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General Philosophy · Lecture 14 of 33 · 8:47

4.1 Scepticism of the External World

4.1 Scepticism of the External World on YouTube

Study guide

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?

Vocabulary

skepticism (noun)
The philosophical position of doubting whether we can really know certain things.
This lecture introduces skepticism about the external world.
external world (noun)
The physical world that exists outside and independently of our minds.
Descartes doubts whether the external world really exists.
vertical (skepticism) (adjective)
Describing doubt that moves from one kind of thing to a different kind of thing.
Vertical skepticism infers from perceptions to external objects.
horizontal (skepticism) (adjective)
Describing doubt that stays within the same kind of thing, like inferring more instances of a pattern.
Induction is an example of horizontal skepticism.
evil demon (noun)
Descartes's hypothetical powerful being that could deceive us about everything we perceive.
The evil demon hypothesis pushes doubt to its extreme.
brain-in-a-vat (noun)
A thought experiment where a brain is kept alive and fed fake sensory signals to simulate reality.
Brain-in-a-vat skepticism is a modern version of the evil demon.
cogito (noun)
Descartes's argument 'I think, therefore I am,' proving one's own existence.
The cogito survives even the evil demon's deception.
clear and distinct perception (phrase)
An idea so obvious and precise that it seems impossible to doubt.
Descartes claims clear and distinct perceptions must be true.
validate (verb)
To confirm that something is true, accurate, or trustworthy.
Descartes tries to validate the senses using God's existence.
Cartesian circle (noun)
The circular reasoning where Descartes uses his mind to prove God, then uses God to trust his mind.
The Cartesian circle undermines Descartes's proof.
generalize (verb)
To extend a specific conclusion into a broader rule.
Descartes generalizes from the cogito to a rule about truth.
orchestrate (verb)
To organize and control something carefully, often secretly.
The evil demon supposedly orchestrates all of Descartes's perceptions.
distinguish (verb)
To recognize or state the difference between two things.
Millican distinguishes vertical from horizontal skepticism.
strengthen (an argument) (verb)
To make a claim or argument more convincing or forceful.
Descartes strengthens ordinary doubt with the dreaming hypothesis.
manipulate (verb)
To control or influence something skillfully, often unfairly.
A scientist manipulates inputs to an isolated brain.
guarantee (verb)
To make something certain to happen or be true.
God's existence is meant to guarantee reliable senses.
undermine (verb)
To weaken the basis or credibility of something.
The Cartesian circle undermines the whole proof.
extreme (doubt) (adjective)
Taken to the furthest or most intense degree.
The evil demon represents extreme doubt.
survive (a challenge) (verb)
To remain true or valid despite a strong objection.
The cogito survives even the most extreme doubt.
hypothetical (adjective)
Imagined or supposed, not necessarily real.
The evil demon is a hypothetical being.

Chapters

From the YouTube description

A series of lectures delivered by Peter Millican to first-year philosophy students at the University of Oxford. The lectures comprise the 8-week General Philosophy course and were delivered in late 2009.

Slides for all his lectures can be found here:
http://podcasts.ox.ac.uk/people/peter-millican

Learn more about Oxford:
http://www.ox.ac.uk

← 3.3 The Problem of Induction · 4.2 Possible Answers to Scepticism of the External World →