4.1 Scepticism of the External World

What this lecture covers

Moving on from induction, this lecture introduces skepticism about the external world, the topic most associated with Descartes's First Meditation. Millican distinguishes this "vertical" skepticism from the "horizontal" skepticism about induction covered previously, then walks through Descartes's dreaming and evil demon hypotheses, his cogito argument, and the circularity problem that undermines his attempt to answer the skeptic.

You'll come away able to distinguish vertical from horizontal skepticism, explain why the cogito survives even the most extreme doubt, and describe the Cartesian circle.

Key ideas

• Vertical vs horizontal skepticism: vertical skepticism infers from one kind of thing (perceptions) to a different kind (external objects), while horizontal skepticism, like induction, infers more of the same kind of thing.
• Dreaming and the evil demon: Descartes strengthens ordinary doubt with the hypothesis that he might be dreaming, then with the hypothesis of an evil demon orchestrating all his perceptions.
• Brain-in-a-vat skepticism: a modern variant where a scientist manipulates the inputs to an isolated brain to simulate a full experienced world.
• The cogito: "I think, therefore I am" survives every skeptical hypothesis, since even a deceived or dreaming thinker must exist to be deceived.
• The general rule from the cogito: Descartes generalizes from his certainty about his own existence to claim that anything clearly and distinctly perceived is true.
• Proving God to validate the senses: Descartes argues a perfect, non-deceiving God exists and uses this to guarantee that clear and distinct perceptions are reliable.
• The Cartesian circle: Descartes uses his faculties to prove God exists, then uses God's existence to validate his faculties—a circularity that is a general problem for any attempt to answer skepticism from a position of total doubt.

Before you watch

• Review the previous lecture on the problem of induction, since Millican explicitly contrasts this "horizontal" skepticism with the "vertical" skepticism covered here.
• Familiarity with Descartes's name and general reputation as a skeptic is useful, though no prior reading is assumed.

Check your understanding

1. What is the difference between vertical and horizontal skepticism, and which type does external-world skepticism belong to?
2. Why does the cogito survive even the evil demon and brain-in-a-vat hypotheses?
3. How does Descartes try to move from certainty about his own existence to a general rule about truth?
4. What makes the Cartesian circle a problem for any attempt to answer skepticism, not just for Descartes specifically?