Who Was That Masked Man? (Formal Fallacies Part 3)

Formal fallacies are arguments that are always wrong, regardless whether the argument's premises (statements claimed as fact) are true or false. For example, in the appeal to probability, someone makes a claim that because something could happen, therefore it will happen. That’s false even if it's true that the something in question could indeed happen.

Masked Man Fallacy

I know who Bruce Wayne is.

I do not know who Batman is.

Therefore, Bruce Wayne is not Batman.

In the masked man fallacy, a substitution of identical designators in a true statement can lead to a false one. The statement "I do not know who Batman is" gets treated as if it excludes Bruce Wayne simply because I do know who he is. Of course, as long as I don’t know that Bruce is actually Batman, both statements can be absolutely true, and yet the conclusion does not follow logically.

The general form of the argument is:

X is known.
Y is unknown.
Therefore, X is not Y.

A similar argument, however, is valid.

Clark Kent is Superman (X is Z).

Batman is not Superman (Y is not Z).

Therefore, Clark Kent is not Batman (therefore, X is not Y).

That’s because being something is different from knowing something. Lack of proof of one proposition doesn’t serve as proof of the counter proposition.