Descartes and Hume on knowledge of the external world

1. Descartes and Hume on knowledge of the external world Michael Lacewing enquiries@alevelphilosophy.co.uk

2. Descartes: Meditation II • At first, our idea of the wax is of something defined by its sensory properties. • But this is muddled: when I melt a piece of wax, it loses all of its original sensory qualities, yet I believe it is the same wax. • This shows our conception of material objects, when clear and distinct, is as changeable and extended.

3. Meditations V and VI • Meditation V: we can know that clear and distinct ideas are true; so material objects really are extended, if they exist at all. • Meditation VI: We have experiences of an external world, which must either be caused by a real external world or God. God is not a deceiver. Therefore material objects do exist. • Note: we can only infer, from the fact that God is not a deceiver, that there really is an extended world because we have done everything possible to avoid error.

4. Descartes’ conclusion • Our idea that material objects are extended and changeable is clear and distinct. • We can know there is an external, material world. • We can know, therefore, that the external world is an extended world. Sensory qualities do not properly belong to material objects (primary/secondary quality distinction).

5. Hume’s sceptical argument • We are naturally disposed to believe in the external world, and at first we think that our impressions are straightforward representations of it, i.e. perfectly resemble it. • On reflection, we don’t suppose a table gets smaller as we move away. • So we must accept that what is immediately available to the mind is only ideas, which don’t resemble objects perfectly; yet we continue to think that the objects represented persist independently of our impressions.

6. Hume’s argument (cont.) • But now we must wonder how we can show that our impressions must be caused by such independent objects! • Experience can’t show this, because all that experience has available is the impressions themselves, not the connexion between impressions and objects.

7. Hume’s argument (cont.) • We cannot use God to prove the existence of the external world. First, if God can never deceive us, then our senses must be infallible – which they are not; and second, we can’t prove the existence of God if we can’t even prove the existence of the external world. • The belief in the external world, therefore, is groundless.

8. Hume on primary and secondary qualities • We have no more reason to think primary qualities belong to material objects ‘in themselves’ than secondary qualities do: • We have nothing but our impressions to go on, and these don’t distinguish between the two. • Our concept of extension is derived from the senses, not the understanding.

9. Contrast • Hume’s attack on using God fails: • God is not part of the external material world • Descartes argues that God’s not being a deceiver does not make us infallible • On extension • Hume: our idea of extension must be formed by abstraction from sense experience • Descartes: it cannot be; but our conception of extension is still about what we sense

10. Compare • Only impressions and ideas are immediately present to the mind • Without God, Descartes also ends up a sceptic. • Arguing for naïve realism undermines both philosophers. • Both allow knowledge of geometry • Hume: relations of ideas • Descartes: knowledge of essential properties of objects

11. Rationalism and empiricism • Descartes’ rationalism: • arguments for God • experiences must have a cause • comprehension of material objects as extended doesn’t derive from the senses • Hume’s empiricism: • the idea of extension derives from the senses • attack on primary/secondary quality distinction • we don’t know experience must have a cause, and could only know the causes of experience from experience itself