EVALUATING PHILOSOPHICAL ARGUMENT VALIDITY AND SOUNDNESS VALIDITY Important

EVALUATING PHILOSOPHICAL ARGUMENT VALIDITY AND SOUNDNESS

VALIDITY ▸ Important tool to evaluate a deductive argument

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

TEXT PHILOSOPHICAL NOTION OF VALIDITY Deductive arguments An argument is valid or invalid What is a valid argument? P 1: All humans are mortal The argument is good? P 2: Socrates is human All the premises and conclusion C: Therefore, Socrates is mortal are true?

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

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

VALIDITY ▸ It’s impossible for a valid argument to have all true premises unless the conclusion is also true ▸ The premises entails the conclusion

SO…. Argument can be

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

VALIDITY DUDE this is soooo abstract….

VALIDITY SOME EXAMPLES… This is a valid argument P 1: P 2: C: True All humans are mortal True Socrates is human True Therefore Socrates is mortal Remember, if the premises of this argument are true, then the conclusion must be true in this case

VALIDITY SOME EXAMPLES… This is a valid argument - With all false premises If True P 1: All humans are immortal P 2: Socrates is human C: If True Therefore Socrates is immortal Necessarily true ** It doesn’t matter if the premises are true or false!

VALIDITY SOME EXAMPLES… This is a valid argument - With premises of which we don’t know the truth P 1: All alliens speak English P 2: Splock is an Alien If True C: Therefore, Splock speaks English Premisses could be true or false Necessarily true

VALIDITY SUMMARY - VALID ARGUMENT ▸ It’s impossible for all the premises to be true while the conclusion is false

VALIDITY INVALID ARGUMENT ▸ The truth of the argument’s premises does not entail the truth of the conclusion…. P 1: All dogs have fur If True P 2: Claire has a lot of fur C: Therefore, Claire is a dog If True Could be false The truth of this conclusion doesn’t follow from the premises because Cats also have a lot of fur.

VALIDITY WHY VALIDITY MATTERS IF THE TRUTH OF THE PREMISES DOESN’T ▸MATTER? Validity determines whether an argument obeys logic/valid inference rules.

VALIDITY Now, write 3 examples of a valid or invalid argument of your own. Share and discuss with a partner

SOUNDNE SS

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

SOUNDNESS EXAMPLE -

SOUNDNESS EXAMPLE -

VALIDITY Now, write 3 examples of a sound argument of your own. Share and discuss with a partner