Chapter 5

  • Logical Strength
    1. We will distinguish between logical strength and factual strength.

    2. Factual strength indicates whether individual statements are true or false.

      For example:
      (1) The moon is made of cheese. [True or False?]
      (2) Everything made of cheese turns green after a while. [True or False?]
      (3) Therefore, the moon must be green by now. [True or False?]

    3. Logical strength indicates whether premises support conclusions.

      For example:
      (1) The moon is made of cheese.
      (2) Everything made of cheese turns green after a while.
      (3) Therefore, the moon must be green by now. [Do (1) and (2) support (3)?]

    4. Validity is another term for logical strength.

    5. Determining whether arguments are Valid or Invalid will be our primary task in this course.

      Is this argument logically strong? Is is factually strong?

      (1) If something landed in my back yard, then it was definitely an extraterrestrial.
      (2) Something has in fact landed in my back yard.
      (3) Therefore, extraterrestrial beings must have landed in my back yard.

    6. If an argument is valid and all of the statements are true, we say the argument is sound.

Indicator Words | Diagrams | Logical Strength | Implicit Premises | Question? | Self-test | Logic Page 1