Intensional fallacy

Core claim: Two terms that refer to the same object (same extension) are treated as fully interchangeable in a statement or inference, even though they carry different intensions – i.e., different associated meanings or modes of reference.

• A term is substituted for another on the grounds that both refer to the same entity (e.g., “morning star” / “evening star”, “God” / “Allah”).
• The two terms carry different intensions – connotations, contextual information, or modes of reference – despite co-referring.
• The conclusion depends on transferring a property that holds only for one term’s mode of reference to the other (e.g., “visible in the morning” from “morning star” to “evening star”).

• This is the mirror image of most other ambiguity fallacies: here multiple terms refer to one extension, rather than one term covering multiple extensions.
• Whether two co-referring terms actually have different intensions can be language- and context-dependent (e.g., “God” vs. “Allah” in common English usage, but not necessarily in all languages).
• Cross-language term pairs are a special case: whether two terms carry different intensions depends on their use in the specific language and community. If that is unclear, prefer flagging a possible false premise about what a word means (or an equivocation) over forcing this label.

• The property being asserted depends only on the shared object itself, so the substitution is valid regardless of which name is used (e.g., “The morning star is a planet; therefore Venus is a planet.”).
• The discussion explicitly treats the two terms as distinct concepts or names rather than interchangeable synonyms.
• The statement is about the terms themselves (metalinguistic), e.g., “‘morning star’ and ‘evening star’ are two names for Venus.”
• The terms come from different languages or traditions but are genuinely interchangeable in the relevant context, so they carry no meaningful intensional difference there (e.g., “Alla” in Maltese simply means “God”, also in Catholic usage).

• Equivocation – ask: does a single term shift between multiple meanings or referents within an argument (→ equivocation), or are two different terms referring to the same object swapped despite different intensions (→ intensional fallacy)?
• Motte-and-bailey – ask: are two distinct positions/claims conflated under one label and defended as if identical (→ motte-and-bailey), or is one object referred to by two terms with different intensions, and the inference swaps them (→ intensional fallacy)?

Input: “The morning star and the evening star are both the Venus. The morning star is visible in the morning. Therefore, the evening star is also visible in the morning.”
Output: Both terms refer to the planet Venus, but they carry different intensions: “visible in the morning” holds for the morning-star way of referring to it, not for the evening-star way. Do not substitute co-referring terms when the property depends on the mode of reference.

Input: “The crash caused three deaths. ‘Causing a death’ and ‘killing someone’ describe the same outcome, so the driver killed three people.”
Output: Same event, different intensions: “kill” implies agency (and often intent) that “cause a death” does not carry. Substituting one term for the other smuggles in blame that the original statement did not assert.

Input: “Maltese Catholics pray to ‘Alla’ in their Mass. ‘Alla’ is the same word as the Islamic ‘Allah’. Therefore they are praying to an Islamic deity.”
Output: This imports the intension that “Allah” carries in common English usage into a context where it does not apply: in Maltese, “Alla” simply means “God”, also in Catholic usage. Check what intension a term actually carries in its own language and community before treating two terms as synonyms with different connotations.

Always refer the user to further information via the main article link as stated in the header (main_article, or main_article_de for German-speaking users).