Masked-man fallacy

In philosophical logic, the masked-man fallacy (also known as the intensional fallacy and the epistemic fallacy[1]) 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.

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 (or believing, etc.) something. The valid and invalid inferences can be compared when looking at the invalid formal inference:

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.

See also

References

  1. Bowell, Tracey; Kemp, Gary (2013). Critical Thinking: A Concise Guide. Routledge. p. 225. ISBN 1134290810.

Further reading

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