Question

Masked man fallacy

Accepted Answer

The name of the fallacy comes from the example:

Premise 1: I know who Bob is.
Premise 2: I do not know who the masked man is
Conclusion: Therefore, Bob is not the masked man.
The premises may be true and the conclusion false if Bob is the masked man and the speaker does not know that. Thus the argument is a fallacious one.

Another example:

Lois Lane believes that Superman can fly.
Lois Lane does not believe that Clark Kent can fly.
Therefore Superman and Clark Kent are not the same person.
In symbolic form, the above arguments are

Premise 1: I know who X is.
Premise 2: I do not know who Y is.
Conclusion: Therefore, X is not Y.
The following similar argument is valid:

X is Z
Y is not Z
Therefore, X is not Y
This is valid because being something is different from knowing (or believing, etc.) something. The valid and invalid inferences can be compared when looking at the invalid formal inference:

Masked Premise I X Man Bob Y Fallacy