The distinction between normal incredulity and philosophical scepticism, local and global scepticism, the role of scepticism in epistemology, Descartes' three waves of doubt and the evil demon, and responses including Descartes' own rationalist reconstruction and reliabilist or externalist replies.

What this dot point is asking

AQA wants you to distinguish philosophical scepticism from ordinary doubt, separate local from global scepticism, explain the constructive role scepticism plays in epistemology, set out Descartes' three waves of doubt culminating in the evil demon, and evaluate the responses, including Descartes' own attempt to rebuild knowledge and externalist or reliabilist replies.

What philosophical scepticism is

Scepticism has a positive role in epistemology: it forces us to identify the foundations of knowledge and to test which beliefs can withstand the strongest possible doubt. This is exactly how Descartes uses it. He treats methodic doubt not as an end in itself but as a method: by deliberately doubting everything that admits of any doubt, he hopes to find an indubitable foundation on which to rebuild certain knowledge, in the way an architect clears unstable ground before laying foundations. So the sceptic and the anti-sceptic can be the same philosopher: scepticism is the tool, certainty the goal. It also sharpens the analysis of knowledge from the neighbouring dot point, because if knowledge requires that we rule out every possibility of error, then the demon shows we have almost none, whereas a fallibilist or externalist account of knowledge can survive the demon untouched.

Descartes' three waves of doubt

Descartes' reconstruction

Descartes finds one belief that survives even the demon: the cogito, "I think, therefore I am", since the very act of doubting proves a thinker exists. From the cogito he argues to the existence of a non-deceiving God (via the trademark and ontological arguments), and uses God's perfection to guarantee that clear and distinct ideas are true, thereby rebuilding knowledge of mathematics and, cautiously, the external world.

Responses and objections

• The Cartesian circle. Descartes uses clear and distinct perception to prove God, then uses God to validate clear and distinct perception; the reasoning looks circular.
• The cogito is too thin. Even granting "I exist", critics (notably Hume) deny we are entitled to a persisting self or substance, only to fleeting thoughts.
• Reliabilism and externalism. If knowledge requires only that beliefs are formed by a reliable process, then we can have knowledge without ruling out the demon from the inside; the sceptic's demand for internal certainty is rejected.
• The demon is self-undermining or too strong. Some argue that a doubt which cannot in principle be resolved by any evidence is not a genuine doubt at all.

Two further responses appear in strong answers. Empiricist replies (Locke, Russell) grant that we cannot achieve Cartesian certainty but deny that knowledge requires it: an external world is the best explanation of the coherence and involuntariness of experience, so we have well-grounded, if fallible, knowledge. Pragmatic and Moorean replies turn the demon argument on its head: G. E. Moore argued it is more certain that he has two hands than that any sceptical premise is true, so we should reject the premise rather than accept the sceptical conclusion. The general evaluative lesson is that the force of scepticism depends entirely on how demanding a conception of knowledge we accept; raise the bar to indubitability and scepticism wins, lower it to reliable-but-fallible belief and most scepticism dissolves.