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Intensional fallacy

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).

For example:

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.

Other names


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.

More examples

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.

See also

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