How is this argument valid? Either Oscar is an octopus or he is a whale. Oscar is a zebra. Therefore, Oscar is an octopus.

Validity in an argument comes down to one question: Is it possible for all the argument's premises to be true and its conclusion false? If no, then the argument is valid. So, assuming it is impossible for Oscar to be both a whale and a zebra, the argument is valid. Even so, the argument is not formally valid, because the following is not a valid form:

Octopus(Oscar) or Whale(Oscar)
Therefore: Octopus(Oscar)

Not all valid arguments are formally valid.

Furthermore, assuming that Oscar is not both an octopus and a zebra, the argument is unsound despite being valid, because in that case the second premise and the conclusion are not both true. The same holds for this argument (on similar assumptions):

Oscar is an octopus, or Oscar is a whale.
Oscar is a zebra.
Therefore: Oscar is a whale.

Valid but unsound. So neither argument establishes its conclusion.

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