Scepticism, Descartes and Hume - TCE Philosophy (Tasmania)
“The limits of knowledge: Cartesian doubt and Humean scepticism about induction”
Scepticism challenges our claim to know things. Rene Descartes uses radical doubt, including the dreaming and evil demon arguments, to strip away everything uncertain, then finds one thing he cannot doubt: that he thinks, so he exists. David Hume raises a different worry, the problem of induction, showing that no reasoning can justify our belief that the future will resemble the past. Both expose limits on what we can be certain of.
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 deliberate use of doubt as a tool. Descartes does not believe everything is false; he withholds assent from anything that could be false in order to discover what, if anything, survives doubt and can serve as a foundation.
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.
The statement that I exist cannot be false at the moment I think it, because thinking presupposes a thinker. This is why Descartes treats it as the one belief immune to even the evil demon.
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.
P1. All justification is either deductive or inductive.
P2. Induction cannot be justified deductively, since the future differing from the past is not a contradiction.
P3. Induction cannot be justified inductively without arguing in a circle.
C. Therefore our inductive beliefs have no rational justification.
The conclusion is sceptical about the rationality of expecting the future to resemble the past.
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.
The cogito establishes only that a thinking thing exists. It does not by itself prove that your body, other people, or the physical world exist. Descartes needs the much weaker and more contested argument about a non-deceiving God to get that far, so do not overstate what the cogito delivers.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Practice6 marksExplain Descartes' method of doubt and the certainty he claims to reach with the cogito.Show worked answer →
A 6 mark response needs the method, its stages, and the cogito as its result.
The method. Descartes deliberately rejects any belief that could possibly be false, seeking an unshakeable foundation. This is methodological, not genuine, scepticism.
The stages of doubt. The senses sometimes deceive, so sensory beliefs are uncertain. The dreaming argument shows you cannot be sure you are not dreaming. The evil demon could deceive you even about mathematics.
The cogito. One thing survives: that he thinks, and so exists, since to be deceived he must exist to be deceived. The cogito is self-verifying because thinking presupposes a thinker.
Markers reward the three stages of doubt and a clear statement that the cogito is the one belief immune even to the evil demon. Strong answers note the cogito establishes only that a thinking thing exists, not the external world.
Practice16 marks"Hume's problem of induction shows that our ordinary beliefs about the world are irrational." Critically evaluate this claim.Show worked answer →
A 16 mark extended argument essay should reconstruct the argument and defend a verdict.
Reconstruction. Set out the problem: inductive beliefs (the sun will rise, fire will burn) reason from observed regularities to unobserved cases. This cannot be justified deductively, since the future differing from the past is no contradiction, and cannot be justified inductively without circularity. So our inductive beliefs appear to lack rational justification.
Case for the claim. If neither deduction nor non-circular induction supports our expectations, they rest on custom and habit, not reason, which looks like a verdict of irrationality.
Objections. Popper abandons induction, proposing conjecture and refutation. Strawson argues the demand for an external justification misunderstands what rational means, since being reasonable just is proportioning belief to evidence. Reichenbach offers a pragmatic vindication: if any method works, induction will.
Judgement. Conclude with a defended position, for example that Hume shows induction cannot be given a non-circular justification, but that this need not entail irrationality if rationality is internal to our evidential practices. Even Hume admitted no one can live as a sceptic. Markers reward an accurate reconstruction, engagement with at least one serious reply, and a reasoned conclusion.