Evaluating Deductive Arguments - Validity & Soundness

When evaluating deductive arguments, we are concerned with two things; Validity and Soundness

• Valid: an argument is valid if and only if it is necessary that if all of the premises are true, then the conclusion is true. If all the premises are true, then the conclusion must be true. It is impossible that all the premises are true and the conclusion is false.

• Invalid: an argument that is not valid.  We can test for invalidity by assuming that all the premises are true and seeing whether it is still possible for the conclusion to be false.  If this is possible, the argument is invalid.

• Sound: an argument is sound if and only if it is valid and contains only true premises.

• Unsound: an argument that contains one false premise, or more. All invalid arguments are unsound.

Here is a nifty flowchart to help you decide if a deductive argument is valid and sound: