The Problem of Skepticism

The discussion focuses on "knowing that," which involves knowing a proposition (p) or a claim.
Examples of "knowing that": knowing that two plus two is four, lemons are yellow, or atoms exist.
Descartes argues that we don't know anything, challenging our assumed knowledge.

Descartes presents a three-part argument to demonstrate that we have no knowledge.
Understanding different types of truth is crucial to grasp his argument.

Truths can be categorized into observable and unobservable truths.
Observable truths can be further divided into directly and indirectly observable truths.

Observable Truths: Truths that can be observed.
- Directly Observable Truths: Truths directly perceived through our senses.
  - Examples: "The grass is green," "The thunder is loud," "The lemon is sour," "The wax is sweet smelling."
  - Evidence is gained through direct sense perception, such as seeing, hearing, tasting, and smelling.
- Indirectly Observable Truths: Truths perceived through their effects on other things.
  - Examples: "There are atoms," "Gravity subsists," "The wax exists."
  - We observe the effects of atoms or gravity on other objects, not the entities themselves directly.

Truths that cannot be observed.
- Examples: "Two plus two is four," "Living things are alive," "Bachelors are unmarried males," "The sum of the interior angles of a triangle is 180 degrees."
- Mathematical truths are about numbers, not physical objects.
- Some truths are based on the meanings of words (e.g., definitions) rather than empirical observation.
- Mathematical truths relate to concepts and representations, not directly observable physical entities.

Premise 1: We have been deceived by sense perception.
- Examples: Pressing on eyelid distorts vision, optical illusions (Muller-Lyer illusion).
Premise 2: Therefore, we don't know what we discover by means of sense perception.
- If our senses have deceived us, we shouldn't fully trust them.
Premise 3: If we don't know what we discover by means of sense perception, then we have no knowledge of the directly observable truths.
Conclusion: We have no knowledge of the directly observable truths.

Premise 1: We have no knowledge of directly observable truths (conclusion from Part 1).
Premise 2: If we have no knowledge of directly observable truths, then we have no knowledge of indirectly observable truths.
- Knowledge of indirectly observable truths depends on knowledge of directly observable truths.
Conclusion: We have no knowledge of the indirectly observable truths.

Premise 1: We have been deceived by mental operations.
- Examples: Making errors in mental math, such as $334,000 \times 256,324$ .
Premise 2: Therefore, we do not know what we discover by means of mental operations.
- If mental operations have deceived us, we shouldn't fully trust them.
Premise 3: If we don't know what we discover by means of mental operations, then we have no knowledge of the unobservable truths.
Conclusion: We have no knowledge of the unobservable truths.

If the argument is successful, we have no knowledge of directly observable, indirectly observable, or unobservable truths.
Since these are the only types of truths, we don't know anything.

Descartes' argument poses a significant problem for our knowledge claims.
To refute skepticism, we need to demonstrate the failure of Descartes' argument.
Two prominent attempts to solve this problem come from G.E. Moore and Stuart Cohen, to be discussed in a later video.