, and is the simplest answer always right?
No. When explanations account for the evidence comparably well, Occam’s razor favors avoiding unnecessary extra assumptions. It does not mean choosing whichever story is easiest to imagine. A short explanation that ignores an important observation is not better merely because it is short. A more complicated explanation may be needed to account for what happened. Philosophers debate exactly how simplicity should guide us.
When is a shift enough?
Separate becoming more confident from making a decision. You might borrow a cable while still unsure what is broken, yet want stronger evidence before replacing the phone. The consequences of a mistake affect what action makes sense; they do not make the explanation more likely.
Before checking, write down an observation that would count against your preferred explanation. Then judge the actual result against that expectation. If neither explanation predicts what you find, consider a third possibility rather than forcing a choice.
How sure can we be?
Bayes’ theorem is mathematics. Applying it to everyday beliefs involves judgment, and its philosophical rules remain debated. Neither Bayesian thinking nor simplicity is a truth guarantee. For related questions, see common thinking traps and extraordinary claims and evidence.
Keep asking
Are the colors in space photos real?
Space photos often show real measurements, but their colors may represent light our eyes cannot see.
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God Plays Dice answers big questions in plain words, with sources and one of six honest labels.
Words worth knowing (2)
- Prior
- A starting probability before considering new evidence.
- Bayesian updating
- Revising probabilities in light of evidence.
Sources & further reading
(3) · Checked Oct 5, 2026
- Bayes’ Theorem ↗Stanford Encyclopedia of PhilosophyOfficial scholarly reference; sections 1–3 support the prior/evidence comparison. Checked October 5, 2026.
- Simplicity ↗Stanford Encyclopedia of PhilosophyOfficial scholarly reference; sections 1–2 and 5 distinguish simplicity from evidential fit and discuss its disputed justification. Checked October 5, 2026.
- Bayesian Epistemology ↗Stanford Encyclopedia of PhilosophyOfficial scholarly reference; sections 1.5,1.8–1.9 explain prior choice, updating disputes and idealization. Checked October 5, 2026.
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