The ākṣepa was – “jīva is qualified by rāga, dveṣa and such kleśas. Brahma is devoid of them. Thus, the viṣaya of this grantha, i.e. aikya of jīva and brahma is not possible”.

While this is true, the aikya between the sākṣi which is devoid of rāga etc, and brahma is indeed appropriate.

You cannot then object with “a sākṣī that is different from the jīva who has dharmas like kartr̥tva, bhoktr̥tva etc would be akin to a son of a barren woman, and does not exist”, since the sākṣi underlies it (is the upahita).

The qualified portion of the samsārī that is qualified by adjectives such as kartr̥tva, bhoktr̥tva etc is the sākṣī. If that sākṣī itself is taken away, the entirety of the samsārī that is kartā and bhoktā itself is also negated.

When we refer to the singular caitanya that is the sākṣī, the antaḥkaraṇa is the upādhi. When we refer to the jīvā which is kartā and bhoktā, the antaḥkaraṇa is the viśeṣaṇa. If the viśeṣaṇa is included, it is called viśiṣṭa. If the upādhi is included, it is called upahita.

*१. The upādhi is included in the lakṣya (kārya), is there, and excludes other things.

For example, in the nyāya mata, the ākāśa limited by the physical ears is the śrotrendriya. There, the physical ears are the upādhi for the śrotra indriya. The ears are used to point to the space in them, but are not a part of it. Thus, the ears are the upādhi for the indriya.

In the same way, the antaḥkaraṇa points to the caitanya that is the sākṣī but has no sambandha with it. Thus, the antaḥkaraṇa is the upādhi for the sākṣī. The sākṣī is the caitanya in the antaḥkaraṇa.