Skepticism

Author
Affiliation

Christopher L. Holland

Saint Louis University

Lecture Date

September 29, 2023

Updated

February 27, 2025

1 Types of Skepticism

  • Global Skepticism: We cannot know anything at all!
  • External World Skepticism: We cannot know anything about the external world.
  • Local Skepticism: We cannot know anything about a specific domain.

2 Arguments for Skepticism in Descartes’s Meditations

  • Illusion Argument
    • How do you know that your senses are not deceiving you?
    • The Illusion can call into question some knowledge of the external world, but not all of our knowledge about the external world.
  • Dream Argument
    • How do you know that you’re not dreaming?
    • The Dream Argument can call the external world into question—including disciplines like physics, astronomy, and medicine.
    • Descartes did not think that the Dream Argument could call into question a priori knowledge. You can still know that \(2+3=5\) and that a square has 4 sides.
  • Evil Demon Argument
    • How do you know that you’re not being deceived by an Evil Genius/Demon?
    • Evil Genius/Demon = A deceiver of supreme power and cunning who is deliberately and constantly deceiving you.
    • The Evil Demon Argument can call the external world into question.
    • The Evil Demon Argument may even call a priori knowledge into question. What if the demon tampers with your memories? Can you still know that \(2+3=5\)?
    • Descartes’s reply: I think, therefore I am.
      • The Evil Demon cannot make us doubt everything, because it cannot make us doubt that we are doubting.
      • Still, if all you can know is that you exists, you know very little.

3 Dealing with External World Skepticism

3.1 G.E. Moore: You can know things you can’t prove

  • Moore’s Three Proof Conditions

    1. The premises are different from the conclusion (i.e., the argument is not circular).
    2. The premises are known to be true.
    3. The conclusion really follows from the premises (i.e., the argument is valid)

  • Moore’s Proof for the External World

    (P1)
    Here is my left hand (points to his left with his right) and here is my right hand (points to his right with his left).
    (C1)
    Therefore, at least two human hands exist.
    (P2)
    If at least two human hands exist, then the external world exists.
    (C2)
    Therefore, the external world exists.

  • Moore’s Argument and the Skeptical Principle

    • The Skeptical Principle: You can only know what you can prove

    • But according to Moore: You can know things you can’t prove (e.g., that you have hands)

    • We can think of this as an inconsistent triad

      1. If you know \(X\) then you can prove \(X\)
      2. You cannot prove \(X\)
      3. You know \(X\)
      • The skeptic argues from A and B to not-C
      • Moore argues from B and C to not-A

3.2 Bertrand Russell: Appeal to Simplicity

  • You cannot prove that there is an external world
  • However, you can argue that skeptical scenarios (e.g., that the world around us is an illusion, dream, demonic deception, or simulation) are less probable than the common sense view that the external world exists
  • According to Russell the external world hypothesis is a better explanation for the evidence than any skeptical scenario.

3.3 David Chalmers: Structural Realism

  • Chalmers’s version of the skeptic’s argument:

    (P1)
    You can’t know that you’re not in a simulation.
    (P2)
    If you can’t know that you’re not in a simulation, then you can’t know anything about the external world.
    (C1)
    Therefore, you can’t know anything about the external world.

  • Chalmers’s Reply: Simulation Realism

    • According to Chalmers simulated reality is reality. If you are in a simulation, then the simulated world is your external world. So, whenever you know something about your simulated world, you know something about your external world.

    • Chalmers’s response relies on a view called “Structural Realism”

      • There are various regularities in the world around us.
      • These regularities are best explained by common structures.
      • These common structures just are the external world.

    • Chalmers agrees with (P1) but rejects (P2)

3.4 References

Chalmers, David J. 2022. Reality+: Virtual Worlds and the Problems of Philosophy. Kindle. New York: W. W. Norton & Company.
Descartes, René. (1614) 2013. Meditations on First Philosophy: With Selections from the Objections and Replies: A Latin–English Edition. Edited and translated by John Cottingham. New York: Cambridge University Press.
Nagel, Jennifer. 2016. “Epistemology: 9. ‘Knowledge First’ Epistemology.” Wireless Philosophy. March 25, 2016. https://www.wi-phi.com/videos/knowledge-first-epistemology/.