Intensional fallacy

Terms that can be used as synonyms in their extensional sense are used in ways that associate them with ambiguous intensions.

For example:

The terms “morning star” and “evening star” both refer to the planet Venus.
The morning star can be seen in the morning.
Therefore, the evening star can also be seen in the morning.

Even though the two terms have the same extension (namely, the planet Venus) and therefore can be used as synonyms in many situations, they carry specific secondary meanings (intensions) because of the way they refer to it, thus making the above conclusion contradictory.

• “Masked man” fallacy
• Illicit substitution of identicals

The intensional fallacy belongs to the group of fallacies of ambiguity, but is effectively the opposite of the other ambiguities here, which are all based on a term referring to more than one possible extension. Here, there are multiple terms which refer to the same extension (i.e. they are synonyms), but in ways that imply different intensions, which can lead to invalid statements.

The Pope believes in God.
The term “God” describes the same entity as “Allah”.
Therefore the the Pope believes in Allah.

Although Christianity, Islam and Judaism, as a consequence of their common history and also according to their self-understanding refer to the same supreme deity, and although “الله” [Allāh ], as well as “יהוה” [Jahwe ] can also be understood as direct translations of the English word “God” (and are also used in such a way, under specific circumstances), the Arabic and Hebrew terms have specific intensions which stand in the way of a conclusion like the above.

• Equivocation
• Extension
• Intension
• Leibniz’ law (of the identity of indiscernibles)

• Masked-man fallacy on Wikipedia
• Illicit Substitution of Identicals on Logically Fallacious
• The Masked Man Fallacy on Fallacy Files