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Descartes and Hume on knowledge of the external world. Michael Lacewing Descartes: Meditation II. At first, our idea of the wax is of something defined by its sensory properties.

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descartes and hume on knowledge of the external world

Descartes and Hume on knowledge of the external world

Michael Lacewing

descartes meditation ii
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.
meditations v and vi
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.
descartes conclusion
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).
hume s sceptical argument
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.
hume s argument cont
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.
hume s argument cont1
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.
hume on primary and secondary qualities
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.
  • 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
  • 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
rationalism and empiricism
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