Week5-Validity&Soundness.ppt
- 2. VALIDITY VALIDITY ▸Important tool to evaluate a deductive argument
- 3. VALIDITY PHILOSOPHICAL NOTION OF VALIDITY (REVIEW) ▸What is an argument? A set of statements, one or more of which is offered in support of some statements ‣ Premises Statement doing the supporting ‣ Conclusion The statement being supported
- 4. TEXT PHILOSOPHICAL NOTION OF VALIDITY Deductive arguments An argument is valid or invalid What is a valid argument? P1: All humans are mortal P2: Socrates is human C: Therefore, Socrates is mortal The argument is good? All the premises and conclusion are true?
- 5. VALIDITY PHILOSOPHICAL NOTION OF VALIDITY Deductive arguments An argument is valid or invalid Validity has nothing to do with truth of the conclusion Or how good the conclusion is
- 6. VALIDITY VALIDITY OF AN ARGUMENT An argument is valid ‣ If and only if the truth of its premises guarantees the truth of its conclusion ‣ Validity is a property of arguments such that if the premises of the arguments are true, then the conclusion must be true
- 7. VALIDITY ▸It’s impossible for a valid argument to have all true premises unless the conclusion is also true ▸The premises entails the conclusion
- 9. VALIDITY ARGUMENT VALIDITY IS CONDITIONAL ▸If an argument is valid, then if its premises are true then the conclusion is true ▸If an argument is valid, then the truth of its conclusion follows from the truth of its premises
- 10. VALIDITY SOME EXAMPLES… This is a valid argument P1: All humans are mortal P2: Socrates is human C: Therefore Socrates is mortal Remember, if the premises of this argument are true, then the conclusion must be true in this case True True True
- 11. VALIDITY SOME EXAMPLES… This is a valid argument - With all false premises P1: All humans are immortal P2: Socrates is human C: Therefore Socrates is immortal ** It doesn’t matter if the premises are true or false! If True If True Necessarily true
- 12. VALIDITY SOME EXAMPLES… This is a valid argument - With premises of which we don’t know the truth P1: All alliens speak English P2: Splock is an Alien C: Therefore, Splock speaks English Premisses could be true or false If True If True Necessarily true
- 13. VALIDITY SUMMARY - VALID ARGUMENT ▸It’s impossible for all the premises to be true while the conclusion is false
- 14. VALIDITY INVALID ARGUMENT ▸The truth of the argument’s premises does not entail the truth of the conclusion…. P1: All dogs have fur P2: Claire has a lot of fur C: Therefore, Claire is a dog The truth of this conclusion doesn’t follow from the premises because Cats also have a lot of fur. If True If True Could be false
- 15. VALIDITY WHY VALIDITY MATTERS IF THE TRUTH OF THE PREMISES DOESN’T MATTER? ▸ Validity determines whether an argument obeys logic/valid inference rules.
- 16. SOUNDNE SS
- 17. SOUNDNESS Soundness of an argument requires: 1. The argument must be valid Conclusion cannot be false if the premises are true 2. The premises must all be true