According to the classical Indian school of logic (Nyāya), which fallacy is…

Concept

In Nyāya, an inference (anumāna) is sound only when the reason (hetu) is genuinely present in the subject (pakṣa) and its invariable concomitance (vyāpti) with the probandum (sādhya) can be corroborated — confirmed by a positive instance (sapakṣa) and secured against a negative instance (vipakṣa).

A hetvābhāsa is a reason that only appears to prove. One inconstant (anaikāntika / savyabhicāra) variety arises when the reason is so exclusive to the subject that it is found nowhere else at all, so no corroborating instance of either kind can ever be produced.

Application

Set the argument out in Nyāya terms: the pakṣa is “anything that is thinkable”, the sādhya is “nameable”, and the hetu offered is “because it is thinkable”.

The hetu is verbally identical with the very property that defines the subject, so it occurs in the pakṣa and nowhere outside it. Corroboration would require a sapakṣa — some locus other than the subject where nameability is already established — in which thinkability is also found; no such locus can be produced, because every thinkable thing has already been swept into the pakṣa itself.

The negative side gives no help either. A vipakṣa is a locus where the sādhya, nameability, is absent; thinkability occurs in no such locus, so there is no counterinstance (vyabhicāra) to be ruled out — but equally nothing against which the concomitance could be put to the test.

Excluded from both sapakṣa and vipakṣa, the reason resides in the pakṣa alone (pakṣa-mātra-vṛtti). The vyāpti between thinkability and nameability can therefore never be established, and the reason does no work beyond restating the subject; it yields doubt rather than proof. This is precisely the asādhāraṇa hetvābhāsa.

Contrast

• Sādhāraṇa: the reason strays into the vipakṣa as well, occurring where the sādhya is absent, so it is too wide to fix the conclusion.

• Āśrayāsiddha: the subject-locus itself is unreal, so the reason has no existing site to reside in — as in “the sky-lotus is fragrant because it is a lotus”.

• Svarūpāsiddha: the reason is not in fact present in the subject at all — as in “sound is a quality because it is visible”, where visibility does not belong to sound.

Cross-check

Now test each defect separately. Thinkability does genuinely belong to the subject, so svarūpāsiddha — in which the reason is absent from the subject — does not arise. The subject “anything that is thinkable” is a real and established locus, so āśrayāsiddha — in which the locus itself is unreal, as with a sky-lotus — does not arise either. Thinkability is found in no locus from which nameability is absent, so the reason is not too wide in the manner of sādhāraṇa. What remains is that it resides in the pakṣa and nowhere else, the same diagnostic pattern as the textbook case “sound is eternal because it possesses sound-ness (śabdatva)”, where śabdatva belongs to sound alone and to nothing eternal or non-eternal outside it.

Hence the argument commits the asādhāraṇa hetvābhāsa.