# Masked man fallacy

*Also called: Intensional fallacy, illicit substitution*

- **Cluster:** Broken structure — The form itself does not carry the conclusion
- **Page:** https://fallacy.wiki/fallacy/masked-man/
- **In Arabic:** https://fallacy.wiki/ar/fallacy/masked-man.md
- **Licence:** CC BY 4.0 — https://creativecommons.org/licenses/by/4.0/

## What it is

Substituting identical things inside a statement about belief or knowledge, where it does not hold.

## In the wild

> I know my colleague; I do not know the thief; so my colleague is not the thief.

## How to answer it

Note that knowing about something is a fact about the knower, not about the thing.

## Origin and naming

Named for a stock example in analytic philosophy: I know who my friend is; I don't know who the masked man across the room is — so my friend is not the masked man. The argument fails because it treats 'known by me' as a property of the person, when it is a fact about me. It is an illicit use of Leibniz's law in an epistemic context.

## More places it shows up

- The board approved Project Falcon; nobody admitted to approving the leaked plan; so the leaked plan is not Project Falcon — and it was, under a different codename.
- Alice knows the sender of the encrypted email; she does not know who runs the anonymous account; so they can't be the same person.
- The witness can describe the driver; he cannot describe the defendant; so the defendant was not the driver.
- Users trust the official app; users do not trust the new SDK; so the SDK can't be part of the official app — both shipped in the same build.

## How to spot it

- Notice when the argument runs 'I know X; I don't know Y; so X isn't Y' — knowledge is a report about the knower, not the known.
- Notice when substituting different descriptions of the same person or thing drives the whole conclusion.
- Notice when two names or two descriptions are treated as two entities with no evidence beyond ignorance of the link.

## Nearby

- [Affirming the consequent](https://fallacy.wiki/fallacy/affirming-consequent.md)
- [Equivocation](https://fallacy.wiki/fallacy/equivocation.md)

## Read more

- [Wikipedia](https://en.wikipedia.org/wiki/Masked-man_fallacy)
- [Fallacies Online](https://fallacies.online/wiki/ambiguity/intensional_fallacy)

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From fallacy.wiki — a bilingual field guide to 91 logical fallacies, published by phi holdings. Licensed CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/): reuse freely with attribution to fallacy.wiki.