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intensional_fallacy
The intensional fallacy occurs when co-referential terms (terms that refer to the same entity) are substituted within intensional (belief, knowledge, desire) contexts as though they were interchangeable. While 'the morning star' and 'the evening star' both refer to Venus, someone can believe something about the morning star without believing it about the evening star, because the cognitive content (intension) of the two descriptions differs. This is a formal error rooted in the distinction between extensional and intensional logic.
"Lois Lane wants to marry Superman. Superman is Clark Kent. Therefore, Lois Lane wants to marry Clark Kent."
A child is told: 'You said you wanted to meet the author of your favourite book. J.K. Rowling is the author of your favourite book. So you must want to meet J.K. Rowling' — ignoring that the child had no idea who wrote it and might strongly object upon learning.
An employee says she admires the CEO of the company that invented the smartphone. Her colleague responds: 'The CEO of Apple invented the smartphone, so you must admire Tim Cook' — even though she was thinking of Steve Jobs and a different origin story entirely.
Binary (yes/no) questions an LLM must answer to identify this aspect:
Does the argument substitute one term for a co-referential term within a belief, knowledge, or desire context?
Type: binaryDoes the argument assume that because two terms refer to the same entity, they are interchangeable in all contexts?
Type: binaryDoes the conclusion depend on treating the belief/knowledge context as transparent to substitution?
Type: binaryCould the person hold different attitudes toward the same entity under different descriptions?
Type: binaryThe intensional fallacy occurs when co-referential terms (terms that refer to the same entity) are substituted within intensional (belief, knowledge, desire) contexts as though they were interchangeable. While 'the morning star' and 'the evening star' both refer to Venus, someone can believe something about the morning star without believing it about the evening star, because the cognitive content (intension) of the two descriptions differs. This is a formal error rooted in the distinction between extensional and intensional logic.
In everyday extensional contexts, substituting co-referential terms is perfectly valid. The fallacy exploits this habitual reasoning pattern, applying it where it fails — in contexts involving beliefs, desires, and other propositional attitudes.
Identify whether the context is intensional (involving beliefs, knowledge, desires, or other mental states). If so, point out that the person's attitude may depend on how the entity is described, not just which entity it is.
Appears in legal reasoning about intent ('the defendant knew the substance was aspirin, and aspirin is acetylsalicylic acid, so the defendant knew it was acetylsalicylic acid'), in philosophy of mind, and in political framing where the same policy is described in different terms to different audiences.
Illicit use of Leibniz's law of identity in intensional contexts.
Using a key term ambiguously – one meaning in premise, another in conclusion.
Using a term with multiple meanings in an argument, shifting between meanings to make the argument appear valid when it is not. Broader category encompassing equivocation.
A categorical syllogism that contains four terms instead of the required three, often due to an ambiguous middle term being used with two different meanings.
Use these tools to detect, analyze, or train this aspect.