The term tuccha means in Classical Sanksrit “worthless”, “insignificant”. In Vedānta, however, it gets a more specific technical meaning, to denote the absolute unreality of chimeral entities, such as the khapuṣpa (flower in the air), which will not and cannot ever exist.
api ca saṃvit siddhyati vā na vā? siddhyati cet, sadharmatā syāt. na cet, tucchatā, gaganakusumādivat (Śrī Bhāṣya, mahāpūrvapakṣa).
Moreover, is consciousness established or not? If it is established, then it must have qualities. If not, it is unreal, like a flower in the sky.
api ca – avidyayā brahmaṇi tirohite tad brahma na kiñcid api prakāśate; uta kiñcit prakāśate? pūrvasmin kalpe, prakāśamātrasvarūpasya brahmaṇo ’prakāśe tucchatāpattir asakṛd uktā (Śrī Bhāṣya, mahāsiddhānta).
Moreover, if the brahman is covered by nescience, would it not shine at all or shine a little bit? In the first case, if the brahman, which is essentially nothing but light, would not shine, it would end up being absolutely unreal —this has been said several times.
In this sense, tuccha seems to have been added to Kumārila’s list of four kinds of absence (previous absence, posterior absence, reciprocal absence and absolute absence). But what is then the difference between tuccha and the fourth type of absence, i.e., atyantābhāva? Notwithstanding Kumārila’s initial understanding of it (as reconstructed by Birgit Kellner, Kellner 1996 available for download here) the latter has ended up covering cases of complete absence which however did not imply a logical impossibility. tuccha, by contrast, covered logical and conceptual impossibilities.
However, tuccha and abhāva (which, if not further specified, means atyantābhāva) are found side by side in an odd passage of Veṅkaṭanātha’s Seśvaramīmāṃsā. The passage attacks the Pramāṇavāda idea that invalidity is intrinsic to cognitions, because, being a sheer absence, it does not need to be created, it is the “by default” situation. This cannot be the case, rebucks Veṅkaṭanātha.
avastutvam iti kiṃ abhāvatvam abhipretam uta tucchatvam? nādyaḥ, pradhvaṃse vyabhicārāt. ahetukavināśasya ca tarkapāde parihariṣyamāṇatvāt. na dvitīyaḥ, asiddheḥ. na hy aprāmāṇyamapramāṇe tuccham api tu prāmāṇyam, yanniṣṭhābhāvapratiyogī yaḥ sa tatra tuccha iti tattvam (SM ad 1.1.5, 1971, pp. 72–73)
Does the expression “it is not real” mean that it does not exist (abhāva), or that it is absolutely inexistent (tuccha)? Not the first one, because there would be a contradiction (vyabhicāra) at the moment of [its] destruction (since, if it never existed, it should not be liable to elimination) and since the destruction of something which has no cause (hetu) will be refuted (parihṛ-) in the Tarkapāda [of the UMS]. Not the second one, since it is not established. In fact, it is not the case that in the case of something which is not an instrument of knowledge (pramāṇa) the invalidity is absolutely inexistent (tuccha). Rather, the validity is. The truth (tattva) is that something is absolutely inexistent in something else if the former is the counter-positive of the absence occurring in the latter.
Now, in this case, the idea that prāmāṇya is said to be tuccha in something which is not an instrument of knowledge makes me think that tuccha cannot mean “conceptually impossible” and that it must rather mean just “absolutely absent”. Validity is in fact not conceptually impossible, but just altogether absent from an invalid cognition. Even the definition which follows seems to support this view, since a genuine tuccha cannot have something existent as its counter positive. But what is then abhāva in the same passage? All that Veṅkaṭanātha has to say about it is that its abhāvatva is contradicted by the fact that invalidity can be eliminated. And something non-existing should not be liable to be eliminated. abhāva might then mean any sort of “absence” and even “conceptually impossible” (but this would be really odd).
Can abhāva in Vedāntic texts refer to something different than atyantābhāva?