The limits of knowledge: Cartesian doubt and Humean scepticism about induction

What this dot point is asking

Unit 1 of the TASC Philosophy course asks you to examine the theoretical limitations of our knowledge through Cartesian and Humean scepticism. You should be able to reconstruct each sceptical argument, explain the conclusion it reaches, and assess how well it can be answered.

Cartesian doubt

In his Meditations on First Philosophy, Descartes adopts a method of doubt: he will reject any belief that could possibly be false, hoping to find an unshakeable foundation for knowledge. He notices first that the senses sometimes deceive us, so sensory beliefs are not certain. He then raises the dreaming argument: since dreams can feel exactly like waking life, you cannot be sure you are not dreaming right now. Finally he imagines an evil demon, a powerful deceiver who manipulates all his experiences, which would undermine even mathematics and logic.

The cogito

Descartes finds his foundation in the act of doubting itself. Even if a demon deceives him about everything, the demon cannot make him think he exists while he does not exist, because to be deceived he must exist to be deceived. This is the cogito, often rendered as I think, therefore I am. From this first certainty Descartes tries to rebuild knowledge, arguing for the existence of a non-deceiving God who guarantees that clear and distinct ideas are true. Many readers accept the cogito but reject this later move as circular, the so-called Cartesian circle, since he seems to use clear and distinct ideas to prove the God who is meant to validate them.

Hume and the problem of induction

David Hume targets a different kind of belief: our expectations about the unobserved. We believe the sun will rise tomorrow, that bread will nourish us, that fire will burn. These rest on induction, reasoning from observed regularities to unobserved cases. Hume asks what justifies this. It cannot be deductively valid, since no contradiction follows from supposing the future will differ from the past. It cannot be justified by experience without circularity, because to say that induction has worked before, so it will work again, is itself an inductive argument. So our most basic expectations rest on custom and habit rather than reason.

Responses

To Cartesian doubt, G. E. Moore offered a robust commonsense reply, holding up his hand and arguing that he knows it exists more certainly than he knows any sceptical premise. To Hume, Karl Popper responded by abandoning induction altogether, proposing that science proceeds by conjecture and refutation rather than confirmation. Others, such as Peter Strawson, argue that asking for an external justification of induction misunderstands what counts as rational, since being reasonable just means proportioning belief to evidence. None of these fully dissolves the sceptical worry, which is part of why both arguments remain central.

Evaluating scepticism

A strong answer distinguishes the two scepticisms. Descartes doubts the external world and his own senses, seeking certainty. Hume grants the past but doubts our leap to the future. Notice that even Descartes' rebuilding depends on rejecting his strongest doubts, and Hume himself admits that in practice no one can live as a sceptic. The philosophical value lies in clarifying what knowledge would require and how far ordinary belief falls short of certainty.