1. Validity and Soundness
Evaluating philosophical argument
Validity and Soundness

2. Validity
Validity
Important tool to evaluate a deductive argument

3. 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. 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?

5. 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 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. 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

8. So….
Argument can be

9. 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
DUDE this is soooo
abstract….

11. 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

12. 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

13. 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

14. Summary - Valid Argument
Validity
Summary - Valid Argument
It’s impossible for all the premises to be true while the conclusion is false

15. 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

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

17. 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

19. Soundness

20. 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

21. Soundness
Example -
This is a valid argument but it is unsound because the premises are false

22. 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.

23. 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