In
philosophical logic, the masked-man fallacy (also known as the
intensional fallacy or epistemic 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 (that is, they have all the same properties). By
modus tollens
In propositional logic, ''modus tollens'' () (MT), also known as ''modus tollendo tollens'' (Latin for "method of removing by taking away") and denying the consequent, is a deductive argument form and a rule of inference. ''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:
* ''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
premise
A premise or premiss is a true or false statement that helps form the body of an argument, which logically leads to a true or false conclusion. A premise makes a declarative statement about its subject matter which enables a reader to either agre ...
s may be true and the conclusion false if Claus is the masked man and the speaker does not know that. Thus the argument is a fallacious one.
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 happens in the reasoning by the speaker "I"; Therefore, in the formal
modal logic form, it'll be
* ''Premise 1:'' The speaker
believes he
knows who X is.
* ''Premise 2:'' The speaker believes he does not know who Y is.
* ''Conclusion:'' Therefore, the speaker believes X is not Y.
''Premise 1''
is a very strong one, as it's
logically equivalent
Logic is the study of correct reasoning. It includes both formal and informal logic. Formal logic is the science of deductively valid inferences or of logical truths. It is a formal science investigating how conclusions follow from premise ...
to
. It's very likely that this is a
false belief:
is likely a false proposition, as the ignorance on the proposition
doesn't 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 Doxastic logic is a type of logic concerned with reasoning about beliefs.
The term ' derives from the Ancient Greek (''doxa'', "opinion, belief"), from which the English term '' doxa'' ("popular opinion or belief") is also borrowed. Typically, a ...
, the above syllogism is:
* ''Premise 1:''
* ''Premise 2:''
* ''Conclusion:''
The above reasoning is inconsistent (not truth-preserving). The consistent conclusion should be
.
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. 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'' (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 relation, unique to the mental, that is directed from concepts, sensations, etc., toward objects.
See also
*
Black box
In science, computing, and engineering, a black box is a system which can be viewed in terms of its inputs and outputs (or transfer characteristics), without any knowledge of its internal workings. Its implementation is "opaque" (black). The te ...
*
Eubulides' second paradox
*
Identity of indiscernibles
The identity of indiscernibles is an ontological principle that states that there cannot be separate objects or entities that have all their properties in common. That is, entities ''x'' and ''y'' are identical if every predicate possessed by ''x'' ...
*
List of fallacies
A fallacy is reasoning that is logically invalid, or that undermines the logical validity of an argument. All forms of human communication can contain fallacies.
Because of their variety, fallacies are challenging to classify. They can be classif ...
*
Opaque context
An opaque context or referentially opaque context is a linguistic context in which it is not always possible to substitute "co-referential" expressions (expressions referring to the same object) without altering the truth of sentences. The expres ...
*
Transitivity of identity
In philosophy, identity (from , "sameness") is the relation each thing bears only to itself. The notion of identity gives rise to many philosophical problems, including the identity of indiscernibles (if ''x'' and ''y'' share all their propertie ...
*
Use–mention distinction
The use–mention distinction is a foundational concept of analytic philosophy, according to which it is necessary to make a distinction between a word (or phrase) and it.Devitt and Sterelny (1999) pp. 40–1W.V. Quine (1940) p. 24 Many philos ...
*
Metonymy
References
Further reading
*
*
{{Formal fallacy
Formal fallacies