3.1.4 - Responses to scepticism

The Response Problem

The sceptic’s challenge is not just that we sometimes make mistakes. Ordinary incredulity doubts a particular claim because of a specific evidential problem. Philosophical scepticism questions whether knowledge is possible in a domain or generally. It is local when restricted to a domain, such as the external world, while leaving mathematics or other knowledge untouched; it is global when it threatens knowledge generally, across its main sources.

A response to scepticism must therefore say what kind of answer is being offered. It might try to prove that the sceptical scenario is false. It might argue that the sceptic is demanding too much. Or it might change the account of knowledge so that a person can know without being able to prove, from the inside, that they are not deceived.

The key terms are:

The shared problem can be reconstructed like this:

1. P1: If global scepticism is correct, then we lack knowledge of the external world.
2. P2: To answer global scepticism, a philosopher must explain why ordinary external-world beliefs can count as knowledge.
3. P3: Different theories answer by using different standards: certainty, sensory evidence, explanatory power, or reliable belief-formation.
4. Therefore, C: The strength of a response depends partly on whether its standard of knowledge is the right one.

Hidden assumption: A response need not satisfy every possible standard; it must justify the standard it uses.
Inference type: conceptual and evaluative.
Main pressure point: whether a weaker standard than certainty is enough to count as knowledge.

So the question for this lesson is not simply Which philosopher beats the sceptic? It is: what kind of victory would count?

Descartes' Own Response

Descartes' response is the most ambitious because it tries to rebuild knowledge after radical doubt. His route is not just the cogito. The cogito gives a first certainty, but Descartes then needs God and clear and distinct perception to get beyond the thinking self.

The structure is:

1. Even if I am deceived, doubting or thinking is occurring, so I cannot coherently doubt my own existence as a thinking thing.
2. I can clearly and distinctly grasp some truths, but I need a guarantee that clear and distinct perception is not systematically misleading.
3. Descartes argues for God's existence in Meditation 3 and again in Meditation 5.
4. Since God is perfect and not a deceiver, what I clearly and distinctly perceive is true.
5. In Meditation 6, sensory ideas are involuntary and I have a strong natural inclination to believe they come from bodies. If there were no bodies at all, God would have allowed me to be deeply misled.
6. Therefore, material things exist, although Descartes is more confident about their mathematically graspable features than about every sensory appearance.

His external-world response can be reconstructed like this:

1. P1: I clearly and distinctly understand myself as a thinking thing and bodies as extended things.
2. P2: God exists and is not a deceiver.
3. P3: I have a strong natural inclination to believe that my involuntary sensory ideas are caused by bodies.
4. P4: If no bodies existed, this natural inclination would make systematic error unavoidable.
5. P5: A non-deceiving God would not create me with such an unavoidable tendency to falsehood.
6. Therefore, C: Bodies exist as the external causes of many sensory ideas.

Hidden assumption: A perfect God would not permit a deep, built-in error about the general existence of bodies.
Inference type: deductive within Descartes' system, because the conclusion is meant to follow once God and clear and distinct perception are secured.
Main pressure point: whether the proof of God can carry the weight placed on it.

Why it is plausible: Descartes does not shrug at scepticism. He tries to meet the global sceptic's demand for certainty by finding something indubitable and then building carefully from it. The cogito also shows a real limit to doubt: doubt itself confirms the existence of a thinker.

The strongest objection is the Cartesian circle. It targets the method, not merely one premise. Descartes seems to need clear and distinct perception to prove God, but he also needs God to guarantee clear and distinct perception. If that is circular, the proof cannot reassure us that our reasoning is trustworthy.

A Cartesian reply is to distinguish between immediate clarity and remembered or general reliability. While I am directly attending to a clear and distinct idea, I cannot rationally doubt it; God is needed to secure the general rule and protect memory from later doubt. This reply reduces the circle, but it does not remove every worry. The residual problem is that Descartes still needs the God proofs to be strong enough, and many empiricists deny that they are.

Locke's Sensitive Knowledge

Locke's response is empiricist: knowledge begins from experience, not from innate principles or purely rational certainty about the world. For Locke, ideas come through sensation and reflection. When he answers scepticism about other things, he does not try to give a Cartesian demonstration that an external world must exist. He argues that present sensation gives a kind of knowledge appropriate to human beings.

Locke's response uses several connected points:

The argument can be reconstructed like this:

1. P1: Some present sensory ideas arrive involuntarily and with a force that imagined or remembered ideas lack.
2. P2: These sensory ideas cohere with other experiences and with practical interaction.
3. P3: The best explanation of these features is that external things are causing the ideas.
4. P4: Human knowledge of external things does not require the same certainty as intuitive or demonstrative knowledge.
5. Therefore, C: We have sensitive knowledge of external things while we are actually perceiving them.

Hidden assumption: A standard of knowledge can be lower than absolute certainty while still being genuine knowledge.
Inference type: partly abductive and partly conceptual, because Locke points to the best explanation of sensory features and classifies it as a legitimate kind of knowledge.
Main pressure point: whether sensitive knowledge is strong enough against global scepticism.

Locke's strength is that he fits the way experience actually functions. I do not choose to feel heat or pain as I choose to imagine a scene. My senses also constrain and correct one another. This makes ordinary perception look less like free-floating inner theatre and more like contact with a world.

The objection is that Locke has not refuted the dream or demon possibility. The target is the inference from involuntary, coherent experience to external causes. A global sceptical scenario could include experiences that are involuntary and coherent.

Locke's best reply is to reject the demand for absolute proof. If the sceptic asks for mathematical certainty about external things, the sceptic is asking for the wrong kind of certainty. Sensitive knowledge is enough for embodied human knowers. What remains unsettled is whether that answer defeats global scepticism or simply refuses its standard.

Berkeley's Idealist Response

Berkeley's response is empiricist but not realist about mind-independent matter. His bold move is to argue that scepticism arises because philosophers invent a gap between what we immediately perceive, ideas, and a supposed material substance behind those ideas. If we remove the unknowable material substance, we remove the sceptical gap.

For Berkeley, ordinary sensible objects are not hidden material things behind experience. They are collections of ideas perceived by minds. This does not mean that ordinary objects are unreal. It means their reality is mind-dependent. A table is not an unknowable object behind all possible experience of tables; it is the ordered set of perceptible ideas available to minds. God explains the continuing order and stability of the world when no finite human mind is perceiving a thing.

Berkeley's anti-sceptical argument can be reconstructed like this: