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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:
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. 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 and the speaker does not know this particular thing about his father. Thus the argument is a fallacy.