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 one object has a certain property, while another object does not have the same property, the two objects cannot be identical. The name of the fallacy comes from the example:

  • Premise 1: I know who Jones is.
  • Premise 2: I do not know who the masked man is
  • Conclusion: Therefore, Jones is not the masked man.

The premises may be true and the conclusion false if Jones is the masked man and the speaker does not know that. Thus the argument is a fallacious one.

Another example:

  • Lois Lane believes that Superman can fly.
  • Lois Lane does not believe that Clark Kent can fly.
  • 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.

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 property of the mind that is directed at an object.

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