The big idea: You say 'I know it's raining' and 'I know my friend's name' without a second thought.
The question: But push on it: what's the difference between really knowing something and just believing it, or getting lucky with a guess? Epistemology — the study of knowledge — starts by trying to pin that down.
There's a famous three-part answer. To know that something is the case, philosophers say three things must line up at once.
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The traditional definition, traced back to Plato, breaks knowledge into three ingredients that must all be present.
Knowledge = justified true belief (JTB)
Belief
You have to actually believe it. You can't know something you don't even think is so.
Truth
It has to be true. You can't know a falsehood — you'd just be mistaken.
Justification
You need a good reason. This rules out lucky guesses that just happen to be right.
Belief · Truth · Justification
Checkpoint — JTB: In one line: you KNOW something when you believe it, it's true, and you have a good reason for it — all three at once. Miss any one and it isn't knowledge.
Go further — higher-level insight: The philosopher Edmund Gettier famously built odd cases where all three boxes are ticked — true, believed, justified — yet it still feels like luck, not knowledge. You don't need the details, but knowing JTB has been challenged shows the definition isn't the last word. It's a great point to nod to for a top-band answer.
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'Know' actually covers three quite different things — and the JTB recipe only really fits the first.
Knowing-THAT (facts)
- 'I know that water boils at 100°C'
- A fact you could put into words
- This is what JTB is about
Knowing-HOW (skills)
- 'I know how to ride a bike'
- A skill in your body, hard to put into words
- You can do it without stating a single fact
A third kind: knowing by acquaintance: There's also knowledge by acquaintance — 'I know Paris' or 'I know my best friend'. This isn't a fact you memorised or a skill you practise; it's knowing something because you've met it directly. You could know every fact about Paris from a book and still not know Paris the way someone who has walked its streets does.
Checkpoint — three knowings: In one line: knowing-that is facts, knowing-how is skills, and knowing-by-acquaintance is direct contact. When people argue about 'knowledge', they usually mean the first.