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René Descartes (1596-1650). Father of modern rationalism. Reason is the source of knowledge, not experience . All our ideas are innate . God fashioned us with these ideas. We discover basic truths by intuition : by grasping basic connections between the ideas we have.

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ren descartes 1596 1650
René Descartes (1596-1650)
  • Father of modern rationalism.
  • Reason is the source of knowledge, not experience.
  • All our ideas are innate.
  • God fashioned us with these ideas.
  • We discover basic truths by intuition: by grasping basic connections between the ideas we have.
  • We deduce or demonstrate more complex truths.
aim and method
Aim and method
  • Descartes wants knowledge.
  • He knows that he has many false beliefs.
  • He needs to weed them out to establish base of indubitable, necessarily true beliefs.
  • Foundationalism – basic beliefs provide the ultimate source of justification.
  • His method is to challenge each thing he believes to see whether it is “completely certain and indubitable”. This is known as the method of doubt.
  • Key reading – Ch. 4; Meditation 1.
scepticism wave i
Scepticism: Wave I

What does this show? Simply that there is a reality and something we could have true beliefs about. But without a test, we can’t know which beliefs are true. So, there’s no way out of the sceptical problem here.

The sceptic says: knowledge is impossible!

So, all that I believe could be false?

A test can only have wrong answers if there are right answers too.

Surely not! A mistake can only exist where there is truth or correctness.

A counterfeit £10 can exist because there are real ones. A counterfeit £15 note can’t.

scepticism waves ii and iii
Scepticism: Waves II and III

The sceptic says: knowledge is impossible!

…and so on, forever. So, even if there were a test, it would be of no use!

Could there not be a test to prove whether I am in the Matrix?

…but then how can I be sure I am really performing T*? I would need another test, T**…

Let’s suppose so: a test T.

I would need another test T* to check that I was performing the first test, T, correctly…

I carry out T to determine whether I am in the Matrix…

…or do I? How can I be sure I really did the test instead of being fooled into thinking I did it?

cogito and self
Cogito and Self

Not reason that tells me – awarneess.

I am a brain./

descartes and the wax
Descartes and the wax

The Empiricist Hume will later argue that Descartes is wrong.

All I can know are the surfaces of things: what my senses tell me.

I simply come to believe that, despite changes in appearance, there is still the same wax because this simplifies my understanding of the world.

Consider a game like Call of Duty. All I see is a two-dimensional grid of pixels…

But my mind interprets certain arrangements of pixels as three-dimensional objects, even though there are no such things really there.

In the same way, perhaps I interpret reality as a three-dimensional world of objects as this is a simpler explanation. It would be impossible to cope if I thought everything was always changing.

descartes and god the ontological argument
Descartes and God: The Ontological Argument

A real football is just as much a football as an imagined one.

Philosophers such as Gassendi, Kant and Russell claim this argument does not work.

What makes (e.g.) a football what it is are properties such as being spherical and being bouncy.

So, we can’t say God has this property either. What makes God God are properties such as:


A real football doesn’t have an extra property of existence.

Existence isn’t a property. It doesn’t make anything the sort of thing it is.