Masked man fallacy

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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. The name 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.

One form of the fallacy may be summarised as follows:

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

The problem arises from the fact that Fact 1 and Fact 2 can be simultaneously true even when X and Y refer to the same person. For example, I know who my father is. I do not know who the thief is. Therefore, my father is not the thief. This is a fallacy, since the conclusion does not necessarily follow from the facts.