masked-man fallacy (also known as the intensional fallacy and the epistemic fallacy) is a logical fallacy that is committed when someone assumes that if two or more names or descriptions refer to the same thing, then they can be freely substituted with one another, in a situation where that’s not the case
Examples
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:
- Premise 1: Lois Lane believes that Superman can fly.
- Premise 2: Lois Lane does not believe that Clark Kent can fly.
- Conclusion: 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:
- X is Z
- Y is Z, or Y is not Z.
- Therefore, X is not Y.
Intension (with an ‘s’) is the connotation of a word or phrase—in contrast with its extension, the things to which it applies. Intensional sentences are often intentional (with a ‘t’), that is they involve a relation, unique to the mental, that is directed from concepts, sensations, etc., toward objects.