VALID and SOUND Deductive Arguments
All human beings will soon die.
Clark is a human being.
Therefore, Clark will soon die.
All German Shepherds are dogs.
Some pets are German Shepherds.
Therefore, some pets are dogs.
VALID but UNSOUND Deductive Arguments
All men are mortal.
Birds are men.
Therefore, birds are mortal.
(Valid, but the premise birds are men is false, making the argument unsound)
All cats have six legs.
A tiger is a cat.
Therefore, a tiger has six legs.
(Valid, but the premise all cats have six legs is false, making the argument unsound)
INVALID Deductive Arguments
Some teachers are musicians.
All instructors are teachers.
Therefore, all instructors are musicians.
Some cats purr.
Some tigers are cats.
Therefore, all tigers purr.
(In both examples, the conclusion does not necessarily follow from the premises)
An important point to consider is that for a deductive argument to be sound, its premises should be true and the whole argument should be valid. For it to be unsound, at least one of its premises should be false. A valid deductive argument may be either sound or unsound. An invalid deductive argument can never be sound. Read more about deductive arguments.
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