Skip to main content
note
This page was translated from the German original, partly by machine. Some passages may read awkwardly or contain inaccuracies. When in doubt, please read the original.

5. Truth and Validity

 

Wireless Philosophy, Fundamentals, 6:54. Original title: CRITICAL THINKING - Fundamentals: Truth and Validity [HD]

Summary​

Julianne Chung (Yale University) examines the two qualities that decide whether we should accept an argument's conclusion. Arguments can be clear, interesting or persuasive, but what matters here is narrower:

  • The premises may be true, that is, in agreement with the facts. In philosophy, truth and falsity are properties of statements, not of arguments.
  • The argument may be valid, that is, its conclusion follows logically from its premises, so that the truth of the premises guarantees the truth of the conclusion. Validity is a property of arguments, not of statements.

Four kinds of example​

The illustrations all involve Split, her friend Julia's Australian Shepherd. True premises and valid:

Premise 1: All Australian Shepherds are dogs.
Premise 2: Split is an Australian Shepherd.
Conclusion: Therefore, Split is a dog.

True premises but invalid. The premises are obviously true, yet the conclusion does not follow, and it is plainly false:

Premise 1: All dogs are animals.
Premise 2: All cats are animals.
Conclusion: Therefore, all cats are dogs.

The more dangerous version has true premises and a true conclusion while still being invalid, which is why the conclusion must always be checked against the premises rather than against our sense of whether it sounds right:

Premise 1: All dogs are animals.
Premise 2: All Australian Shepherds are animals.
Conclusion: Therefore, all Australian Shepherds are dogs.

A false premise but valid reasoning. The conclusion may happen to be true, though this argument does not establish it:

Premise 1: You can't teach an old dog new tricks.
Premise 2: Split is an old dog.
Conclusion: Therefore, you can't teach Split new tricks.

A false premise and invalid:

Premise 1: I like Split.
Premise 2: Training dogs is easy.
Conclusion: Therefore, I'll win a lot of awards for teaching Split how to roll over.

The four possibilities​

Truth and validity combine into four cases: our facts may be right or wrong, and we may use them properly or improperly.

  1. Facts right, used properly: true premises, valid reasoning.
  2. Facts right, used improperly: true premises, invalid reasoning.
  3. Facts wrong, used properly: false premises, valid reasoning.
  4. Facts wrong, used improperly: false premises, invalid reasoning.

We should accept an argument's conclusion only in the first case. Philosophers call such arguments sound.

Why valid arguments with false premises are still useful​

If only case one licenses the conclusion, why bother with the rest? Because we are often not in a position to know whether our premises are true, and validity can help us find out. A false conclusion cannot validly be deduced from true premises, so validly deriving a conclusion we know to be false tells us that one of the premises must be false.

Premise 1: John is in bed with a terrible case of the flu.
Premise 2: If John is in bed with a terrible case of the flu, then he is not bowling.
Conclusion: Therefore, John is not bowling.

John has phoned in sick, and his boss constructs this valid argument. If she then sees him bowling, the second premise is untouched, but the first is in danger: she may conclude that John is not in bed with the flu after all. It seems he lied.

Chung recommends Paul's video on validity and Aaron's video on soundness for more.

Key takeaway​

Truth belongs to statements, validity to arguments, and the two vary independently. Only their combination, soundness, licenses the conclusion; but reasoning validly from premises we are unsure of is one of our best tools for testing whether those premises are true.