Validity
This video presents the same text shown beside it, spoken and on screen. It adds nothing the text does not say.
State
An argument is valid when it is impossible for the premises to be true and the conclusion false. The form guarantees the transfer of truth.
Show
"All dogs are mammals; Rex is a dog; so Rex is a mammal." No possible world breaks this.
Watch for
Validity is all-or-nothing. There is no "mostly valid."