Explain Descartes’ a priori deduction of the external world and how an empiricist might respond to this argument using Hume’s Fork

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AQA A-Level Philosophy | Paper 1 | 2027 Predicted Paper | Mark Scheme Question 4

Question 4

Explain Descartes’ a priori deduction of the external world and how an empiricist might respond to this argument using Hume’s Fork.

[12 marks]

What is expected in the answer?

This is a 12-mark AO1 question, so students should explain rather than evaluate. They need to reconstruct Descartes’ deduction clearly, then explain how Hume’s Fork can be used as an empiricist challenge to the possibility of gaining factual knowledge of the external world through a priori reasoning.

Both parts matter. AQA examiner reports on 12-mark questions repeatedly stress that answers need relevant, accurate material on both components to move into the upper bands, and the strongest answers logically integrate the two rather than presenting two disconnected blocks.

The current specification explicitly includes Descartes’ arguments for God and his proof of the external world as examples of a priori deduction, together with empiricist responses and the possible application of Hume’s Fork.

Levels of response

Marks	Descriptor
10–12	A precise, detailed and fully integrated explanation. Descartes’ deduction and the application of Hume’s Fork are both accurately explained, with clear logical links and sustained relevance.
7–9	A clear, integrated and logical response showing detailed understanding of both parts. There may be some minor imprecision, omission or redundancy.
4–6	A largely correct and coherent response. Relevant points are made about both parts, but there may be gaps, limited development or weak integration.
1–3	Some relevant knowledge is shown, but points are fragmented, significantly incomplete or imprecise. One part of the question may be largely absent.
0	Nothing written worthy of credit.

This follows the structure AQA uses for its 12-mark AO1 questions. AQA’s examiner reports emphasise that simply knowing both topics is not enough for the highest marks, the relationship between them needs to be made clear.

Indicative content

Descartes’ a priori deduction of the external world

Students may begin by locating the argument within Descartes’ intuition and deduction thesis. Descartes holds that knowledge can be gained through a priori intuition and a priori deduction. His proof of the external world is one of the deductions explicitly required by AQA.

AQA’s own published mark scheme reconstructs the argument along the following lines:

Descartes has perceptual experiences as if there are physical objects.
These perceptual experiences must have a cause.
Possible causes include:
- Descartes’ own mind
- God
- external physical objects.
The experiences cannot simply be caused by Descartes’ own mind because they are involuntary, they occur independently of his will.
Descartes also has a very strong natural inclination to believe that these experiences are caused by physical objects.
God exists and is not a deceiver.
If God directly caused Descartes to have these experiences while allowing him to be systematically mistaken in his strong belief that external objects cause them, God would be deceptive.
Therefore the best remaining explanation is that perceptual experiences are caused by external physical objects.
Descartes concludes that an external world exists.

The argument depends on earlier stages of Descartes’ reasoning, particularly his claimed proof of the existence of a perfect and non-deceiving God.

A strong student should make clear that this is intended by Descartes as an a priori deduction, rather than an inductive inference from repeated sensory experience.

Hume’s Fork

An empiricist can challenge this through Hume’s Fork.

Hume divides knowledge into:

relations of ideas, which are a priori, necessary and analytic
matters of fact, which are a posteriori, contingent and synthetic.

Relations of ideas can be known without experience. Their denial produces a contradiction.

For example, a mathematical or definitional truth may be known simply through reasoning.

Matters of fact, however, concern how the world actually is. Their opposites can be conceived without contradiction, and they therefore require experience.

The existence of an external physical world appears to be a matter of fact rather than a relation of ideas.

There is no contradiction in supposing:

“There is no mind-independent external world.”

The claim that an external world exists is therefore not analytically or necessarily true merely in virtue of concepts.