Why this concept? Because there is a difference between not knowing something that appears in empirical transactional reality (vyāvahārika satya), and the creation of īśvara (īśvara sṛṣṭi) and something that really does not occur in empirical reality.

It falls under the category of three ontological levels. Satya (the independent truth of everything), mithyā (the objects that do appear in a neutral world, but which are changeable and impermanent, and especially dependent on satya) and thus tuccham. The point is that we can now say that ordinary objects do exist, but are not what they appear to be, but in their true nature consciousness.

A well-known example of tuccha is a square circle. Well-known examples in vedānta classes are:

śaśaviṣāṇa śaśaviṣāṇāḥ “The horns (viṣāṇāḥ) of a hare (śaśa)”.

naraviṣāṇa “The horns (viṣāṇāḥ) of a human being (nara)”.

vandhyāputra “The son (putra) of a barren (vandhyā) woman”.

gaṅgājalāntarvahni “Fire in Ganges water” Fire (vahni) that consists in (antar) the flowing water (jala) of the Ganges.

utpalam agnau “A lotus (utpalam) blooms in fire (agnau)”.

ākāśa puṣpam “A flower (puṣpam) in the sky (ākāśa), an aerial flower”.

nīlātaṇḍula “Blue (nīlā) rice (taṇḍula)”. In India used metaphorically to indicate impossibility.