Validity and Soundness Evaluating philosophical argument Validity and Soundness

Validity Validity Important tool to evaluate a deductive argument

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

Philosophical notion of Validity 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?

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

The premises entails the conclusion 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

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….

Some examples… This is a valid argument P1: All humans are mortal 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

Some examples… This is a valid argument - With all false premises 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

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

Summary - Valid Argument 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…. 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

Why validity matters if the truth of the premises doesn’t matter? Validity determines whether an argument obeys logic/valid inference rules.

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

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 - This is a valid argument but it is unsound because the premises are false

Soundness Example - Not every unsound arguments have false premises. To have a sound argument, we need to have both conditions - A valid argument and all the premises true.

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