Summary of Descartes' argument

In essence, Descartes' argument is this:

  1. I have an idea of an infinite substance, i.e. I have an idea of God.
  2. An idea of an infinite substance has as much (objective) reality as an infinite substance would have formal reality if it existed.
  3. There must be at least as much reality in the cause as in the effect.
  4. Therefore, my idea of God (an infinite substance) must have been caused by an actual infinite substance.
  5. Therefore God exists.

 

In the course of the argument Descartes also makes the following points:

  • Ideas are either (i) innate, i.e. inborn or known from one's own nature, (ii) adventitious, i.e. come from outside me, or (iii) made by me.
  • If I can be sure that the objective reality of one of my ideas is so great that it isn't in me either formally or eminently and hence that I cannot be the cause of that idea, I can infer that I am not alone in the world—that there exists something else that is the cause of the idea.
  • I could have composed my ideas of animals, other men and angels. (There is a brief argument to support this.)
  • I could have composed my ideas of physical objects without these existing. (There is another argument to show that this is true.)
  • The idea of God is completely clear and distinct and contains more objective reality than any other idea.
  • If I were greater than I have assumed I could be the cause of the idea of a being with all perfections but the gradual increase in my knowledge shows that I am imperfect so that cannot be.
  • If one concentrates carefully, all this is quite evident by the natural light.