What Is Knowledge?

Every lesson before this one has asked you to weigh evidence, spot bad reasoning, and decide what to believe. But underneath all of it sits a question we have not yet asked directly: what is it actually to know something, as opposed to just being confident, or lucky, or loud? The word "know" gets thrown around loosely — people say they know a diet works, know a stock will rise, know a friend would never lie. Often they mean they strongly believe it. The two are not the same.

Getting clear on the difference is practical, not academic. If you can tell knowing from believing from guessing, you can calibrate how much weight a claim deserves — your own claims included. You stop mistaking the feeling of certainty for the fact of knowledge, and you start asking the more useful question: not "am I sure?" but "what would make this genuine knowledge rather than a hunch that happened to pay off?"

Believing, guessing, and knowing

A guess is a claim you hold with little or no support — you pick an answer and hope. A belief is a claim you actually accept as true, whether or not you have good grounds for it; you can believe something firmly and be completely wrong. Knowledge is the strongest of the three: it is belief that has met further tests. Crucially, you can believe without knowing, but you cannot know without believing — knowing something is a special, well-earned kind of believing it.

Justified true belief

For over two thousand years the standard answer, traced back to discussions in Plato, has been that knowledge is justified true belief. This sets three conditions, all of which must hold. First, belief: you have to actually accept the claim — you cannot know something you do not even think is so. Second, truth: the claim has to be correct — you cannot know a falsehood, only wrongly believe it. Third, justification: you need good reasons or evidence, not a lucky stab. A guess that turns out right satisfies belief and truth but fails justification, which is why we do not call it knowledge.

Why justification matters

Justification is the condition that does the real work. It is what rules out being right by accident. If you correctly guess a coin will land heads, your belief is true, but you did not know it would — nothing about your reasons connected to the outcome. Justification is meant to be that connection: the reasons that make your belief not just true, but reliably, non-accidentally true.

Three people each say it will rain tomorrow. Ana glanced at the sky and simply felt like saying it — a guess. Ben is certain because he had a vivid dream about rain — a belief, but with no real support. Chen checked three forecasts, all showing an incoming front, and reasoned from them — a justified belief. Suppose it does rain. All three turned out right, so all three had a true belief. But only Chen had anything like knowledge: his reasons tracked the actual cause of the rain. Ana and Ben were simply lucky. Being right is not the same as knowing, and this is why justification earns its place.

You might think justified true belief must therefore be knowledge — tick all three boxes and you are done. That felt airtight for centuries. But consider justification that happens to line up with the truth by sheer chance. You look at a wall clock, it reads three o'clock, and it is three o'clock, so you believe it justifiably and truly. Yet suppose the clock stopped exactly twelve hours ago and you had no idea. You have a justified true belief that it is three o'clock — but did you know? Most people say no. Something has gone missing, even with all three conditions met.

In 1963 the American philosopher Edmund Gettier published a three-page paper in the journal Analysis, titled "Is Justified True Belief Knowledge?" It is one of the most cited short papers in philosophy, and it did something rare: it broke a definition that had stood, in some form, for millennia.

Gettier offered simple cases. In one, Smith and Jones apply for the same job. The company president assures Smith that Jones will get it, and Smith has also counted ten coins in Jones's pocket. So Smith forms a justified belief: "the man who will get the job has ten coins in his pocket." As it happens, Smith himself gets the job — and, unknown to him, Smith also has ten coins in his own pocket. His belief was true, and it was justified. Yet it was true for a reason entirely different from the reason he believed it. We would not say Smith knew that the man who would get the job had ten coins in his pocket; he was right by luck. Cases like this, now called Gettier cases, show that justified true belief can be present while knowledge is absent — the right answer reached for the wrong reason. Philosophers have argued ever since about what fourth ingredient is missing, without full agreement, which is itself a useful lesson in how a good counterexample can outlast any tidy fix.

• "If I'm certain, I know it." Certainty is a feeling; knowledge requires truth and good justification, and confident people are often confidently wrong.
• "If it turned out true, I knew it." Being right by luck — a good guess — is true belief without knowledge; the reasons have to do the work.
• "Knowledge means one hundred percent proof." Justification can be strong without being absolute; we know plenty of things on very good, though not conclusive, evidence.
• "Gettier proved knowledge is impossible." He showed the standard definition is incomplete, not that we cannot know things.

Write down five things you would normally say you "know" — about your health, your work, a person, the news, anything. For each, ask the three questions in turn: Do I actually believe it? Is it in fact true, so far as I can tell? And what is my justification — could I be right here only by luck? Move any that fail the third test out of the "know" column and into "believe" or "guess." Notice how much of what felt like knowledge is really confident belief.

Think Like a Maester: Before you claim to know a thing, ask whether your reasons made you right, or whether you were merely right and lucky.

1. What are the three conditions in the "justified true belief" account of knowledge?
2. How does a guess that turns out correct differ from knowledge?
3. Why is justification the condition that rules out being right by accident?
4. In Gettier's job-and-coins case, why do we hesitate to say Smith knew his belief?
5. Give an everyday example where you had a justified true belief but were arguably right only by luck.

Knowing, believing, and guessing are not the same. A guess has little support, a belief is any claim you accept whether or not it is warranted, and knowledge is belief that has earned its status. The classic account, going back to Plato, says knowledge is justified true belief: you must accept the claim, the claim must be true, and you must have good reasons rather than a lucky stab. Justification is the crucial part, because it is meant to guarantee you are right non-accidentally. In 1963 Edmund Gettier showed, in a famous three-page paper, that this is still not quite enough — you can have a justified true belief and yet be right for the wrong reason, which we would not count as knowledge. The lasting lesson is humility about the word "know": ask not just whether you are sure, but whether your reasons truly made you right.

• Claims, Evidence and Arguments
• Where Knowledge Comes From