In philosophical logic, the masked-man fallacy (also known as the intensional fallacy and the epistemic fallacy) is committed when one makes an illicit use of Leibniz's law in an argument. Leibniz's law states that if A and B are the same object, then A and B are indiscernible (that is, they have all the same properties). By modus tollens, this means that if one object has a certain property, while another object does not have the same property, the two objects cannot be identical. The fallacy is "epistemic" because it posits an immediate identity between a subject's knowledge of an object with the object itself, failing to recognize that Leibniz's Law is not capable of accounting for intensional contexts.
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.
- 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.
- Black box
- Eubulides' second paradox
- Identity of indiscernibles
- List of fallacies
- Opaque context
- Transitivity of identity
- Use–mention distinction