1. RENE DESCARTES
French philosopher, mathematician and physical scientist (optics, physics, physiology)
Father of Early Modern Rationalist Philosophy
Early Modern Philosophy is characterized by the thesis that genuine knowledge can be achieved by humans through the use of their rational and perceptual faculties independent of any form of divine revelation.
A priori knowledge is possible
A priori knowledge is required in order for there to be a posteriori, empirical or scientific knowledge.

2. Descartes’ Scientific Contemporaries
Copernicus (Polish; )
Astronomy: Heliocentric solar system
Challenge to Church-endorsed Geocentric universe
Francis Bacon (English; )
Development of the scientific method
Galileo (Italian; )
Mathematician, Physicist & Astronomer; Copernican
Challenge to Church’s claims of divine revelation of natural laws
Kepler (German; )
Discovered laws of planetary motion
Boyle (Irish; )
Developed experimental chemistry; worked in mechanics, medicine, hydrodynamics
Newton (English; )
Fundamental laws of physics; classical mechanics
Develops the calculus (independently, so too does Leibniz (1646–1716))

3. Descartes as a Foundationalist
Descartes is a Foundationalist who repudiates the Augustinian doctrine of Illumination and affirms that the human mind is capable of genuine knowledge of universal and necessary truths
Foundationalism: By appropriate use of their rational faculties for a priori reasoning, humans can autonomously come to know with appropriate certainty the fundamental truths evidentially basic to the empirical or a posteriori sciences of both the world and ourselves

4. Descartes’ Refutation of Skepticism
As a Foundationalist, Descartes refutes Skepticism
The skeptic maintains that
Certainty is required for knowledge
However, humans are incapable of certainty
Hence, humans are incapable of knowledge
Thus, science of both the world and ourselves is impossible

5. The Skeptic’s Argument
Meditation I: Descartes’ provisional argument on behalf of the skeptic
Knowledge requires certainty.
Certainly is either empirical or a priori
Empirical certainty is impossible because of
Illusion: hence, no empirical certainty regarding attributes of material substances
Hallucination : hence, no certainty regarding the existence of any particular material substance
Dream Hypothesis: hence, no certainty regarding the existence of the material universe generally
A priori certainty is impossible because of
Evil Demon hypothesis
Hence, certainty is impossible
Hence, knowledge is impossible

6. The Cogito as Descartes’ Reply to the Skeptic
Cogito, ergo sum!
No evil demon could delude one about one’s own existence
Thus, some a priori knowledge is possible!
Each person can be a priori certain and have genuine a priori knowledge about
His/her own individual existence as a thinking substance
The existence and content of his/her own current ideas (i.e. psychological or mental attributes).
Thus, one can know with a priori certainty what one believes about
God
The material universe.

7. From the Cogito, through the Ontological Argument, to A Priori Knowledge
Thus, one may know with certainty the content of one’s idea of God as the perfect being.
Thus, Anselm’s Ontological Argument is certain and sound.
Hence, God exists!
God’s existence implies that the Demon Hypothesis is false.
Hence, a priori reasoning can provide certainty.
Hence a priori knowledge of all of logic and mathematics is possible.

8. Empirical Knowledge is Possible Too
Perceptual ideas are known reflections within the mind of the (yet) unknown character and existence of the external material
The existence of God implies that sensation and perception – if employed according to God’s design and as characterized by science - produce generally reliable, even if not absolutely certain, representations of the material universe.
Hence, empirical knowledge of the material universe is possible through science

9. Problems with the Cogito as the Foundation of Knowledge
Circularity in arguments for the existence of God in refuting the Demon Hypothesis
Descartes appeals to the Ontological Argument to refute the Demon Hypothesis.
But, the Ontological Argument is an instance of a priori reasoning.
All a priori reasoning is dubious according to the skeptic’s evil demon hypothesis
So, the Ontological Argument circularly presupposes that the evil demon hypothesis is false
In that case, the Ontological Argument is not a legitimate refutation of the evil demon hypothesis.

10. Other Problems with the Cogito
The Cogito aims to demonstrate that we know with certainty our current thoughts.
But empirical evidence suggests the opposite:
Evil Sophomores and the effects of psychological priming:
I might falsely believe that I am experiencing pain
The unattended channel and confabulation