Lecture: Knowledge I: Doubt, Reason, and Experience
Lecture script · about 15–20 minutes spoken · Week 4
Opening
Have you ever woken up sure something had happened, and then slowly realized it was a dream? Your heart's still racing. You missed the bus, you ran, you watched it leave. None of it was real, and while it was going on, nothing told you so. Hold on to that feeling. This whole lecture grows out of it.
It isn't only a puzzle about sleep. A nursing student deciding how far to trust a monitor's reading is asking a version of it. So is a journalist checking whether a video call is real, or a business major forecasting next month's sales from last year's. Each one is asking what they can know, and whether it comes from careful reasoning or from experience.
In Week 1 you learned to test a belief with a counterexample. In Week 2 you learned standard form and what makes an argument valid. In Week 3, Plato asked whether what we see might be shadows on a wall. In this lecture I'll take that worry to its sharpest point with René Descartes, and then turn to two thinkers who rest everything on experience, John Locke and David Hume. Next week, in Week 5, we ask what knowledge is in the first place.
1. Two answers about where knowledge comes from
Start with two things you know. If three friends sit at a table and two more join them, five people are at the table. And the coffee at the corner shop is bitter. You learned the coffee fact by tasting it. The table fact feels different. You could work it out with your eyes shut.
That difference gives us two big answers. Rationalism is the view that some substantive knowledge of the world comes from reason independent of experience. Empiricism is the view that all knowledge of the world comes from experience, through the senses and through noticing the workings of your own mind.
Here's what neither side says. A rationalist doesn't call the senses useless. An empiricist doesn't call reasoning useless. They disagree about the source. Can reason alone ever hand you real knowledge of the world? A rationalist says yes, sometimes. An empiricist says every piece starts in experience.
Pause and try this. Someone says, "Empiricists think we shouldn't reason about anything." Is that a fair statement of empiricism?
Think about what empiricism says comes first.
Here's the answer. No, it isn't fair. Empiricism is a claim about where knowledge of the world comes from, and empiricists reason all the time; they hold that reasoning works on material experience supplied.
2. Methodical doubt
Picture a cook sorting strawberries before a party. A few have soft spots, and mold spreads. The careful move is to tip the whole basket out and put back only the berries that pass a close look.
Descartes does that with his beliefs. In the Meditations on First Philosophy, from 1641, he sets aside every belief he can find any reason to doubt, so he can look for one that can't be doubted. That's 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" matters most. Setting a belief aside isn't the same as calling it false. It's like putting a berry on the counter until you've checked it. The bar is high on purpose. If one belief clears it, you've got solid ground.
Pause and try this. A classmate says, "Descartes believed nothing was real." What would you say back?
Keep the method and the result apart.
Here's the answer. I'd say doubt is Descartes' tool, not his conclusion. He sets beliefs aside to test them, and setting a belief aside doesn't mean believing it's false.
3. The dream argument
Back to the bus dream. You felt the cold air and heard the brakes. Every sense reported something, and none of it was real.
Here's the dream argument: because no sure sign tells waking experience from a vivid dream, any belief that rests on the senses can be doubted. Put it in standard form. 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. That's modus ponens, so it's valid.
Notice what it doesn't conclude. It doesn't say you're dreaming right now. It says your senses alone can't settle the question with certainty. For Descartes, that's enough, because his method sets aside anything open to doubt.
Pause and try this. Does the dream argument show that your life is a dream?
Read the conclusion word for word.
Here's the answer. No. It shows that beliefs from the senses can be doubted, and a belief that can be doubted may still be true; the argument removes certainty, not reality.
4. The evil deceiver
You might think a few beliefs survive even a dream. In a dream, two plus three still seems to make five. So Descartes needs a stronger test, and it's the case at the center of this week.
He asks you to suppose that a powerful deceiver is faking everything you perceive. That's the evil deceiver, the thought experiment in the Meditations that supposes a powerful deceiver is faking everything a person perceives, to ask what could still be known. Its question is: what can I know if everything I perceive might be faked?
Here's a modern variation of my own, so treat it as a variation. Imagine a video call with someone you trust, and suppose software is making every frame. Nothing in the call could prove it's real. The deceiver stretches that worry to everything at once.
Pause and try this. Did Descartes believe an evil deceiver really exists?
Remember what a thought experiment is for.
Here's the answer. No. It's a supposition, an imagined case used to test which beliefs still stand, so the point is the test, not the deceiver.
5. What survives the doubt
So suppose the deceiver is real and working hard. It fools you about the sky and your own hands. Is anything left?
The Second Meditation concludes, in words, that the proposition that he exists is necessarily true whenever he thinks it. In plain terms: to be fooled, you have to be there to be fooled. That finding has a name, the cogito, short for the Latin cogito, ergo sum.
Here's the course's reconstruction. P1, which we add and mark as 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. That's modus ponens, so it's valid. And here's the clever part. The harder the deceiver works, the more you think, because doubting is thinking.
A critic can still ask whether that added premise is earned, or whether "I am thinking" shows a lasting self or only that thinking is going on. A defender replies that a thought with no thinker makes no sense.
Pause and try this. The cogito argument is valid. Does that, by itself, prove that I exist?
Recall the two questions from Week 2.
Here's the answer. Not by itself. Valid means the conclusion can't be false if the premises are true, so you still have to ask whether the premises are true, and critics press on the added one.
6. The famous words and their source
Now a slip almost everyone makes. "I think, therefore I am" isn't a line from the Meditations. In John Veitch's translation, it comes from Part IV of the Discourse on the Method, René Descartes' work of 1637, written in French. The French reads "je pense, donc je suis". Veitch's English adds the Latin in parentheses, and that Latin is his own insertion. The Latin "cogito, ergo sum" shows up in later Latin works, among them the Principles of Philosophy, from 1644.
Put the books in order. The Discourse is from 1637, and the Meditations are from 1641. That's a gap of 4 years. So the famous wording is older than the Meditations and isn't printed in them. The Meditations say the same idea in different words.
Pause and try this. A friend writes, "In the Meditations, Descartes wrote the English words 'I think, therefore I am.'" How many mistakes are in that sentence?
Count the book, then count whose words they are.
Here's the answer. Two. The wording comes from the Discourse, not the Meditations, and the English words are Veitch's translation of Descartes' French.
7. An argument about certainty
Descartes wanted certainty. Is certainty what knowledge needs? Here's argument 1 from Dataset B, the course's illustrative argument set. P1: if knowledge requires certainty, then I know very little. P2: knowledge requires certainty. C: so I know very little.
Its form is if P then Q; P; so Q. That's modus ponens, so it's valid. If both premises were true, the conclusion couldn't be false.
Is it sound? That would need both premises to be true. And P2, whether knowledge requires certainty, is an open question. So the course keys this argument as valid and stops there. Next week, when we ask what knowledge is, that question comes back.
Pause and try this. Should argument 1 be keyed "sound"?
Ask the second Week 2 question.
Here's the answer. No. It's valid, but soundness depends on whether knowledge requires certainty, which is open, so it's keyed for validity only.
8. Locke and the white paper
Watch a toddler taste a lemon for the first time. The face scrunches, and now there's an idea of sour that wasn't there a minute ago. Where did it come from? From the tongue.
John Locke's Essay Concerning Human Understanding, with a title page dated 1690, builds on cases like that. Locke compares the mind at the start to white paper: blank, with no innate ideas, until experience writes on it. Every idea, he says, comes through 2 sources. Sensation gives you ideas from the senses, like red or warm. Reflection gives you ideas from noticing your mind's own workings, like thinking, doubting, and willing.
That second source keeps the view strong. The mind isn't idle. And the Essay's title page is dated 49 years after the Meditations, so Locke wrote with Descartes' search already in the world.
Pause and try this. Where would Locke say your idea of doubting comes from?
Think about his two sources.
Here's the answer. From reflection. You get the idea of doubting by noticing yourself doubt, which is experience of the mind's own workings.
9. Hume and the problem of induction
A forecaster looks at the records. On every past morning when the barometer fell overnight, rain followed. So, rain by noon. But there's an unspoken premise, which we add: the future will resemble the past.
David Hume's Enquiry Concerning Human Understanding, from 1748, presses on that premise. That's the problem of induction: no non-circular argument shows the future will resemble the past. Try to support the premise with experience: "The future has always resembled the past, so it will again." That uses the premise to prove itself. It's a circular argument, one that assumes the very conclusion it sets out to support.
The Enquiry came out 58 years after Locke's Essay, and 111 years after the Discourse. Notice what the problem doesn't say. It doesn't tell the forecaster to stop forecasting. It asks what justifies the forecast.
Pause and try this. Does Hume's problem show that science is useless?
Ask whether it's about usefulness or justification.
Here's the answer. No. It asks what justifies reasoning from past patterns, which is a question about justification, not a verdict that science or forecasting doesn't work.
10. Weighing the two positions
Now let's set the two views side by side, each at its strongest. Rationalism's best example is the cogito, reached by thinking while every sense was in doubt. The empiricist objects that the mind begins as white paper, so even the ideas of thinking and existing came from experience. The rationalist replies that, even if the idea came from reflection, reason is what makes the cogito certain.
Empiricism traces every idea to sensation and reflection. Its strongest objection is Hume's problem, since experience shows only what has happened. The empiricist replies that the problem asks what justifies a prediction, not whether to keep predicting, and that past patterns are still how we learn from evidence.
So where do they disagree, in one plain sentence? Whether reason alone can ever be a source of knowledge of the world. This course gives no verdict.
Pause and try this. Does one strong objection defeat a position?
Look for the reply.
Here's the answer. No. Both positions face serious objections and offer serious replies, and that's what an open question looks like.
Closing
Let me pull it together. Rationalism says some knowledge of the world comes from reason alone; empiricism says it all comes from experience. Descartes' methodical doubt sets aside every doubtful belief. The dream argument puts the senses in doubt, and the evil deceiver goes further. What survives is the cogito: whenever he thinks, he exists. The famous wording comes from the Discourse, in Veitch's translation, not from the Meditations. Argument 1 is valid and never keyed for soundness. Locke's mind starts as white paper, filled by sensation and reflection. And Hume's problem asks what justifies a prediction.
Next in this module come the slide deck, the Readings and Resources page, the lecture tutorial, the practice set, the quiz, the discussion, and the assignment. In Week 5, Knowledge II: What Knowledge Is, we test a famous definition of knowledge against the cases built to break it.
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