The big question and its setting
Epistemology is the part of philosophy that asks: what is knowledge, and how do we get it? In the 1600s, the Scientific Revolution was changing old ideas. Telescopes showed things the old books never mentioned. Thinkers asked: if so much we believed was wrong, what can we trust?
Two big answers grew:
- Rationalism: reason is the main source of sure knowledge. Some ideas are innate (born with us). Maths is the model.
- Empiricism: all ideas come from experience through the senses. Nothing is in the mind that was not first in the senses.
These are tendencies, not two football teams. Rationalists did use experience, and empiricists did use reasoning. The quarrel is about which comes first and which is surer.
Descartes: doubt, the cogito and two substances
René Descartes (1596–1650), a French philosopher and mathematician, wanted knowledge as firm as geometry. His method (in the Discourse on Method, 1637) had simple rules: accept only what is clear and distinct, break problems into small parts, go from simple to complex, and check that nothing is left out.
Methodical doubt
He decided to doubt everything that could possibly be false, not because he thought it was false, but to find what survives:
- The senses sometimes trick us (a stick looks bent in water).
- Maybe I am dreaming right now.
- Maybe a powerful evil demon deceives me even about maths.
The cogito
But while I doubt, I am thinking. And to think, I must exist. So: "I think, therefore I am" (Latin: cogito, ergo sum). This is the first certainty.
Rebuilding
Descartes then argued that he has the idea of a perfect being, which could not come from an imperfect mind, so God exists. A perfect God would not deceive us. So what we perceive clearly and distinctly is true, and the outside material world exists.
Dualism
He split reality into two substances: mind (thinking, not taking up space) and body/matter (taking up space, not thinking). This is mind–body dualism. Its puzzle: how can a non-physical mind move a physical body?
Descartes' followers: Spinoza and Leibniz
Baruch Spinoza (1632–1677, Netherlands) wrote his Ethics like a geometry book, with definitions and proofs. He said there is only one substance, which he called "God or Nature". Mind and body are two sides of that one reality, so the dualism puzzle disappears. Freedom means understanding why things happen.
Gottfried Wilhelm Leibniz (1646–1716, Germany) also co-invented calculus. He said the world is made of countless simple units called monads. He split truths into truths of reason (necessary, like 2 + 2 = 4) and truths of fact (could have been otherwise, like "it rained today"). He held that ours is the best of all possible worlds, and that every fact has a sufficient reason.
All three trusted reason and believed in innate ideas.
British empiricism: Locke, Berkeley and Hume
John Locke (1632–1704)
In his Essay Concerning Human Understanding (1690), Locke attacked innate ideas: if they were inborn, every child would know them. The mind at birth is a blank slate (tabula rasa). Ideas come from sensation (the five senses) and reflection (noticing our own thinking). Simple ideas (red, sweet, round) are joined into complex ideas (apple). He split qualities into primary (shape, size, motion: really in the object) and secondary (colour, taste, smell: produced in us). In politics Locke argued that people have natural rights to life, liberty and property, and that government needs the consent of the governed.
George Berkeley (1685–1753)
Berkeley pushed further: if all we know are ideas, then even shape is in the mind. Objects exist only by being perceived: "to be is to be perceived". This is idealism. Things continue when no human looks because God always perceives them.
David Hume (1711–1776)
Hume split mental contents into impressions (strong, direct: the heat you feel) and ideas (faint copies: remembering that heat). Every idea must trace back to an impression. Then he asked: from which impression comes the idea of cause? Watch one billiard ball hit another: you see contact, then motion, again and again. You never see a hidden power. Our belief that the first makes the second happen comes from habit (custom). So we cannot prove by reason that the sun will rise tomorrow: this is the problem of induction. Hume also doubted a fixed self: looking inward, he found only a bundle of changing perceptions.
Try it: drop a pen ten times. Write what you actually saw each time. Did you ever see "gravity" itself, or only one thing after another?
Rationalists vs empiricists side by side, and Kant's bridge
| Rationalism | Empiricism | |
|---|---|---|
| Main source | Reason | Sense experience |
| Innate ideas? | Yes | No, blank slate |
| Model of knowledge | Maths, deduction | Observation, induction |
| Thinkers | Descartes, Spinoza, Leibniz | Locke, Berkeley, Hume |
| Weak spot | Can build castles in the air | Can end in scepticism |
Immanuel Kant (1724–1804) said Hume woke him up. His answer: all knowledge begins with experience, but not all of it comes from experience. The mind supplies built-in forms (space, time, cause) that shape every experience. In short: ideas without experience are empty, and experience without ideas is blind.
Key formulas and definitions
- Rationalism: reason first; innate ideas (Descartes, Spinoza, Leibniz)
- Empiricism: experience first; tabula rasa (Locke, Berkeley, Hume)
- Cogito: "I think, therefore I am"
- Methodical doubt: senses → dream → evil demon
- Locke: simple ideas → complex ideas; primary vs secondary qualities
- Berkeley: to be is to be perceived
- Hume: impressions → ideas; cause = constant conjunction + habit
Worked examples
1. Why could Descartes not doubt "I think, therefore I am" even if an evil demon fooled him?
To be fooled, there must be someone being fooled. Doubting is itself a kind of thinking, so the doubter must exist while doubting.
2. Sort these as rationalist or empiricist claims: (a) "A baby already knows that a thing cannot both be and not be." (b) "A person born blind cannot have the idea of red."
(a) Rationalist: an innate logical idea. (b) Empiricist: no sense impression of red, so no idea of red (Locke and Hume both used this kind of case).
3. Use Hume to explain why a chicken that is fed every morning may be wrong about tomorrow.
The chicken has seen feeding follow sunrise many times and expects it by habit. But past repetition cannot prove the future; one day the farmer comes for a different reason. Induction gives expectation, not certainty.
Common mistakes
- Saying rationalists ignored experience. They used it, but held that the surest truths come from reason.
- Translating the cogito as "I think so I should exist". It is a claim of certainty: while I think, I exist.
- Mixing up Berkeley and Hume: Berkeley says things are ideas kept in being by God; Hume doubts we can prove causes, the self or the outside world.
- Thinking Hume said causes do not exist. He said reason cannot prove necessary connection; our belief comes from habit.