Argument
Masked-man fallacy
Overview
In philosophical logic, the masked-man fallacy (also known as the intensional fallacy or epistemic fallacy) is the false assumption that knowledge or a belief about an object (an 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 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, 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.
Overview adapted from Wikipedia (CC BY-SA 4.0; see the article history for authors).
Possible passages
Found by search and not yet checked by a person; they may only mention the idea in passing.
He would acquiesce in the sufficiency of the reason first given, “bodies which contain more particles are heavier.” It is when he finds this questioned, and is called upon to prove it, without knowing how, that he tries to establish his premise by supposing proved what he is attempting to prove by it. The most effectual way, in fact, of exposing a petitio principii, when circumstances allow of it, is by challenging…
John Stuart Mill, A System of Logic, Ratiocinative and Inductive
States what Masked-man fallacy is.
Consequently he is likely to set the sort of object or material which has place and significance only in one of the serial functional situations of experience, over against the active attitude which describes part of the structural constitution of another situation; or with equal lack of justification to assimilate terms characteristic of different stages to one another. He sets the agent, as he is found in the…
John Dewey, Studies in Logical Theory
States what Masked-man fallacy is.
In this case, however, the predicate and subject are not merely reversed, but one of them is changed. Instead of [A] and [B], the terms of the new proposition are [a thing which is not B], and [A]. The original proposition, Some A _is not_ B, is first changed into a proposition æquipollent with it, Some A _is_ "a thing which is not B;" and the proposition, being now no longer a particular negative, but a particular…
John Stuart Mill, A System of Logic
States what Masked-man fallacy is.
Philosophers who discussed Masked-man fallacy
Related ideas
- Intension
- Logical equivalence
- Modus tollens
- Validity (logic)
- Philosophical logic
- Modal logic
- Syllogism
- Identity of indiscernibles
- Premise
- Doxastic logic
- Epistemic modal logic
Place in the tradition
Part of Epistemic modal logic.