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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. Valid All parrots are birds All birds have feathers So all parrots have feathers
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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
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
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
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
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.
