The limits of knowledge — AQA A-Level Philosophy

This topic requires you to characterise philosophical scepticism precisely and to separate it from ordinary incredulity. Philosophical scepticism is a systematic, theoretical doubt about whether knowledge is possible at all in a given domain; it questions the justification of whole classes of belief rather than any single claim. Normal incredulity is everyday, local doubt: you disbelieve a particular report (a friend's excuse, a news item) because of specific evidence, and you accept that the belief could in principle be settled. The distinction matters because philosophical scepticism cannot be answered by pointing to one reliable source, since it challenges the standards of justification themselves. Use examples: doubting whether any perception reliably reveals an external world (philosophical) versus doubting that the milk in the fridge is still fresh (normal).

Philosophical scepticism is not the everyday attitude of doubting everything, nor a refusal to believe anything. Within epistemology it is a methodological and evaluative tool: it tests whether a claimed kind of knowledge can survive the strongest available challenge. Its function is threefold. First, it is a challenge device: by suspending assent where justification is insufficient, it exposes whether a belief is genuinely warranted or merely assumed. Second, it is a diagnostic device: it separates claims that rest on empirical evidence from those that rest on a priori reasoning, and shows where each is vulnerable. Third, it is a constructive device: responding to sceptical pressure forces epistemologists to clarify what knowledge requires, for example whether it needs certainty, infallibility or only justification plus truth.

Local scepticism targets a restricted domain: it doubts knowledge of, say, other minds, the past, or the external world, while leaving other domains untouched. Global scepticism targets knowledge as such, questioning whether any belief can be known at all. The distinction matters because a local doubt can often be answered by appealing to knowledge from another domain, whereas a global doubt removes every such foothold. The question of possible global application asks whether sceptical method can coherently be extended from particular cases to all knowledge claims at once. Extending it globally raises a self-application problem: if the sceptic claims to know that nothing can be known, that claim itself falls under the doubt. So global scepticism is best treated as a limiting test of the structure of justification rather than a position that can be asserted without tension.

Descartes builds doubt in stages to test whether any belief survives. First wave: the senses sometimes deceive me, so I cannot trust them in matters where they might be deceiving me now. Second wave: the dream argument — I cannot be certain I am awake, since dreaming experience can seem as vivid as waking experience, so even my body and surroundings may be doubted. Third wave: the evil demon (or malicious deceiver) could make me go wrong even about simple arithmetic and geometry, so those too are suspended. Note the ordering and the increasing scope: senses, then the external world, then reason itself. The doubt is methodological, used to find an indubitable foundation, not a claim that nothing exists.

Descartes answers his own scepticism by finding a belief the demon cannot undermine: even if I am deceived, I must exist to be deceived, so 'I think, therefore I am' is certain whenever I think it. From this he argues that the idea of a perfect being could not come from an imperfect thinker, so God exists, and a perfect God would not allow systematic deception, so clear and distinct perceptions can be trusted. Learners should apply this as a response to scepticism: it aims to defeat the demon by securing a foundation and then rebuilding knowledge. Note the structure — cogito, then God, then the reliability of clear and distinct ideas — and be ready to assess whether the response succeeds.

In 'The limits of knowledge', philosophical scepticism raises doubts about whether we can know anything beyond appearances. Empiricist responses appeal to experience as the source and limit of knowledge. Locke holds that the mind begins as a blank slate and all ideas derive from sensation and reflection; knowledge is constrained by what experience supplies. Berkeley argues that we know only ideas and that material substance is unnecessary; experience itself is reliable because it is given by God, though we still lack knowledge of mind-independent matter. Russell, in this topic, treats knowledge of the external world as inferred from sense-data; we are directly acquainted with appearances and construct beliefs about physical objects, so certainty about the external world is limited. Each response addresses sceptical doubt by grounding knowledge in experience while accepting limits.

Reliabilism is a theory of knowledge that drops the demand for internal justification and instead asks whether a belief was produced by a reliable process. A process such as perception, memory or sound reasoning counts as reliable when it generally yields true beliefs; a lucky guess does not, even if it happens to be correct. This lets the reliabilist answer sceptical worries by pointing out that ordinary perceptual beliefs can be knowledge because perception is a truth-conducive process, without requiring the believer to prove that it is. Learn to distinguish reliabilism from justified-true-belief accounts, to identify what makes a process reliable, and to recognise the objection that a believer may have no access to the reliability of their own process.