2.1: Validity & Soundness

Validity & Soundness

Argument 1

  1. New York is east of Philadelphia.
  2. Philadelphia is east of Los Angeles.
  3. Therefore, New York is east of Los Angeles.

Argument 2

  1. Either grass is green, or the sky is pink.
  2. Grass is not green.
  3. Therefore, the sky is pink.

Validity & Soundness

(VALIDITY) A deductive argument is valid just in case if the premises are true, the conclusion must be true (i.e., it is impossible for the premises to be true and the conclusion to be false).

  • Validity pertains to the quality of the reasoning involved in an argument.
  • When judging validity, we simply assume that the premises are true. 

Validity & Soundness

 

  1. Paris is the capital of France.
  2. If Paris is the capital of France, then French is spoken in Paris.
  3. Therefore, London is a famous city.
  • Even if an argument contains all true sentences, that doesn't make it valid.

NOT VALID

Validity & Soundness

(SOUNDNESS) A deductive argument is sound just in case it is valid and the premises are true.

  • Soundess = Validity + True premises

Validity & Soundness

  1. All fish are mammals.
  2. All mammals are cold-blooded.
  3. Therefore, all fish are cold-blooded.

VALID but NOT SOUND

Validity & Soundness

  1. Whales are mammals.
  2. Whales live in the ocean.
  3. Therefore, some mammals live in the ocean.

VALID and SOUND

Key Concepts

  • Validity
  • Soundness

2.1: Validity & Soundness

By Jesse Rappaport

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2.1: Validity & Soundness

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