1. Validity & Invalidity
Valid arguments guarantee true conclusions but only when all of their premises are true
Invalid arguments do not guarantee true conclusions even when all their premises are true

2. Valid
All parrots are birds
All birds have feathers
So all parrots have feathers
Invalid
All crocodiles are reptiles
All turtles are reptiles
So all crocodiles are turtles

3. Be Careful!
Not all invalid arguments have a false conclusion. The key point is: does the conclusion logically follow from the premises?
Birds have wings
A parrot has wings
Therefore, a parrot is a bird

4. Validity & Soundness
A sound argument = a valid structure + true premises
To describe an argument as sound is a much more fundamental claim than to say it is valid

5. Why this is important! All inferior races should be eliminated
Jews are an inferior race
Thus, all Jews should be eliminated
This valid but unsound argument was used to justify the elimination of six million Jews.