# Masked-man fallacy > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Masked-man_fallacy > Markdown URL: https://mediated.wiki/source/Masked-man_fallacy.md > Source: https://en.wikipedia.org/wiki/Masked-man_fallacy > Source revision: 1347639881 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) Formal fallacy about knowledge of objects In [philosophical logic](/source/Philosophical_logic), the **masked-man fallacy** (also known as the **intensional fallacy** or **epistemic fallacy**)[1] is the false assumption that knowledge or a belief about an object (an [intension](/source/Intension)) can be used to correctly tell it apart from another object (as opposed to facts, that can be used to correctly tell two objects apart). It is committed when one makes an illicit use of [Leibniz's law](/source/Identity_of_indiscernibles) 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](/source/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. ## Examples The name of the fallacy comes from the example: - *Premise 1*: I know who Claus is. - *Premise 2*: I do not know who the masked man is. - *Conclusion*: Therefore, Claus is not the masked man. The [premises](/source/Premise) may be true, yet the conclusion is false if Claus is the masked man and the speaker does not know that. Though the speaker is aware of a large part of Claus's identity, it would not logically follow that Claus is not the masked man, seeing as the speaker cannot account for those parts of Claus's identity that are not known to them. Thus, the argument is a fallacious one. The fallacy results from the speaker's confusion of their own knowledge with complete factuality. 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. Note, however, that this [syllogism](/source/Syllogism) happens in the reasoning by the speaker "I"; Therefore, in the formal [modal logic](/source/Modal_logic) form, it would be: - *Premise 1:* The speaker [believes](/source/Doxastic_logic) they [know](/source/Epistemic_modal_logic) who X is. - *Premise 2:* The speaker believes they do not know who Y is. - *Conclusion:* Therefore, the speaker believes X is not Y. *Premise 1* B s ∀ t ( t = X → K s ( t = X ) ) {\displaystyle {\mathcal {B_{s}}}\forall t(t=X\rightarrow K_{s}(t=X))} is a very strong one, as it is [logically equivalent](/source/Logical_equivalence) to B s ∀ t ( ¬ K s ( t = X ) → t ≠ X ) {\displaystyle {\mathcal {B_{s}}}\forall t(\neg K_{s}(t=X)\rightarrow t\not =X)} . It is very likely that this is a [false belief](/source/False_belief): ∀ t ( ¬ K s ( t = X ) → t ≠ X ) {\displaystyle \forall t(\neg K_{s}(t=X)\rightarrow t\not =X)} is likely a false proposition, as the ignorance on the proposition t = X {\displaystyle t=X} does not imply the negation of it is true. Another example: - *Premise 1:* Lois Lane thinks Superman can fly. - *Premise 2:* Lois Lane thinks Clark Kent cannot fly. - *Conclusion:* Therefore, Superman and Clark Kent are not the same person. Expressed in [doxastic logic](/source/Doxastic_logic), the above syllogism is: - *Premise 1:* B L o i s F l y ( S u p e r m a n ) {\displaystyle {\mathcal {B}}_{Lois}Fly_{(Superman)}} - *Premise 2:* B L o i s ¬ F l y ( C l a r k ) {\displaystyle {\mathcal {B}}_{Lois}\neg Fly_{(Clark)}} - *Conclusion:* S u p e r m a n ≠ C l a r k {\displaystyle Superman\neq Clark} The above reasoning is inconsistent (not truth-preserving). The consistent conclusion should be B L o i s ( S u p e r m a n ≠ C l a r k ) {\displaystyle {\mathcal {B}}_{Lois}(Superman\neq Clark)} . The following similar argument *is* [valid](/source/Validity_(logic)): - 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](/source/Intension)* (with an 's') is the connotation of a word or phrase—in contrast with its [extension](/source/Extension_(semantics)), 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. ## See also - [Black box](/source/Black_box) - [Eubulides](/source/Eubulides)' second paradox - [Identity of indiscernibles](/source/Identity_of_indiscernibles) - [List of fallacies](/source/List_of_fallacies) - [Opaque context](/source/Opaque_context) - [Transitivity of identity](/source/Transitivity_of_identity) - [Use–mention distinction](/source/Use%E2%80%93mention_distinction) - [Metonymy](/source/Metonymy) ## References 1. **[^](#cite_ref-1)** Bowell, Tracey; Kemp, Gary (2013). *Critical Thinking: A Concise Guide*. Routledge. p. 225. [ISBN](/source/ISBN_(identifier)) [978-0-415-47182-4](https://en.wikipedia.org/wiki/Special:BookSources/978-0-415-47182-4). ## Further reading - Shatz, Itamar. ["The Masked-Man Fallacy"](https://effectiviology.com/masked-man-fallacy/). *Effectiviology*. - Curtis, Gary N. ["The Masked Man Fallacy"](http://www.fallacyfiles.org/illisubs.html). *The Fallacy Files*. v t e Common fallacies (list) Formal In propositional logic Affirming a disjunct Affirming the consequent Conflation Denying the antecedent Argument from fallacy Masked man Mathematical fallacy In quantificational logic Existential Illicit conversion Proof by example Quantifier shift Syllogistic fallacy Affirmative conclusion from a negative premise Negative conclusion from affirmative premises Exclusive premises Existential Necessity Four terms Illicit major Illicit minor Undistributed middle Informal Equivocation Equivocation False equivalence False attribution Moral equivalence Conflation Quoting out of context Loki's Wager No true Scotsman Semantic argument Reification Map–territory relation Question-begging Circular reasoning / Begging the question Loaded language Leading question Compound question / Loaded question / Complex question No true Scotsman Correlative-based False dilemma Perfect solution Denying the correlative Suppressed correlative Illicit transference Composition Division Ecological Secundum quid Accident Converse accident Faulty generalization Anecdotal evidence Sampling bias Cherry picking McNamara Base rate / Conjunction Double counting False analogy Slothful induction Overwhelming exception Ambiguity Accent False precision Moving the goalposts Quoting out of context Slippery slope Sorites paradox Syntactic ambiguity Questionable cause Animistic Furtive Correlation implies causation Cum hoc Post hoc Gambler's Inverse Regression Single cause Slippery slope Texas sharpshooter Appeals Law/Legality Stone / Proof by assertion Consequences Argumentum ad baculum Wishful thinking Emotion Children Fear Flattery Novelty Pity Ridicule In-group favoritism Invented here / Not invented here Island mentality Loyalty Parade of horribles Spite Stirring symbols Wisdom of repugnance Genetic fallacy Ad hominem Appeal to motive Association Reductio ad Hitlerum Reductio ad Stalinum Bulverism Poisoning the well Tone Tu quoque Whataboutism Authority Accomplishment Ipse dixit Poverty / Wealth Etymology Nature Tradition / Novelty Chronological snobbery Other fallacies of relevance Arguments Ad nauseam Sealioning Argument from anecdote Argument from incredulity Argument from silence Argument to moderation Argumentum ad populum Cliché The Four Great Errors I'm entitled to my opinion Ignoratio elenchi Invincible ignorance Moralistic / Naturalistic Motte-and-bailey fallacy Psychologist's fallacy Rationalization Red herring Two wrongs make a right Special pleading Straw man Category --- Adapted from the Wikipedia article [Masked-man fallacy](https://en.wikipedia.org/wiki/Masked-man_fallacy) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Masked-man_fallacy?action=history)). 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