“Cogito, ergo sum.” “I think, therefore I am.”

 chief architect of 17 th C intellectual revolution  laid foundations of ‘modern scientific age’

 Born Touraine, France  “well-to-do” family  Sent to school: studied mathematics, logic, philosophy  Impressed by “certainty” of mathematics  Disappointed by philosophy, contained “no point which was not disputed & hence doubtful.”

 True knowledge could only emerge from the power of human reason  At the age of 25, he decided: ◦ He would no longer rely on ideas of other philosophers (just because they were accepted authorities) ◦ To use his own rational powers to determine truth  His major works: ◦ Discourse on Method (1637) ◦ Meditations on the First Philosophy (1641) ◦ Principles of Philosophy (1644) ◦ Passion of the Soul (1649)

 From Intuitive Certainty to Truth  Recall: philosophers have long sought truth & certainty  Descartes eventually rejected ideas from commonly accepted sources: they were uncertain & contradictory methods for determining truth  His conclusion: all knowledge began with ideas in the mind

 “reject as absolutely false everything concerning which I could imagine the least ground of doubt.”  i.e, it is possible to doubt everything (shades of Chuang Tzu?)  He continued to seek one solid certainty upon which he could build knowledge

 He must exist  In the act of doubting everything, there is a being who doubts:  “I think, therefore I am,” “Cogito, ergo sum.”  Certain that he exists, what else can he know with certainty?

 In his thoughts, Descartes realized that he found the idea of God as an infinitely independent, all-knowing, and all-powerful being.  From where could this idea come?  Since it cannot come from himself: ◦ an imperfect being such as himself could not have developed such an idea of perfection ◦ The idea must have come from God ◦ Therefore, God must exist

 We have impressions of physical objects (sometimes against our will)  Is it possible that we are being fooled?  Would God be deceiving us?  No! ◦ God is perfect & wouldn’t want to deceive ◦ Therefore external objects must exist!

 He existed  God exists  External objects exist

 Orderly & systematic thinking  Two major elements: ◦ Intuitive certainty ◦ Deductive logic  Descartes believed: we can be certain of a truth when we recognize it intuitively, absolutely, and without question  E.g., our consciousness of our own existence, ◦ = 4, ◦ one cannot be in two different places at the same time, ◦ the parts of an object split in two pieces are smaller than the original whole

 Intuitive certainty is an essential characteristic of truth  We use deductive reasoning to move with certainty from one truth to another  Deductive reasoning would mean if one accepts the premises, one would accept the conclusion

 Do not begin with what others say: begin with what is clear and certain  Reduce each step to its simplest components  Move step by step from easiest to more difficult ideas  Review carefully being careful to omit nothing  Stop when you arrive at a step is unclear  (read Meditations of First Philosophy excerpt on page 112)