1. Descartes and Hume on knowledge of the external world
Michael Lacewing

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: On extension
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