Rene Descartes
“I think, therefore a whole mess of things are certain”
  • Father of Modern Philosophy
  • Descartes and Fermat independently founded Analytic Geometry in the 1630s
    • Analytic Geometry uses algebra to represent geometric objects
  • Philosophic Works
    • Discourse on Method (1637)
    • Meditations (1641)
    • Principles of Philosophy (1644)
    • Passions of the Soul (1649)
  • Themes
    • Methodic Doubt
    • Cogito Ergo Sum
    • Principle of Clear and Distinct Ideas
    • Ontological Proof
    • Mind-Body Dualism
Methodic Doubt
  • Descartes’ objective was to put knowledge on a certain foundation by logically deriving what we naturally accept as true from absolutely certain truths.
    • If A is certain and B logically follows from A, B is also certain.
  • His approach was to set aside everything he could doubt and see what remained.  He set aside:
    • knowledge from tradition, because authorities disagree
    • knowledge of the physical world, because of illusions, hallucinations, and dreams
    • mathematical knowledge, because people make errors calculating
  • He went further, concocting the idea a powerful evil genius whose mission was to fool Descartes.
Cogito Ergo Sum

After meditating a while, Descartes realized the evil genius couldn’t fool him into mistakenly thinking he was thinking.  So, while thinking, Descartes was certain he was thinking. He was also sure he existed.

Principle of Clear and Distinct Ideas
  • Descartes had found two things he was sure of: that he thought and that he existed.  Were there other certainties? Descartes observed that he also clearly and distinctly perceived that he thought and existed.  So Descartes entertained the hypothesis:
    • Whatever is perceived clearly and distinctly is true. (C&D Principle)
  • The idea required proof.  Descartes noted that the principle would be true if God existed and was not a deceiver.  To establish the principle, therefore, Descartes had to prove that a non-deceiving God existed. The proof also had to be a priori, since at this point in his thinking Descartes had set aside the existence of the physical world.  Descartes thus put forth a variant of Anselm’s a priori Ontological Argument.
Descartes’ Ontological Argument
  • A supremely perfect being would have every perfect attribute.
  • Necessary existence is a perfect attribute.
  • Thus, a supremely perfect being would have the attribute of necessary existence.
  • A supremely perfect being, therefore, would necessarily exist.
  • Which implies that a supremely perfect being exists.
  • A supremely perfect being is not a deceiver, since being a deceiver is an imperfection
  • Hence, finally, whatever is clearly and distinctly perceived is true.
Descartes’ use of the C&D Principle
  • Descartes used his C&D Principle, that whatever is perceived clearly and distinctly is true, to establish:
    • The truths of mathematics (Principles, Part 1,XXX)
    • That the will is free (Principles, Part 1,XXXIX)
    • What our senses clearly and distinctly perceive is the case (Principles, Part 1,XXX)
    • There are three basic kinds of substance
      • Matter, whose essence is extension in space
      • Mind, whose essence is thought
      • God, whose essence is perfection
Mind-Body Dualism
  • Descartes held that a person was an immaterial mind interacting with a human body, making it move and experiencing sensations through its organs.
  • But Descartes went further: he pinpointed the place of interaction, the tiny pineal gland in the middle of the brain. When you bend a finger, your will causes changes in the pineal (PI-knee-ul) gland that radiate through the brain and down the arm, contracting muscles in the hand. When you see a tomato, light waves bouncing off it travel into your eyes, sending signals to the pineal gland that cause a red, tomato-shaped sensation.
Criticisms of Descartes
  • Descartes’ Ontological Argument is fallacious.
    • Kant’s criticism that “existence is not a predicate”
  • It’s inconceivable that an immaterial mind can causally interact with a material brain. 
    • Spinoza and Leibniz put forth alternatives to Cartesian Interaction
      • Spinoza: Monism
      • Leibniz: Pre-established Harmony
  • Cartesian Circle
    • Antoine Arnauld, a contemporary of Descartes, accused Descartes of circular reasoning (or begging the question);
      • Descartes’ idea was to establish the C&D Principle by proving that God existed and was not a deceiver. But look at two premises of Descartes’ Ontological Proof:
        • A supremely perfect being has every perfect attribute.
        • Necessary existence is a perfect attribute.
      • How did Descartes know these things?  Presumably because he clearly and distinctly perceived them; and per the C&D Principle they were therefore true. But he was trying to prove the C&D Principle.  He thus used the principle in proving the principle, a case of circular reasoning.
Addendum: Course of Descartes’ Reasoning
  • It’s certain I think
    • It’s impossible I mistakenly think I’m thinking.
  • It’s certain I exist
    • Cogito Ergo Sum
  • It’s certain God exists and is not a deceiver.
    • Ontological Argument
  • It’s certain that whatever is perceived clearly and distinctly is true. (C&D Principle)
    • God is not a deceiver.
  • The truths of mathematics are certain
    • C&D Principle
    • The truths of mathematics are perceived clearly and distinctly.
  • It’s certain that the will is free
    • C&D Principle
    • That the will is free is perceived clearly and distinctly.
  • It’s certain that what our senses clearly and distinctly perceive is the case
    • C&D Principle