Ask someone how they know the sun will rise tomorrow and you get a shrug, because the question sounds like a trick. Ask how they know they are not dreaming and the shrug turns into something warier. Both are requests for exactly the same thing, and the fact that they feel so different is the first evidence that the word "know" is carrying more than it looks.
This course spends fifteen lessons on three questions people argue about badly: what is actually real, whether there is a God, and whether we are anything special. None of them can be argued about well until the standard for a good answer is fixed, and that is the job of this lesson. It is the least dramatic in the course and the one the other fourteen lean on.
What a claim to know contains
Knowing is not the same as believing, because a belief can be false and knowledge cannot. If you believed the meeting was at three and it was at four, you did not know it was at three. You were wrong. So truth is one condition, and it is a condition about the world rather than about you.
A true belief is not enough either. Someone who backs a horse because its name reminds them of a childhood dog, and wins, held a true belief and knew nothing. They were lucky. So something has to connect the believing to the truth, and the traditional name for that something is justification: a reason, evidence, a reliable route by which the belief arrived.
Together these give the analysis that held the field for two thousand years. You know that p when
- p is true,
- you believe that p,
- you are justified in believing that p.
It is usually credited to Plato, though the credit is loose. In the Theaetetus, written around 369 BC, Socrates puts up the proposal that knowledge is true judgement with an account, takes it seriously for several pages, and then rejects it without finding anything to put in its place. The dialogue ends in failure, which is worth knowing: the analysis is not handed down as doctrine, it is the best surviving candidate from an argument that Plato thought he had lost.
The Meno gives the better image. True opinions, Socrates says, are like the statues of Daedalus, which were reputed to run away in the night unless somebody tied them down, and what ties an opinion down is working out the reason for it. Justification is a tether. An untethered true belief is not worthless, it is just liable to be gone in the morning.
Notice what each condition does on its own, because the sceptical arguments in the next lesson go after them one at a time. Truth is a fact about the world. Belief is a fact about you. Justification is the relation between the two, and it is the only one of the three you can inspect from where you are sitting. That asymmetry is the entire opening through which scepticism gets in.
Example. Three people each say "there is milk in the fridge". The first checked an hour ago and there is. The second has not looked but always buys milk on Tuesdays, and today is Tuesday, and there is milk. The third insists there is milk because they feel there is, and there is. Which of them knows?
The first clearly does: true, believed, and justified by the right sort of route. The third clearly does not, because the belief is true and nothing connects it to the truth, which is the horse and the childhood dog again. The second is the interesting one and the honest answer is that it depends on how good the Tuesday habit is. If they have bought milk every Tuesday for ten years, the inference is a decent one and most people would grant knowledge. If they have done it twice, it is a guess wearing a reason. Justification is not a switch, it comes in degrees, and the analysis above hides that by writing condition three as though it were yes or no.
Now you. A doctor reads a scan and concludes the patient has a fracture. The scan is genuine, the reading is competent, the patient does have a fracture. Later it emerges the scan was mislabelled and belongs to a different patient, who also has a fracture in the same place. Did the doctor know?
Answer
No, though all three conditions are met. The belief is true, it is believed, and the doctor's justification is as good as a doctor's justification ever gets. What has gone wrong is that the justification is not connected to the fact that makes the belief true: it reaches through to the other patient's bone. Had the second patient's fracture been anywhere else, the doctor would have said the same thing and been wrong. The truth of the belief is an accident relative to the reason for it, and that is exactly the pattern of the next section.
Three pages that broke it
In June 1963 Edmund Gettier published a paper in Analysis, volume 23, running from page 121 to page 123. It was three pages long, it was his only significant publication, and it ended the two thousand year consensus.
His first case. Smith and Jones have both applied for a job. The company president has told Smith that Jones will get it. Smith has also counted the coins in Jones's pocket and found ten. Smith therefore has strong evidence for
the man who will get the job has ten coins in his pocket
and believes it on that basis. As it happens, Smith gets the job, and, unknown to him, Smith has ten coins in his own pocket. The belief is true. It is believed. It is justified, by the president's word and a count. And it is plainly not knowledge, because Smith is right about a proposition that turns out to be about himself for reasons that had nothing to do with him.
Bertrand Russell had constructed the same shape fifteen years earlier, in Human Knowledge: Its Scope and Limits (1948), with a cleaner example that is now the standard teaching case. A clock on the wall stopped exactly twenty four hours ago. You glance at it at precisely the moment it is right, read the time off it, and form a true belief about the hour, justified by the ordinary and normally excellent practice of looking at clocks. You do not know the time. You would have said the same thing a minute earlier and been wrong.
The common structure is this: justification that does not reach through to the truth, so that the belief is true by accident. That matters because ruling out accident was supposed to be justification's entire job. The third condition was added precisely to exclude the horse named after the dog, and Gettier's cases show that it does not exclude it, it only makes the luck harder to see.
Example. Someone owns a car and you have seen them driving it for years, so you believe "somebody in this office owns a car". In fact they sold it last week and are driving a hire car, but a new colleague you have never spoken to owns one. Is this a Gettier case, and what makes the answer yes or no?
Yes, and the diagnostic is the same as before. The belief is true, believed, and justified. But the justification runs through a false intermediate claim, that your colleague still owns a car, while the truth of your belief rests on a completely separate fact you know nothing about. Take away the new colleague and your evidence is untouched while your belief becomes false, which shows the evidence was never tracking the thing that made it true. That test, asking whether the belief could have been false with the evidence unchanged, is the most useful thing to carry away from this section.
Now you. A stopped clock is on the wall. You know it is stopped, but you also know it stopped at exactly 6:00 and you have independent reason to think it is now within a minute or so of six. You look, read 6:00, and believe it is about six. It is. Is this a Gettier case?
Answer
No, and the difference is instructive. Here the clock face is not what is doing the work: your independent reason for thinking it is about six is. The reading is a confirmation rather than a source, and had the time been half past five you would not have been fooled, because you would have noticed the conflict with what you already had reason to believe. Apply the test from the example: could the belief have been false with your evidence unchanged? Not easily, since the evidence includes the independent estimate. The belief is safe, not lucky, and safety turns out to be one of the standard repairs discussed next.
Sixty years of repairs
The obvious first patch was to require that your reasoning contain no false step. Michael Clark proposed it within months, in 1963, and it handles both of Gettier's cases, since Smith's inference runs through the falsehood that Jones will get the job. It fails on cases built without any false step, which were duly built.
Reliabilism, developed by Alvin Goldman from 1967, drops justification as a matter of having reasons and requires instead that the belief be produced by a process that mostly yields truths. Robert Nozick proposed sensitivity in Philosophical Explanations (1981): you know only if, had p been false, you would not have believed it. Ernest Sosa preferred safety: you know only if you could not easily have believed p falsely. Timothy Williamson, in Knowledge and Its Limits (2000), took the more radical line that the whole project is misconceived, that knowledge is not built out of belief plus extras but is a basic state in its own right.
Sixty years on there is no agreed analysis of knowledge. That sounds like a failure and for the purposes of this course it is not, because nothing later depends on having a definition. What the course needs is the three parts, the recognition that luck defeats knowledge, and the safety test. Those survive every version of the dispute.
Knowing is not being certain
Here is the distinction the next three lessons live on, and it is the one most often collapsed in ordinary argument.
Certainty is a maximum. To be certain of p is for there to be no possibility whatever that you are wrong. Descartes, in the Meditations of 1641, went looking for beliefs with that property, on the reasonable-sounding ground that a house built on anything less could fall down. He found almost nothing, and the next lesson follows him into that hole.
Knowledge, on almost every modern account, demands much less. Fallibilism is the position that you can know that p while it remains possible that you are wrong about p. You know your own name, and you could conceivably have been swapped at birth. You know you had breakfast, and you could conceivably be misremembering. Requiring certainty for knowledge does not make knowledge more rigorous, it makes the word useless, since on that standard almost nothing outside mathematics and your own current experience is ever known by anyone.
Why this matters so much: the sceptical arguments coming in the next lesson are excellent at establishing that you cannot be certain. If knowledge required certainty, that would be the end of the matter and the course could stop at lesson two. Because it does not, the sceptic has further work to do, and watching exactly how that further work is attempted is what lessons two to four are about.
Example. A friend says: "You can't be one hundred percent sure the external world exists. So you don't really know it does. So believing in it is just faith, like religion." Where does this go wrong?
At the second step, and the error is an equivocation on "really know". The first claim is true and almost nobody disputes it. The second slides from an absence of certainty to an absence of knowledge, which only follows if knowledge requires certainty, and that premise is never stated because stating it would reveal how implausible it is. The third step compounds it by treating everything short of certainty as equally unsupported, which throws away the entire difference between a belief with overwhelming evidence and one with none. Lesson five is about how to grade that difference properly, and lesson twelve returns to this exact move, because it is the commonest bad argument in debates about God, deployed by both sides.
Now you. A second friend replies: "Fine, but if you admit you might be wrong about the external world, you have to admit you might be wrong about anything, and then no belief is better than any other." What has been assumed?
Answer
That fallibility is all-or-nothing, when it comes in degrees. Conceding that you might be wrong about the external world does not commit you to thinking you might be wrong about it to the same degree you might be wrong about next week's weather. The argument treats "not certain" as a single category containing everything, and once you are in it, nothing distinguishes its members. That is the flattening the previous answer identified, stated as a principle. Notice too that the conclusion is self-undermining in the usual way, since if no belief is better supported than any other then the friend's own conclusion has nothing going for it either.
The standard an argument has to meet
One paragraph of housekeeping, because the rest of the course is arguments and they are all judged the same way. An argument is valid when its premises cannot all be true while its conclusion is false, and sound when it is valid and its premises are in fact true. Validity is about shape and is settled without leaving your chair; soundness needs the world. The practical consequence, which the logic course develops and this one simply uses, is that if you find an argument valid and you reject its conclusion, you are obliged to name a premise you deny. "I do not find it convincing" is not a move. Every major dispute in this course, including the existence of God and whether we are alone, can be laid out as one valid argument with three or four premises, and each position is a choice about which premise to give up. The final lesson does that explicitly.
What philosophers actually think
The 2020 PhilPapers Survey, run by David Bourget and David Chalmers, asked close to 1800 professional philosophers where they stood on a long list of questions. On the external world, 79.5 percent accepted or leaned towards non-skeptical realism, 6.6 percent towards idealism, and 5.4 percent towards skepticism, with the rest choosing something else. The proportions in the 2009 round were close to the same.
Two things follow. First, scepticism is not a live position among people who spend their careers on this, which is worth knowing before spending a lesson on it. Second, and more important, that is not an argument, and you should not take it as one. The reason to read the next lesson is that the sceptical arguments are genuinely hard to answer, and the interesting question is not whether professionals believe the conclusion but what exactly they think is wrong with the reasoning. Four out of five philosophers rejecting a conclusion while still teaching the argument every year is a signal that the argument is doing something useful.
The three conditions are now on the table, along with the fact that the third one leaks, and the distinction between knowing and being certain. The next lesson takes the strongest arguments ever built for the claim that condition three never holds for anything outside your own head, and states them at full strength before laying a finger on them.