1. 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.

2. The Scientific Revolution
Lived during the Scientific Revolution (17th century).
This was a time of great intellectual change. Most famously, people started to believe that the earth orbited the sun thanks to Copernicus (in 1543).
Descartes’ predecessors believed that knowledge begins with experience (empiricism)
Descartes argued that this view must change too.
His Meditations (1641) is effectively a manifesto for rationalism.

3. 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.

4. Scepticism: Wave I
But that our senses can let us down sometimes is no good reason to believe they always do. I would be mad to deny that I am sitting here at my desk looking at a screen!
The basis of our ordinary beliefs is sensory experience.
But we all know that our senses are fallible.
The pencil is not really bent.

5. Scepticism: Wave II But what if I were dreaming?
And whether I’m dreaming or awake, 2+2=4 and a square has four sides.
If there are no clear signs by which to distinguish waking experiences from dream experiences, then I cannot be certain that any sense-based belief is true.
In the same way, there must be colours and shapes I have experienced in order to create dreams. So I can be certain that there is a world out there even if I can’t be sure I’m experiencing it now.
But dreams are like paintings. A painting may be of something that does not exist (like a winged badger) but the raw ingredients – colours and shapes – must.

6. If only I could find just one thing of which I could be certain!
Scepticism: Wave III
But what if I were in the grip of all-powerful demon – like the Matrix.
If only I could find just one thing of which I could be certain!
He could not just delude me into thinking I have a body…
If so, I cannot be certain of anything. The sceptic has shown that philosophy’s goal of obtaining certainty cannot be met.
…but deceive me about everything!
The existence of space and the ‘truths’ of mathematics and geometry could all be powerful illusions!

7. Descartes and the Cogito
“So after considering everything very thoroughly, I must finally conclude that this proposition, I am, exist is necessarily true whenever it is put forward by me or conceived in my mind.” Meditation II.
Cogito, ergo sum
I think, therefore I am
Descartes’ first certainty is established through the application of reason alone: I exist. He has put into doubt all beliefs arrived at by the senses through the use of the ‘evil demon’. His certain existence results from his awareness of himself as the subject of his thoughts.

8. I am a thinking thing – a non-physical conscious self
Descartes and the Mind
I know that I exist.
But what am I?
I am a thinking thing – a non-physical conscious self
Descartes argues that he can clearly doubt or conceive of himself lacking a body whilst still existing but not a mind. So, he and his body are distinct things.
It is reason that tells him that he exists and what he is.

9. …but I know it is the same wax.
Descartes and the wax I
BEFORE
Sight: bright yellow
Smell: flowers
Taste: pollen
Sound: dull thud
Feel: solid
If all I had were senses, I would have to conclude the old wax had been destroyed and that something new had appeared….
AFTER
Sight: dull yellow
Smell: none
Taste: none
Sound: none
Feel: soft, malleable
…but I know it is the same wax.

10. Descartes and the wax II
How do I know it is the same wax if my senses tell me that everything about it has changed?
Perhaps my power of imagination – my ability to create mental pictures – is at work.
I can picture all the ways the wax could look and hence am able to track the wax as the same wax.
This can’t be right. There are far too many ways the wax can appear for me to have imagined them all.
I can’t imagine 80 dots in a line and picture them dividing into two groups…
Furthermore, my powers of imagination are extremely limited!
…but I can think of 80 dots and divide them into two groups using my powers of reason.

11. Descartes and the wax III
What enables me to see the wax is my mind – reason.
Reason understands that the essence of material things is extension: being (filled) bits of three-dimension space.
I thus have the idea of a 3D object. My senses present me with the surface appearances and reason makes me see them as features of a persisting object.
I say, “I see the wax!”
In reality, I see the colours and judge that there is wax there.
In the same way, I say, “I see the men in the square.”
In reality, I just see clothes but I judge that there are men (and not automatons) underneath.

12. Descartes and God: The Ontological Argument
We understand the concept of a unicorn. We grasp the essence of a unicorn as a magical white horse with a horn.
Essence is one thing and existence another. Just because the idea of a unicorn makes sense, it doesn’t mean there are any.
Except in the case of God. God is an infinitely perfect being. He is…
…omnipotent: maximally powerful.
4: necessarily existent: God
3: contingently existent: you
2: contingently non-existent: unicorn
1: necessarily non-existent: round square.
…omniscient: maximally knowledgeable.
…omnibenevolent: maximally good.
So if it is part of God’s essence to exist necessarily, God must exist!
…necessarily existent: maximally real.

13. The naïve view of our perception of the world
The redness in the cube is transmitted to my mind where I perceive it
Red light travels to my eye.
Redness is in the cube. It’s a feature of the world.

14. Descartes and our perception of the world
Colourless matter:
Our innate ideas of colours are triggered in the mind by the information in the brain sent from the eyes. The colour is in me.
Colourless signals are sent to my brain.:
Colourless light is sent to my eye.:
The cube is red in the sense that it has a chemical structure that our brains react to with a sensation of red. It is not red in the way we experience it as red.
Colourless matter:

15. Descartes’ Rationalist Conclusions.
Cogito, ergo sum: I think, therefore I am. My ability to think proves I exist.
The senses are fallible and could be deceived.
Reason leads me to the conclusion that I am a thinking thing.
It is also thanks to reason that I see the world. The senses inform us of a changing world. Reason grasps the underlying constancy of the world. It enables us to see things. (Think of the wax example.)
Reason is needed for science. Science is not concerned with the sensible properties of things. It is concerned with quantifiable properties. Reason is required to work out the mathematical structure of the universe: e.g. the laws of physics.
Reason proves that God exists too.
And all the ideas I have are innate!