Week 4 · Chapter

Chapter 4 — Knowledge I: Doubt, Reason, and Experience

Introduction to Philosophy Free sample edition

The module's primary reading · ~40–50 minutes · Objective 3 (explain central positions in metaphysics and epistemology — Plato's Forms, rationalism and empiricism, skepticism, and the analysis of knowledge — and evaluate the arguments for them), with Objective 2 (identify arguments, put them in standard form, judge whether they are valid, sound, or inductively strong, and test claims with counterexamples and thought experiments) carried forward · Read it first this week; the lecture, the slide deck, the tutorial, the quiz, and the Philosophy Workshop all build on it.

In plain language

This week asks two old questions: what can you know for sure, and where does your knowledge come from? One answer, rationalism (the view that some real knowledge of the world comes from thinking alone), says careful reasoning can take you there. The other answer, empiricism (the view that all knowledge of the world comes from experience), says everything you know started with what you saw, heard, felt, and noticed. You follow one thinker who doubted almost everything to see what was left, and two thinkers who traced knowledge back to experience. You also meet a puzzle about predictions, and your goal is to state each side fairly and test it, not to pick a winner.

Why you already care about this

You wake up with your heart racing. In the dream you overslept, ran three blocks, and watched the 7:40 bus pull away without you. For a few seconds you are sure it happened. Then you see the clock, and it is 6:15. Nothing was real, yet it felt as real as this page.

That leaves a strange question behind. While you were dreaming, you had no idea you were dreaming. So what sign tells you that you are awake right now? You probably feel sure. But can you name the test you used? In 1641 the philosopher René Descartes asked this kind of question on purpose, in a book called the Meditations on First Philosophy. He wanted to find out whether anything at all could survive his doubts.

The other side of the week starts somewhere more ordinary: a weather forecaster looking at past records and saying "rain by noon". The forecaster trusts that the future will look like the past. Most of us do, every hour of the day. The philosopher David Hume asked what gives us the right to.

By the end of this chapter you will be able to:

state rationalism and empiricism in their strongest forms, each with its strongest objection and a reply;
explain Descartes' methodical doubt as a method, not a belief that nothing is real;
walk through the dream argument and the evil deceiver;
put the cogito in standard form and judge it valid;
name the right source for "I think, therefore I am", with its translator;
key Dataset B's argument 1 for validity without keying it for soundness;
explain Locke's white paper and his two sources of ideas;
state Hume's problem of induction and say what it does not claim.

Carry this line with you: doubt is the tool, not the conclusion.

Where we are

Already covered: Week 1, What Philosophy Is: Socrates and the Examined Life, gave you the habit of testing a belief with a counterexample. Week 2, Arguments and Thought Experiments, gave you standard form, validity, and the thought experiment as a test. Week 3, Plato: Reality and the Forms, asked whether what we perceive might be like shadows on a cave wall.
This week: where knowledge comes from, reason or experience; Descartes' methodical doubt, the dream argument, the evil deceiver, and the cogito; Locke's white paper; and Hume's problem of induction.
How they connect: Plato's cave asked whether appearances could fool us. Descartes asks the same worry in its sharpest form, and you use Week 2's standard form to test the belief he finds that survives.
Where we're going next: Week 5, Knowledge II: What Knowledge Is, asks what knowledge is in the first place, and puts a famous definition of it to the test.

Two tools from earlier weeks do the heavy lifting here. From Week 2, you need standard form: premises numbered P1 and P2, the conclusion last, marked C and introduced by "So", and any premise you add marked "(added)". You also need valid, which means the conclusion cannot be false if the premises are true. From Week 1, you need the habit of asking who wrote the words on the page. That habit matters a lot this week, because the most famous sentence in the chapter has a surprising source.

1. Name the two answers: rationalism and empiricism

Think about two things you know. First: if you have 3 friends at a table and 2 more sit down, there are 5 people at the table. Second: the coffee at the corner shop is bitter. You learned the second one by tasting it. The first one seems different. You could work it out with your eyes closed. Philosophers have argued for centuries about which kind of knowing comes first.

Here are the two answers, stated the way their best defenders would.

Rationalism — the view that some substantive knowledge of the world comes from reason independent of experience. "Substantive" means the knowledge tells you something real about the world, not only how words are used.
Empiricism — the view that all knowledge of the world comes from experience. Experience here includes what you sense and what you notice going on inside your own mind.

A rationalist does not say the senses are useless. A rationalist says reason can reach some truths the senses could never give you, and that those truths can be the firmest ones you have. An empiricist does not say reasoning is useless. An empiricist says reasoning only works on material that experience supplied first.

This week, Descartes carries the reason side. His search turns up a first truth he reaches by thinking, at a moment when he is doubting every sense he has. Locke and Hume carry the experience side. Locke traces every idea to experience, and Hume asks how far experience can take us.

Misconception check — reason against experience? Students often think empiricists reject reasoning and rationalists reject the senses. It is tempting because the two names sound like opposite teams. The cure is to ask what each view says about the source of knowledge of the world. Both sides reason, and both sides use their senses. They disagree about whether reason alone can ever be the source.

2. Use doubt as a method: Descartes' starting move

Picture a cook sorting a basket of strawberries before a party. A few berries have soft spots, and mold spreads. The cook could check each berry one at a time while the party waits. Or the cook could tip the whole basket out, set every berry aside, and put back only the ones that pass a close look.

Descartes treats his beliefs the second way. In the Meditations on First Philosophy (1641), he sets aside every belief he can find any reason to doubt, so that he can look for one that cannot be doubted.

Methodical doubt — Descartes' method of setting aside every belief that has any reason for doubt, in order to find a belief that has none.

The word "method" is the key. Descartes is not reporting that he has stopped believing in the world. He is running a test. A belief does not have to be false to be set aside. It only has to be open to some doubt, however small. The bar is high on purpose. If even one belief clears it, that belief can serve as solid ground.

Here is an honesty note about this move: methodical doubt is a strict standard, and many people think everyday knowledge does not need to meet it. The course keeps that question open. You will see it again in section 7, where an argument depends on whether knowledge really requires certainty.

Misconception check — doubting is not denying. Students often think Descartes believed that nothing is real. It is tempting because he doubts so much. The cure is to keep the method and the result apart. Setting a belief aside "for now, until it passes the test" is not the same as believing it is false. Doubt is his tool, not his conclusion.

3. Test your senses with the dream argument

Go back to the bus dream. In the dream, you felt the cold air, heard the brakes, and saw the bus number. Every sense reported something. None of it was real. And while it was happening, nothing in the experience said "this is a dream".

That is the heart of the dream argument. Dreams can feel as real as waking life, and there seems to be no sure sign that tells the two apart. So any belief that rests on the senses can be doubted.

Dream argument — Descartes' argument that, because no sure sign tells waking experience from a vivid dream, any belief that rests on the senses can be doubted.

Worked example — the dream argument in standard form. Find the conclusion first: beliefs that rest on the senses can be doubted. Then find the reasons and number them.

P1: If no sure sign tells waking from dreaming, then any belief that rests on the senses can be doubted.

P2: No sure sign tells waking from dreaming.

C: So any belief that rests on the senses can be doubted.

In words: if P1 and P2 are true, the conclusion cannot be false. The form is modus ponens, from Week 2: if P then Q; P; so Q. So the argument is valid. Whether P2 is true is a separate question, and a fair one to argue about.

Notice what the conclusion does not say. It does not say you are dreaming now. It says your senses alone cannot settle the matter with certainty. That is enough for Descartes' method, because the method sets aside any belief open to doubt.

Misconception check — "so I am dreaming right now"? Students often read the dream argument as claiming that life is a dream. It is tempting because the example is so vivid. The cure is to read the conclusion word for word: beliefs from the senses can be doubted. A belief that can be doubted may still be true. The argument removes certainty, not reality.

4. Suppose everything is faked: the evil deceiver

You might think a few beliefs survive even a dream. In a dream, 2 plus 3 still seems to make 5. Descartes' method needs a test that reaches beliefs like that too, and the week's thought experiment, the Week 4 case on the course's thought-experiment roster, supplies it.

Descartes asks you to suppose that a powerful deceiver is faking everything you perceive. The deceiver might even be fooling you about the things that seem most basic. This is the evil deceiver, from the Meditations on First Philosophy (1641).

Evil deceiver — the thought experiment in Descartes' Meditations on First Philosophy (1641) that supposes a powerful deceiver is faking everything a person perceives, to ask what could still be known.

The roster gives the case a question: What can I know if everything I perceive might be faked? It tests two things this chapter teaches, methodical doubt and the cogito.

A modern variation makes the case feel close. (This variation is the course's own, labelled as a variation; Descartes did not write it.) Imagine a video call with someone you trust. Their face, voice, and timing all look right. Now suppose software is generating every frame. Nothing on the screen could prove it is real. The evil deceiver stretches that worry to everything you perceive, all at once.

Misconception check — a supposition, not a belief. Students often think Descartes believed an evil deceiver exists. It is tempting because the case is described in such detail. The cure is to read it as a thought experiment: an imagined scenario used to test a claim. Descartes supposes the deceiver so he can see which beliefs still stand. The point is the test, not the monster.

5. Find what survives: the cogito in standard form

Here is the turn. Suppose the deceiver is real and working hard. It fools you about the sky, the floor, and your own hands. Is there anything it cannot fool you about?

The Second Meditation concludes, said in words, that the proposition that he exists is necessarily true whenever he thinks it. Put in the course's own words: to be fooled, you have to be there to be fooled. Every doubt is a thought, and every thought needs someone doing the thinking.

Cogito — short for the Latin cogito, ergo sum, the name for Descartes' finding that he must exist whenever he thinks; the Second Meditation puts the finding as the proposition that he exists being necessarily true whenever he thinks it.

Worked example — the cogito in standard form. This is the course's reconstruction (illustrative), written to teach the step. The first premise is one the reconstruction adds, so it is marked.

P1 (added): If I am thinking, then I exist.

P2: I am thinking, even if a deceiver is fooling me about everything else.

C: So I exist.

Now judge it. The pattern is if P then Q; P; so Q. That is modus ponens. So the argument is valid: if P1 and P2 are true, C cannot be false. In one plain sentence: methodical doubt leaves Descartes one belief that no deceiver can shake, that he exists whenever he thinks.

Look at P2 closely, because it is the clever part. The more the deceiver fools you, the more you think. Doubting is thinking. So the deceiver's own work keeps P2 true.

Here is a caveat worth keeping. A critic can ask whether P1, the added premise, is itself something Descartes is entitled to at this stage. Another critic asks whether "I am thinking" shows a lasting "I", or only that some thinking is going on. A defender of the cogito replies that a thought with no thinker makes no sense, so the thinking and the thinker come together. You do not need to settle that debate. You do need to state both sides fairly.

Misconception check — valid is not the same as proven. Students often think that once an argument is valid, its conclusion is proven. It is tempting because "valid" sounds like a seal of approval. The cure is Week 2's distinction. Valid means the conclusion cannot be false if the premises are true. Whether the premises are true is a second question. Here, most readers accept P2, and critics press on P1.

6. Get the famous words from the right book

Ask ten people where "I think, therefore I am" comes from, and many will say the Meditations. That is the most common slip in the whole week, and it is easy to make. Here is the record.

The wording comes from Descartes' earlier book, the Discourse on the Method (1637), Part IV. In John Veitch's English translation, the line reads:

"I think, therefore I am (COGITO ERGO SUM)" — René Descartes, Discourse on the Method, Part IV, trans. John Veitch (Dataset C, Q2)

Three facts travel with that line.

The words on the page are Veitch's English, not Descartes' own. Descartes wrote the Discourse in French, and the 1637 French reads "je pense, donc je suis".
The Latin in parentheses is Veitch's own insertion. It is not in Descartes' French at that spot.
The Latin "cogito, ergo sum" appears in later Latin works, among them the Principles of Philosophy (1644).

Worked example — putting the books in order. Line up the two dates the chapter pins for Descartes' main works.

The Discourse on the Method: 1637.
The Meditations on First Philosophy: 1641.
The gap: 1641 − 1637 = 4 years.

In words: the Discourse came out 4 years before the Meditations. So the famous wording is older than the Meditations and is not printed in them. The Meditations carry the same idea, said in different words: the proposition that he exists is necessarily true whenever he thinks it.

Discourse on the Method — René Descartes' work of 1637, written in French, whose Part IV is the source of the wording rendered in English as I think, therefore I am.
Meditations on First Philosophy — René Descartes' work of 1641, which uses methodical doubt, the dream argument, and the evil deceiver to search for a belief that cannot be doubted.

Misconception check — two mix-ups in one line. Students often place "I think, therefore I am" in the Meditations, and they often read the English as Descartes' own words. Both are tempting because the line is so famous that its source fades from view. The cure is to say all three layers out loud: Descartes wrote the idea in French in the Discourse (1637); Veitch put it into English and added the Latin in parentheses; the Meditations (1641) state the idea in different words.

7. Test an argument about certainty

Descartes wanted certainty: a belief no doubt could touch. Is certainty what knowledge requires? Here is an argument from Dataset B, the argument set (illustrative), that turns on that question. Its premises were written for teaching, and the course does not endorse them.

Worked example — Dataset B, argument 1, in standard form. Lay the argument out line by line, then name its form.

P1: If knowledge requires certainty, then I know very little.

P2: Knowledge requires certainty.

C: So I know very little.

Name the form: if P then Q; P; so Q. It is modus ponens. Key it: valid. If both premises were true, the conclusion could not be false.

Now the harder question. Is it sound? Sound means valid with all premises true. P1 looks hard to deny, since very few of your beliefs could pass Descartes' strict test. P2 is where the real disagreement lives. Whether knowledge requires certainty is an open question, and next week takes up what knowledge is. So the course keys this argument for validity only, never for soundness.

Misconception check — valid does not mean sound. Students often call argument 1 sound because it is valid and its conclusion sounds gloomy but plausible. It is tempting because the shape is so tidy. The cure is to ask the two Week 2 questions separately: would the conclusion have to be true if the premises were? Yes. Are both premises true? That depends on an open question about certainty, so the argument is keyed valid and nothing more.

8. Trace ideas to experience: Locke's white paper

Watch a toddler meet a lemon for the first time. The face scrunches. Now the toddler has an idea of "sour" that was not there a minute ago. Where did it come from? From the tongue, from experience.

John Locke's An Essay Concerning Human Understanding (title page dated 1690) builds a whole view of the mind on cases like this. Locke compares the mind at the start to white paper: blank, with no innate ideas, ideas present from birth. Experience writes on the paper. Every idea comes from experience through 2 sources:

Sensation — the source of the ideas a person gets from the senses, such as the ideas of red, warm, or loud.
Reflection — the source of the ideas a person gets by noticing the mind's own workings, such as thinking, doubting, and willing.

This is empiricism in its strongest form. It does not ask you to believe the mind is passive. Reflection means the mind's own activity is part of experience too. When you notice yourself doubting, you gain the idea of doubt.

Worked example — the timeline so far. Place Locke's Essay against Descartes' Meditations.

The Meditations: 1641.
The Essay's title page: 1690.
The gap: 1690 − 1641 = 49 years.

In words: Locke's Essay is dated 49 years after the Meditations, so Locke wrote with Descartes' search for certainty already in the world.

Misconception check — blank is not idle. Students often think "white paper" means the mind does nothing until the world pushes on it. It is tempting because paper is passive. The cure is Locke's second source. Reflection is the mind noticing its own thinking, doubting, and willing. The paper starts blank, but the mind that holds it is busy.

9. Question every prediction: Hume's problem of induction

Back to the forecaster. Here is the forecaster's reasoning (illustrative), laid out in standard form.

P1: On every past morning in the records when the barometer fell overnight, rain followed.

P2 (added): The future will resemble the past.

C: So rain will follow this time.

The forecaster never said P2 out loud. But without it, P1 says only what happened before. P2 is what carries the record into tomorrow.

David Hume's An Enquiry Concerning Human Understanding (1748) raises the problem of induction: no non-circular argument shows that the future will resemble the past. Here is why. Suppose you try to support P2 with experience: "The future has resembled the past every time before, so it will again." That reasoning uses P2 to prove P2. It is a circular argument, one that assumes what it sets out to support.

Problem of induction — the problem, raised in David Hume's An Enquiry Concerning Human Understanding (1748), that no non-circular argument shows the future will resemble the past, so reasoning from past patterns to new cases lacks a clear justification.
Circular argument — an argument that assumes, among its premises, the very conclusion it sets out to support.

Worked example — the whole timeline. Add Hume's Enquiry to the dates the chapter has pinned.

The Essay's title page to the Enquiry: 1748 − 1690 = 58 years.
The three gaps together: 4 + 49 + 58 = 111 years.
A check from the two ends: 1748 − 1637 = 111 years.

In words: both ways of counting give 111, so the week's works span 111 years, from the Discourse to the Enquiry.

The problem bites empiricism hardest. If all knowledge of the world comes from experience, and experience only shows what has happened, then knowledge of what will happen needs something experience cannot supply on its own.

Misconception check — the problem does not say science is useless. Students often think the problem of induction means forecasts, medicine, and science are worthless. It is tempting because the problem sounds so total. The cure is to ask what the problem is about. It asks what justifies reasoning from past patterns. It does not tell the forecaster to stop forecasting or the nurse to ignore a person's medical history. A question about justification is not a verdict on usefulness.

10. Weigh the two positions, each at its strongest

You now have both sides in hand. The fair way to compare them is the course's way: each view in its strongest form, its strongest objection, and a reply from its own side.

Rationalism and empiricism compared: central claim, strongest objection, and a reply from each side
Position	Central claim	Strongest objection	Reply from its side
Rationalism	Some substantive knowledge of the world comes from reason independent of experience.	The mind begins as white paper, so even the ideas of thinking and existing must come from experience.	Even if the idea came from reflection, what makes the cogito certain is reason, not trust in the senses.
Empiricism	All knowledge of the world comes from experience, through sensation and reflection.	The problem of induction: experience shows only what has happened, and no non-circular argument shows the future will resemble the past.	The problem asks what justifies a prediction, not whether to keep predicting; reasoning from past patterns is still how we learn from evidence.

Read the table one row at a time. For rationalism, the cogito is the best example: Descartes reaches it by thinking, while every sense is in doubt. The empiricist objection says the raw ideas still came from somewhere, and Locke says that somewhere is experience. The rationalist reply grants that point and moves the weight to certainty: the idea may come from reflection, but reason, not the senses, is what makes it certain.

For empiricism, the strongest form traces every idea to sensation or reflection. Hume's problem is the strongest objection, and it comes from inside the experience side. The empiricist reply separates two questions: what justifies a prediction, and whether to rely on one. A nurse, a forecaster, or a scientist still learns from evidence while philosophers argue about the first question.

Where do the two sides disagree, in one plain sentence? They disagree about whether reason alone can ever be a source of knowledge of the world, or whether every such piece of knowledge starts in experience.

A limitation to keep in view: this week compares only two positions, and the course keys no verdict on which one is right. Your job is to state each one so well that its own defenders would sign it.

Misconception check — one objection does not end a view. Students often think that once a view has a strong objection, it is defeated. It is tempting because objections sound decisive when you first hear them. The cure is to look for the reply. Both positions here have serious objections and serious replies. That is what an open question looks like.

Key terms

Rationalism — The view that some substantive knowledge of the world comes from reason independent of experience. (first used: 1. Name the two answers: rationalism and empiricism)
Empiricism — The view that all knowledge of the world comes from experience, through the senses and through noticing the workings of one's own mind. (first used: 1. Name the two answers: rationalism and empiricism)
Methodical doubt — Descartes' method of setting aside every belief that has any reason for doubt, in order to find a belief that has none; it is a tool for testing beliefs, not a belief that nothing is real. (first used: 2. Use doubt as a method: Descartes' starting move)
Dream argument — Descartes' argument that, because no sure sign tells waking experience from a vivid dream, any belief that rests on the senses can be doubted. (first used: 3. Test your senses with the dream argument)
Evil deceiver — The thought experiment in Descartes' Meditations on First Philosophy (1641) that supposes a powerful deceiver is faking everything a person perceives, to ask what could still be known. (first used: 4. Suppose everything is faked: the evil deceiver)
Cogito — Short for the Latin cogito, ergo sum, the name for Descartes' finding that he must exist whenever he thinks; the Second Meditation puts the finding as the proposition that he exists being necessarily true whenever he thinks it. (first used: 5. Find what survives: the cogito in standard form)
Discourse on the Method — René Descartes' work of 1637, written in French, whose Part IV is the source of the wording rendered in English as I think, therefore I am. (first used: 6. Get the famous words from the right book)
Meditations on First Philosophy — René Descartes' work of 1641, which uses methodical doubt, the dream argument, and the evil deceiver to search for a belief that cannot be doubted. (first used: 6. Get the famous words from the right book)
White paper — Locke's comparison, in An Essay Concerning Human Understanding (title page dated 1690), for the mind at the start of life: blank, with no innate ideas, until experience writes on it. (first used: 8. Trace ideas to experience: Locke's white paper)
Innate idea — An idea that would be present in the mind from birth rather than gained through experience; Locke's Essay denies that there are any. (first used: 8. Trace ideas to experience: Locke's white paper)
Sensation — In Locke's empiricism, the source of the ideas a person gets from the senses, such as the ideas of red, warm, or loud. (first used: 8. Trace ideas to experience: Locke's white paper)
Reflection — In Locke's empiricism, the source of the ideas a person gets by noticing the mind's own workings, such as thinking, doubting, and willing. (first used: 8. Trace ideas to experience: Locke's white paper)
Problem of induction — The problem, raised in David Hume's An Enquiry Concerning Human Understanding (1748), that no non-circular argument shows the future will resemble the past, so reasoning from past patterns to new cases lacks a clear justification. (first used: 9. Question every prediction: Hume's problem of induction)
Circular argument — An argument that assumes, among its premises, the very conclusion it sets out to support, so it gives no independent reason to accept that conclusion. (first used: 9. Question every prediction: Hume's problem of induction)

Summary

Rationalism: some substantive knowledge of the world comes from reason independent of experience.
Empiricism: all knowledge of the world comes from experience.
Methodical doubt sets aside every doubtful belief to find one that cannot be doubted; doubt is the tool, not the conclusion.
The dream argument: no sure sign tells waking from dreaming, so beliefs from the senses can be doubted.
The evil deceiver asks what you can know if everything you perceive might be faked.
The Second Meditation concludes that the proposition that he exists is necessarily true whenever he thinks it.
"I think, therefore I am (COGITO ERGO SUM)" is Veitch's translation of the Discourse on the Method, Part IV, not a line from the Meditations; the Latin in parentheses is Veitch's.
Dataset B's argument 1 is valid by modus ponens and is never keyed for soundness.
Locke: the mind starts as white paper; ideas come from sensation and reflection.
Hume's problem of induction asks what justifies reasoning from the past; it does not say science is useless.
Each position gets its strongest form, its strongest objection, and a reply.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

A museum guard says, "If the painting in the dark hall is a fake, the alarm would have gone off. The alarm did not go off. So the painting is not a fake." Put the argument in standard form, name its form, and say whether it is valid.
A classmate writes, "Descartes said in the Meditations, 'I think, therefore I am.'" Name the two errors in that sentence and rewrite it correctly.
A barista notices that a regular customer has ordered oat milk on each of the last 12 visits, and starts the oat milk before the customer reaches the counter. Write the barista's reasoning in standard form with the added premise marked, and say which premise Hume's problem targets.
A student says, "Locke's white paper means the mind cannot get the idea of doubting, because nobody can see or touch a doubt." Explain why Locke's view has an answer to this.

Answers (worked reasoning — read after attempting)

P1: If the painting in the dark hall is a fake, the alarm would have gone off. P2: The alarm did not go off. C: So the painting is not a fake. P is "the painting is a fake" and Q is "the alarm would have gone off". The pattern is if P then Q; not Q; so not P, which is denying the consequent. That form is valid, so if both premises are true, the painting cannot be a fake. Whether P1 is true, whether the alarm really is wired to catch fakes, is a separate question about soundness.
First error: the wording is not from the Meditations; it comes from Part IV of the Discourse on the Method (1637). Second error: the English words are not Descartes' own; they are John Veitch's translation of Descartes' French. A correct version: "In John Veitch's translation of the Discourse on the Method, Part IV, Descartes' line reads 'I think, therefore I am (COGITO ERGO SUM)'; the Meditations state the same idea in other words."
P1: On each of the last 12 visits, the customer ordered oat milk. P2 (added): The future will resemble the past. C: So the customer will order oat milk this visit. Hume's problem targets P2. Any argument from experience for P2 would assume P2, so it would be circular. Notice that the problem does not tell the barista to stop guessing; it asks what justifies the guess.
Locke names 2 sources of ideas, not 1. Sensation gives ideas from the senses, but reflection gives ideas from noticing the mind's own workings, and doubting is one of those workings. So the idea of doubting comes from experience after all: the experience of catching yourself doubting.

Next up in this module: the Readings & Resources page, the lecture tutorial, the practice set, the quiz, the discussion, and the assignment. Next week, the rule that knowledge might require certainty meets a famous definition of knowledge, justified true belief, and the cases that test it.

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