Modern Philosophy — Descartes and Locke

Week 5 · Week 5: Modern Philosophy — Descartes and Locke · Ch. 4: The Rationalists, Ch. 5: The Empiricists · Download .docx

Objectives

Explain Descartes' method of doubt and his three waves of doubt
State the cogito and explain why it survives every doubt
Distinguish rationalism from empiricism
Explain Locke's tabula rasa and his distinction between primary and secondary qualities

Key terms

Method of doubt: Descartes' plan: reject anything that can possibly be doubted to find what cannot.
Dream argument: I cannot prove I am not dreaming right now, so the senses alone cannot ground certainty.
Evil demon: Descartes' worst case: a powerful deceiver who could make even math seem true when it is false.
Cogito: 'I think, therefore I am' — the one truth that survives every doubt.
Wax argument: Wax keeps its identity while every sensory quality changes — so the mind, not sense, knows what it is.
Rationalism: The view that reason alone can be a source of knowledge.
Tabula rasa: 'Blank slate' — Locke's picture of the mind at birth, with no innate ideas.
Sensation and reflection: Locke's two sources of all ideas: the outer senses and the mind observing its own operations.
Primary qualities: Properties in the object itself: shape, size, motion, solidity, number.
Secondary qualities: Powers in objects to cause sensations in us: color, sound, taste, smell.
Empiricism: The view that all knowledge begins in experience.
Innate ideas: Ideas present in the mind from birth — what Locke denies and Descartes assumes.

The concept

The Modern Question: Where Does Knowledge Come From?

Descartes and Locke are the two poles of a debate that still runs: can reason alone give us knowledge, or must all knowledge begin in experience? Their disagreement is the modern version of the Plato–Aristotle contrast covered in earlier weeks.

Descartes: Doubt Everything to Find What Cannot Be Doubted

Descartes wants knowledge as certain as mathematics. His method: reject everything that can possibly be doubted — not just what is probably false, but what could conceivably be false under any hypothesis. Three waves: first, the senses sometimes deceive (optical illusions, misperceptions); second, I might be dreaming right now and cannot prove from my experience that I am not; third and most radical, an evil demon might be deceiving me about everything — even arithmetic. What survives? 'I think, therefore I am.' Even the demon must have something to deceive; the very act of doubting proves that something exists to do the doubting. Descartes calls this thing res cogitans — a thinking thing. The wax argument reinforces this: a piece of wax melts and loses every sensory property (hardness, shape, scent, sound) yet we recognize it as the same wax. The senses cannot identify what persists; only the intellect can. Conclusion: the mind is better known than the body.

Locke: Knowledge Begins in Experience

Locke opens his Essay by rejecting innate ideas. There is no idea — not God, not justice, not mathematical truth — that is present in the mind from birth. The mind at birth is a blank slate (tabula rasa). All simple ideas come from two sources: sensation (outer experience — color, heat, sound) and reflection (the mind noticing its own operations — thinking, doubting, willing). From simple ideas the mind builds complex ones. Locke also distinguishes primary from secondary qualities. Primary qualities — shape, size, motion, solidity, number — are in the object itself and our ideas of them resemble the object. Secondary qualities — color, taste, sound, smell — are merely powers in the object to produce certain experiences in us; the experience of red is not 'in' the apple the way its shape is.

Why This Debate Still Matters

If Descartes is right, some knowledge — mathematical truths, logical principles — is available through reason alone and is therefore certain. If Locke is right, every claim must ultimately be tested against experience and is therefore fallible. Kant, studied in Week 6, tries to combine them: experience provides the content, the mind provides the structure.

Worked examples

Example 1: Example 1 — The dream argument applied: You are reading this sentence. How do you know you are not dreaming right now? If you pinch yourself and feel pain, that is itself a sensory experience that could be part of the dream. If you look at the clock and it shows 2:17 PM, you could be dreaming a clock showing 2:17 PM. Descartes' point: no sensory check can rule out the dream hypothesis. The only thing that cannot be inside the dream is the thinking that is doing the checking. That is why 'I think, therefore I am' is the foundation — it is not a sensory report but a rational insight.

Example 2: Example 2 — Primary vs. secondary qualities of an orange: Primary (in the orange itself): shape (roughly spherical), size (fits in your palm), solidity (resists pressure), number (one). Secondary (powers to produce sensations): the color orange (your visual system interprets reflected light as orange — an object in total darkness has none of our color experience but still has the same molecular structure), the smell (volatile compounds trigger your olfactory receptors; the smell-experience is in you, not the orange), the taste (acids and sugars cause taste-sensations; a person with anosmia tastes differently the same orange). Locke's point: secondary qualities are real powers in the object but the experiences they produce are in the perceiver.

Common mistakes

Thinking the cogito proves Descartes' body exists — the cogito only proves that a thinking thing exists; Descartes spends later Meditations trying to prove the external world and his body exist.
Confusing the dream argument with the evil demon — the dream argument doubts external-world beliefs; the evil demon is more radical and doubts even mathematical beliefs.
Thinking the tabula rasa means the mind is passive — Locke allows the mind to combine, compare, and abstract from simple ideas; the blank slate concerns origin, not processing.
Thinking secondary qualities are not real — they are real powers in the object; what is not in the object is the experience of them, not the power that produces the experience.
Treating rationalism and empiricism as opposites about all knowledge — both agree that mathematical truths are necessary; they disagree about whether this knowledge is innate (rationalism) or derived from experience (empiricism).

Self-check

Try each one before you look. A miss here costs nothing and tells you exactly what to reread.

1. What are Descartes' three waves of doubt in Meditation I, and what does each wave doubt?

2. Why does 'I think, therefore I am' survive even the evil demon hypothesis?

3. What is the wax argument and what does it prove?

4. What does Locke mean by 'tabula rasa' and why does it matter for his disagreement with Descartes?

5. Give one example of a primary quality and one secondary quality of the same object, and explain the difference.

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