An informal fallacy of ambiguity, in which terms that can refer to the same extension (synonyms) are used interchangeably in a context in which they have non-identical connotations because of the way and manner in which they refer to it (intensions).
The terms “morning star” and “evening star” are both used to refer to the planet Venus.
The morning star can be seen in the morning.
Therefore, also the evening star can be seen in the morning.
Even though the extension of the two terms is identical (they both refer to the planet Venus), they are not true synonyms in all aspects, since they each each refer to it in different ways (i.e. they have different intensions), namely that they allude to the morning and evening, respectively. In case of the “evening star”, it can only reasonably referred to as appearing in the evening sky.
- Illicit substitution of identicals
- Intensional fallacy
The intensional fallacy belongs to the group of fallacies of ambiguity, but is in some sense the opposite of the other fallacies in this group, which are all based on equivocations: while those are about a single expression, which is used in two or more different meanings (extensions), this fallacy is base on multiple terms which appear to refer to the same extension, but with different intensions.
Intensions in the concept of God
The pope believes in God.
The term “God” describes the same entity as “Allah”.
Therefore, the pope believes in Allah.
Even though Christianity, Islam and Judaism, due to their common history, indeed refer to the same deity; And even though “الله” [Allāh ], as well as “יהוה” [JHWH ] can be regarded as direct translations of “God” (and are at least in some context also used in this way), the Arabic and Hebrew terms have further intensions, which stand in the way of a conclusion like the above.