The Limits of Knowledge

Normal incredulity

Doubting a claim because the available evidence is insufficient, while remaining willing to change one’s belief if new evidence appears.

Philosophical scepticism

The view that knowledge claims can be doubted even when evidence is presented, because the sceptic can question the reliability of the evidence itself.

Purpose of sceptical arguments

Sceptical arguments test the reliability of sources of knowledge such as perception, reason, and memory.

Local scepticism

A belief which raises doubt with one particular area of knowledge, It is scepticism about some specific claim, or more commonly some area/branch of supposed knowledge.

Global scepticism

The view that knowledge of any kind may be impossible, raising doubt about all knowledge claims.

Illusion (first wave of doubt)

Because our senses sometimes deceive us through illusions, we cannot be completely certain that sensory knowledge is reliable.

Dreaming (second wave of doubt)

Since dreams can feel indistinguishable from waking experience, we cannot be certain that the physical world we perceive is real.

Evil demon (third wave of doubt)

Descartes imagines a powerful deceiver who manipulates his thoughts, meaning even logic and mathematics could be false.

Cogito ergo sum

I think, therefore I am (the only piece of certain knowledge).

John Locke’s response to scepticism

Locke claims the mind gains knowledge through experience of the external world which is mind-independent.

Bertrand Russell’s response to scepticism

Russell argues we are directly aware of sense-data, which provide indirect evidence for the existence of external objects because the sense-data are produced by something.

Practical response to scepticism

Although certainty may be impossible, the best explanation of our consistent experiences is that they are caused by a real external world.

Sceptical criticism of empiricism

Sceptics argue that sense-data do not prove that external objects exist because the same experiences could occur in dreams or illusions.

Reliabilism

A subject knows that a proposition is true if and only if a subject believes that the proposition is true, the proposition is true, and the subject's belief that the proposition is true is formed by a reliable cognitive process.

Reliable cognitive process

A process which produces a higher proportion (over 50%) of true beliefs.

Criticism of reliabilism

Reliabilism may fail to defeat scepticism because it is difficult to prove that our cognitive processes are genuinely reliable.