3.1.4 - Philosophical scepticism and waves of doubt

The Problem Of Ordinary Knowledge

Start with something ordinary: you say that you know there is a desk in front of you, that you are awake, or that two plus three equals five. Most of the time, no one challenges you. Epistemology becomes difficult when someone asks what would have to be ruled out before those claims count as knowledge.

You might doubt a rumour because the source is unreliable, or doubt a weather forecast because the evidence is weak. The doubt depends on some specific defect in the evidence.

Philosophical scepticism is more radical because it aims at a whole class of beliefs. Instead of asking whether this particular claim is poorly supported, it asks whether any claim of that kind can be known at all.

This distinction matters for AQA because Descartes is not simply being cautious. He is using extreme doubts to test whether any belief can survive as certain knowledge.

The Function Of Scepticism

The role of scepticism in epistemology is not just to make knowledge look impossible. It functions as a pressure test for theories of knowledge. A theory that explains easy cases but collapses when sceptical possibilities are raised has left something important unclear.

Scepticism brings hidden standards into the open. It asks questions such as:

• Does knowledge require certainty, or is strong justification enough?
• Must I rule out every possible error before I know?
• Are sense experience, memory, and reason trustworthy sources of knowledge?
• If a belief is true but could easily have been false, is it knowledge?

This is why scepticism is useful even if it is not finally accepted. It exposes assumptions about justification, certainty, evidence, and the connection between appearance and reality.

The basic sceptical pressure can be reconstructed like this:

1. P1: If I know that p, then my grounds for p must be strong enough to rule out relevant error.
2. P2: There are possible errors that my ordinary grounds cannot rule out.
3. Therefore, C: My ordinary grounds may not amount to knowledge that p.

Hidden assumption: Knowledge requires a standard of justification that excludes the relevant sceptical alternatives.
Inference type: Deductive if P1 and P2 are accepted; the argument moves by applying a condition on knowledge.
Main pressure point: Whether the sceptic is allowed to demand that such possibilities must be ruled out.

The phrase relevant error is doing important work. If knowledge required ruling out every logically possible mistake, almost nothing would count as knowledge. If knowledge only required ruling out ordinary practical doubts, scepticism would lose much of its force. Descartes' waves of doubt push this standard as far as possible.

Local And Global Scepticism

Someone might be a local sceptic about moral knowledge, knowledge of the future, other minds, or the external world. Local scepticism can leave many other kinds of knowledge untouched.

Global scepticism is wider. It questions whether knowledge is possible in general, or at least across most of what we ordinarily take ourselves to know. The more sources of belief the sceptical argument attacks, the more global it becomes.

The possible global application of philosophical scepticism is the key issue in Descartes. Each wave of doubt has a wider reach than the previous one. The first wave challenges some sensory beliefs. The second wave challenges sensory knowledge of the external world more generally. The third wave threatens even reason, mathematics, and apparently obvious truths.

The pattern can be put like this:

1. P1: If a doubt undermines every source I use for a class of beliefs, then I cannot claim knowledge in that class.
2. P2: Descartes' waves of doubt appear to undermine sense experience, then waking experience, then even reason.
3. Therefore, C: Descartes' method creates a possible route from local doubt to global scepticism.

Hidden assumption: A source of belief cannot ground knowledge if the subject cannot rule out a systematic error affecting that source.
Inference type: Cumulative sceptical reasoning; each wave broadens the target.
Main pressure point: Whether mere possibility of systematic error is enough to defeat knowledge.

Descartes' Method Of Doubt

Descartes' scepticism is methodological. He is not saying that he casually disbelieves everything in ordinary life. He is asking what can survive the strictest possible test when the aim is certain knowledge.

The method has three features:

This explains why he does not need to examine every belief one by one. If many beliefs depend on a common source, such as sense experience, then challenging that source challenges all the beliefs built on it.

Descartes' project can be reconstructed as a method:

1. P1: If a belief can be doubted, it should not be accepted as a foundation for certain knowledge.
2. P2: Many beliefs depend on basic sources such as the senses, memory, and reason.
3. P3: If a basic source can be doubted, the beliefs depending on it can be doubted.
4. Therefore, C: To find certain knowledge, test the basic sources of belief with the strongest doubts available.

Hidden assumption: Certain knowledge needs indubitability, at least at the foundational stage.
Inference type: Practical-methodological reasoning about how to search for certainty.
Main pressure point: Whether the search for certainty sets the standard for knowledge too high.

Meditation 1 contains the main sceptical pressure. Descartes' rebuilding of knowledge in the later Meditations is covered in the next lesson, Responses to scepticism.

First Wave: The Senses

The first wave begins with a familiar fact: the senses sometimes deceive us. A straight stick can look bent in water; distant objects can look smaller than they are; poor lighting can make colours misleading. This is enough to show that sense experience is not infallible.

The argument is limited, but it starts the sceptical movement:

1. P1: A source that has deceived me before is not completely trustworthy.
2. P2: The senses have deceived me before.
3. Therefore, C1: The senses are not completely trustworthy.
4. P3: If a belief rests on a source that is not completely trustworthy, it may fail to be certain knowledge.
5. Therefore, C2: Some sensory beliefs may fail to be certain knowledge.

Hidden assumption: A foundation for certainty must be incapable of deception.
Inference type: Inductive support for distrust, followed by a conditional challenge to certainty.
Main pressure point: Occasional sensory error does not show that all sensory beliefs are doubtful.

This wave is not yet global. It works best against difficult or uncertain cases, such as distant, small, faint, or unusual objects. It is much weaker against apparently obvious present perceptions, such as that you are sitting by a desk or holding a pen.

That weakness is exactly why Descartes needs the second wave. If the first wave only shows that the senses sometimes deceive us, it does not yet threaten ordinary sensory knowledge as a whole.

Second Wave: Dreaming

The second wave is the dream argument. Dreams can present experiences that feel like waking experiences while they are happening. If a dream can seem real from the inside, then the mere fact that an experience feels vivid and coherent may not prove that it is waking perception.