Renés Descartes
Descartes (1596 - 1650)
Lecture Notes: Raw form

He discovered Analytical Geometry.
This fulfilled the Pythagorean dream of demonstrating the relation between plane geometry and pure algebra.

Context of his writing:
Serious challenges threatened the prevailing world view that Man was the center of the universe.

Galileo Galilei discovered that Jupiter had three moons.

The significance of this is that it was argued that because the moon clearly orbited the earth, that everything else did too.

Galileo’s discovery pulled out the last support for the geocentric (Earth is the center) theory of the universe.

In 1633 Descartes was about to publish a manuscript on physics, but held back when Galileo was arrested by the Inquisition for teaching similar ideas.

Descartes was a good Catholic, but a modern Catholic.

Implications of science on inflexible religion.

The purpose of Meditations.

The plan was to embed his ideas in the Meditations, hoping the Church leaders would accept his ideas without realizing it.

Meditations On First Philosophy
Important stylistic points

Intensely individualistic
Written in first person

Instead of a formal exposition, he takes the reader on a dramatic voyage from doubt to certainty.

The title Meditations conveys the character of the work -- a private mental exercise that the reader must “make his own.”

Descartes insists on beginning philosophy afresh; starting from scratch by ridding the mind of all accumulated preconceptions, through his . . .


First, Testimony of the senses is scrutinized.

Senses sometimes deceive
However, some sensory evidence is so clear that only a madman would doubt it, “for example that I am here seated by the fire wearing a dressing gown.”

Second doubt; one might be dreaming, so even the belief about the fire and dressing gown could be false.

What about mathematics?
Perhaps it can be a candidate for absolute certainty (consider Plato’s Forms).

“Whether I am asleep or awake, 2 and 3 are 5, and a square has no more than four sides.”

Radical doubt requires that even this be doubted.

The Evil Genius, the malevolent demon

:...who has employed all his energies in deceiving me. I shall consider the sky, earth, colours, shapes, sounds, and all external things are not more than the delusions of dreams . . I shall consider myself having no hands, no eyes,. . .”

Descartes even considers the possibility that the entire world was nothing but the dream of the evil genius.

Ultimately, any judgment about the external world (any existential claim) must be suspect.

The First Meditation ends with Universal Doubt.

The Second Meditation
One proposition cannot be doubted.

“However great the demon’s deceptions, he can never cause me to be nothing so long as I think I am something.”

Descartes concludes, “I am, I exist, is necessarily true as often as I put it forward or conceive of it in my mind.”

This leads to “I think, therefore I am.”

Cogito ergo sum

There is at least ONE existential truth -- that something which he calls “I” exists.

He then begins to build

He must find a way of escaping the confines of his own subjectivity and establish the existence of an external world.

He first examined the contents of his own mind and discovered it contained certain innate ideas such as,
1. Self
2. Identity
3. Substance
4. God

The First argument for God’s existence
1. men know that they are imperfect, yet find in their minds an idea of supreme perfection.

2. A cause must contain at least as much reality as its effect (again, remember Plato), to the conclusion that . . .

3. the cause of the idea of perfection must itself be a perfect being -- God.

4. A perfect, omnipotent, omnibenevolent God would not allow the evil Deceiver to exist.

Nex he used a version of St. Anselm’s “ontological argument” which as a priori, i.e., making no appeal to the external world--(a world which for Descartes did not yet exist), he “proved” God’s existence, thereby disposing of the Evil Genius

1. Existence, being a perfection, can no more be separated from the concept of a supremely perfect being than the fact that its angles equal two right angles can be separated from the concept of a triangle.

2. So, the supremely perfect being (God) must by definition exist.

Building upon this Descartes recovered math into his system (the only objection to math was the Evil Genius hypothesis.)

Applying math to his innate idea of corporeal substance, Descartes came up with what he thought to be the correct account of reality -- the world as known by mathematical physics (Again, remember Plato’s Forms).

1. Scientific knowledge is proven to be possible.

2. For once God’s existence is established, one is no longer limited to the private momentary certainty of one’s own existence;

3. one can now, since God is benevolent and non-deceiving, have a reasonable degree of confidence in the existence of an external world, in one’s powers of memory, in the normal reliability (subject to careful scrutiny) of the senses.

4. The construction of a systematic body of truths becomes possible.

He thus proved that you can have both God and Galileo!

Some interesting observations

Descartes assumes basic logical truths cannot be doubted.
The Law of Non-Contradiction
Descartes never discusses this.

The world is not “what you see is what you get” (naive realism), but a bunch of mathematically measurable quantities (a cold, colorless, odorless, soundless, tasteless world of matter in motion.)
All perceivable qualities (“red,” “blue,” “sweet,” “warm,” “melodious”) are in the mind.

From the proposition “I think, therefore I am,” Descartes proceeds to “I am a being whose whole essence or nature is to think, and whose being requires no place and depends on no material thing.”

For Descartes “thinking” includes intellectual and volitional activities, such as willing and affirming, and even the mental awareness involved in the operations of imagining and perceiving.

“Thought is a world that covers everything that exists in us in such a way that we are immediately conscious of it.”

The result is that the world is made up of two kinds of substances

Mind or consciousness (res cogitans), which is unextended and indivisible, and . . .

Matter (res extensa), which is extended and divisible.

* Substance is defined as “an existent thing which requires nothing but itself in order to exist.”

The mystery of “intermingling” was never unravelled by Descartes

“Cartesian dualism” (mind/soul are interchangeable terms)
The mind is one substance and the body is another.

Descartes was a RATIONALIST

He attempted to construct a world system purely a priori, in contrast to the observational methods of the empiricists.