Masked Man Fallacy

The masked man fallacy is a fallacy of formal logic in which substitution of identical designators in a true statement can lead to a false one.

One form of the fallacy may be summarized as follows:

• Premise 1: I know who X is.
• Premise 2: I do not know who Y is.
• Conclusion: Therefore, X is not Y.

The problem arises because Premise 1 and Premise 2 can be simultaneously true even when X and Y refer to the same person. Consider the argument, "I know who my father is. I do not know who the thief is. Therefore, my father is not the thief." The premises may be true and the conclusion false if the father is the thief but the speaker does not know this about his father. Thus the argument is a fallacious one.

The name of the fallacy comes from the example, "I do not know who the masked man is", which can be true even though the masked man is Jones, and I know who Jones is.

If someone were to say, "I do not know the masked man," it implies, "If I do know the masked man, I do not know that he is the masked man." The masked man fallacy omits the implication.

Note that the following similar argument is valid:

• X is Z
• Y is not Z
• Therefore, X is not Y

But this is because being something is different from knowing (or believing, etc.) something.