Masked-man fallacy


In philosophical logic, the masked-man 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. 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.

Examples

The name of the fallacy comes from the example:
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:
In symbolic form, the above arguments are
The following similar argument is valid:
This is valid because being something is different from knowing something. The valid and invalid inferences can be compared when looking at the invalid formal inference:
Intension is the connotation of a word or phrase—in contrast with its extension, the things to which it applies. Intensional sentences are often intentional, that is they involve a relation, unique to the mental, that is directed from concepts, sensations, etc., toward objects.