sarvasya cāhaṁ hṛdi sanniviṣṭo mattaḥ smṛtirjñānamapohanaṁ ca | vedaiśca sarvairahameva vedyo vedāntakṛd vedavideva cāham ||15.15||
I have entered the hearts of all. From me (have come) memory, knowledge, and forgetfulness. I alone am the one to be known by all the Vedas and I alone am the author of the Vedānta (sampradāya) and indeed the knower of the Vedas.
The basis for all which are here and because of which all transactions take place is told from verse 12. In this verse Lord Krsna says sarvasya cāhaṁ hṛdi sanniviṣṭo - I have entered into the hearts of all, as the most inner-self. Entry is in figurative sense. In the Taittirīyopanisad, it is said, “having created that, he entered into it, and having entered he remains there as ātmā” , the inner-self of all, means there is no separation at all between jīva and Īśvara. Just like one space is conditioned by various upādhi, consciousness seemingly conditioned by the knower-known-knowledge. Conditioned by mind, it becomes the knower due to the superimposed attribute upon ātmā. And when the mind sees another upādhi, he says it is the known. Actually the consciousness that obtains in the knower and known is indeed the only consciousness caityanya which is brahman.
mattaḥ smṛtirjñānamapohanaṁ ca - from me the memory, knowledge and forgetfulness have come. Knowledge of the entire creation is Īśvara alone, and knowledge within the creation is also Īśvara, because without consciousness, there is no knower and known at all, and therefore no knowledge. Similarly from the standpoint of the faculty of remembering, Īśvara is the memory. Even forgetfulness/suspension of idea is him, since in the order of karma, forgetful can be in the form of pāpa if I need to write an exam, or it also can be punya when one forgets any pain one experienced. Otherwise one has no capacity to pick up a moment of joy if suspension of the pain is not there.
vedaiśca sarvairahameva vedyo - I alone am the one to be known by all the Vedas. We understand that everything that is said in the Veda is nothing but Īśvara. From karma-kanda, the ritual, deities, materials, the giver of the result, and the receiver all are Īśvara. And jñāna-kanda reveals that Īśvara is me alone.
Who is the author of Veda? vedāntakṛd vedavideva cāham - I am the author and the knower of the Veda. The entire tradition of teaching (sampradāya) is him alone. The teacher-student-knowledge are no different from paramesvara.
dvāvimau puruṣau loke kṣaraścākṣara eva ca | kṣaraḥ sarvāṇi bhūtāni kūṭastho’kṣara ucyate ||15.16||
These two persons, the perishable and the imperishable, (exist) in the world. All beings and elements are called the perishable, the changeless (is called) the imperishable.
uttamaḥ puruṣastvanyaḥ paramātmetyudāhṛtaḥ | yo lokatrayamāviśya bibhartyavyaya īśvaraḥ ||15.17||
But the other superior person is called the paramātmā, limitless self, the changeless Lord who, having entered the three worlds, sustains (them).
These two verses give the total insight of the creation. All the manifested names and forms are kṣara - perishable. And the un-manifest potential māyā is akṣara - indestructible. Both kṣara and akṣara are sustained by that uttamaḥ puruṣaḥ - paramātmā - brahman, the truth of everything. There is no object that exists apart from satyam jñānam anantam brahman which is the basis for everything. That is why the word puruṣaḥ - that which filled up, is used. The kṣara-upādhi, the manifest world becomes unmanifest resolving into the cause - akṣara, and again comes into manifestation. But these two are not having independent reality out of brahman. Just like the dream projected and resolved back into the dreamer, but my real nature neither the dream objects nor the dreamer. I am the basic consciousness who witnessing the dream and dreamer come and go, and even the waker. They are my projection but I am not them. Their attributes are not inherent to me, and I am not effected by them.
uttamaḥ puruṣastvanyaḥ - apart from those two is the most exalted. That is brahman who is entirely distinct from kṣara and akṣara. Just like the pot is clay but the clay is not pot.
It is called the limitless self - paramātmetyudāhṛtaḥ. The self which we thought being confined in this body-mind-senses, but actually it is the same self which limitlessly filled up the entire creation (space wise limitless). The one who is above kṣara and akṣara is the only inner consciousness of all being and entire manifestation which is represented by three world bhūḥ, bhuvaḥ, suvaḥ. Thus when we say lokatraya - three world, all the fourteen worlds are included. The same “is-ness” which is the real content of time is also not bound by time therefore changeless (time wise limitless).