Masked man fallacy
Encyclopedia
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:
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 premise
s 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
:
But this is because being something is different from knowing (or believing, etc.) something.
Formal logic
Classical or traditional system of determining the validity or invalidity of a conclusion deduced from two or more statements...
in which substitution
Substitution (logic)
Substitution is a fundamental concept in logic. Substitution is a syntactic transformation on strings of symbols of a formal language.In propositional logic, a substitution instance of a propositional formula is a second formula obtained by replacing symbols of the original formula by other formulas...
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 premise
Premise
Premise can refer to:* Premise, a claim that is a reason for, or an objection against, some other claim as part of an argument...
s 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
Validity
In logic, argument is valid if and only if its conclusion is entailed by its premises, a formula is valid if and only if it is true under every interpretation, and an argument form is valid if and only if every argument of that logical form 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.