In the Bhagavad-gītā (14.27), the Lord explains His personal rays (brahma-jyotir), the dazzling effulgence of His personal form, in this way:
brahmaṇo hi pratiṣṭhāham
śāśvatasya ca dharmasya
"I am the basis of the impersonal Brahman, which is immortal, imperishable and eternal and is the constitutional position of ultimate happiness." Brahman, Paramātmā and Bhagavān are three aspects of the same Absolute Truth. Brahman is the aspect most easily perceived by the beginner; Paramātmā, the Supersoul, is realized by those who have further progressed; and Bhagavān realization is the ultimate realization of the Absolute Truth. This is confirmed in the Bhagavad-gītā (7.7), where Lord Kṛṣṇa says that He is the ultimate concept of the Absolute Truth: mattaḥ parataraṁ nānyat. Therefore Kṛṣṇa is the source of the brahma-jyotir as well as the all-pervading Paramātmā. Later in the Bhagavad-gītā (10.42) Kṛṣṇa further explains:
atha vā bahunaitena
kiṁ jñātena tavārjuna
viṣṭabhyāham idaṁ kṛtsnam
ekāṁśena sthito jagat
"But what need is there, Arjuna, for all this detailed knowledge? With a single fragment of Myself I pervade and support this entire universe." Thus by His one plenary expansion, the all-pervading Paramātmā, the Lord maintains the complete material cosmic creation. He also maintains all manifestations in the spiritual world. Therefore in this śruti-mantra of Śrī Īśopaniṣad, the Lord is addressed as pūṣan, the ultimate maintainer.
The Personality of Godhead, Śrī Kṛṣṇa, is always filled with transcendental bliss (ānanda-mayo 'bhyāsāt). When He was present at Vṛndāvana in India five thousand years ago, He always remained in transcendental bliss, even from the beginning of His childhood pastimes. The killings of various demons—such as Agha, Baka, Pūtanā and Pralamba—were but pleasure excursions for Him. In His village of Vṛndāvana He enjoyed Himself with His mother, brother and friends, and when He played the role of a naughty butter thief, all His associates enjoyed celestial bliss by His stealing. The Lord's fame as a butter thief is not reproachable, for by stealing butter the Lord gave pleasure to His pure devotees. Everything the Lord did in Vṛndāvana was for the pleasure of His associates there. The Lord created these pastimes to attract the dry speculators and the acrobats of the so-called haṭha-yoga system who wish to find the Absolute Truth.
Of the childhood play between the Lord and His playmates, the cowherd boys, Śukadeva Gosvāmī says in Śrīmad-Bhāgavatam (10.12.11):
itthaṁ satāṁ brahma-sukhānubhūtyā
dāsyaṁ gatānāṁ para-daivatena
sākaṁ vijahruḥ kṛta-puṇya-puñjāḥ
"The Personality of Godhead, who is perceived as the impersonal, blissful Brahman by the jñānīs, who is worshiped as the Supreme Lord by devotees in the mood of servitorship, and who is considered an ordinary human being by mundane people, played with the cowherd boys, who had attained their position after accumulating many pious activities."
Thus the Lord is always engaged in transcendental loving activities with His spiritual associates in the various relationships of śānta (neutrality), dāsya (servitorship), sakhya (friendship), vātsalya (parental affection) and mādhurya (conjugal love).
Since it is said that Lord Kṛṣṇa never leaves Vṛndāvana-dhāma, one may ask how He manages the affairs of the creation. This is answered in the Bhagavad-gītā (13.14-18): The Lord pervades the entire material creation by His plenary part known as the Paramātmā, or Supersoul. Although the Lord personally has nothing to do with material creation, maintenance and destruction, He causes all these things to be done by His plenary expansion, the Paramātmā. Every living entity is known as ātmā, soul, and the principal ātmā who controls them all is Paramātmā, the Supersoul.
This system of God realization is a great science. The materialistic sāṅkhya-yogīs can only analyze and meditate on the twenty-four factors of the material creation, for they have very little information of the puruṣa, the Lord. And the impersonal transcendentalists are simply bewildered by the glaring effulgence of the brahma-jyotir. If one wants to see the Absolute Truth in full, one has to penetrate beyond the twenty-four material elements and the glaring effulgence as well. Śrī Īśopaniṣad points toward this direction, praying for the removal of the hiraṇmaya-pātra, the dazzling covering of the Lord. Unless this covering is removed so one can perceive the real face of the Personality of Godhead, factual realization of the Absolute Truth can never be achieved.
The Paramātmā feature of the Personality of Godhead is one of three plenary expansions, or viṣṇu-tattvas, collectively known as the puruṣa-avatāras. One of these viṣṇu-tattvas who is within the universe is known as Kṣīrodakaśāyī Viṣṇu. He is the Viṣṇu among the three principal deities—Brahmā, Viṣṇu and Śiva—and He is the all-pervading Paramātmā in each and every individual living entity. The second viṣṇu-tattva within the universe is Garbhodakaśāyī Viṣṇu, the collective Supersoul of all living entities. Beyond these two is Kāraṇodakaśāyī Viṣṇu, who lies in the Causal Ocean. He is the creator of all universes. The yoga system teaches the serious student to meet the viṣṇu-tattvas after going beyond the twenty-four material elements of the cosmic creation. The culture of empiric philosophy helps one realize the impersonal brahma-jyotir, which is the glaring effulgence of the transcendental body of Lord Śrī Kṛṣṇa. That the brahma-jyotir is Kṛṣṇa's effulgence is confirmed in the Bhagavad-gītā (14.27) as well as the Brahma-saṁhitā (5.40):
yasya prabhā-prabhavato jagad-aṇḍa-koṭi-
koṭiṣv aśeṣa-vasudhādi vibhūti-bhinnam
tad brahma niṣkalam anantam aśeṣa-bhūtaṁ
govindam ādi-puruṣaṁ tam ahaṁ bhajāmi
"In the millions and millions of universes there are innumerable planets, and each and every one of them is different from the others by its cosmic constitution. All of these planets are situated in a corner of the brahma-jyotir. This brahma-jyotir is but the personal rays of the Supreme Personality of Godhead, Govinda, whom I worship." This mantra from the Brahma-saṁhitā is spoken from the platform of factual realization of the Absolute Truth, and the śruti-mantra of Śrī Īśopaniṣad under discussion confirms this mantra as a process of realization. The Īśopaniṣad mantra is a simple prayer to the Lord to remove the brahma-jyotir so that one can see His real face. This brahma-jyotir effulgence is described in detail in several mantras of the Muṇḍaka Upaniṣad (2.2.10-12):
hiraṇmaye pare kośe
virajaṁ brahma niṣkalam
tac chubhraṁ jyotiṣāṁ jyotis
tad yad ātma-vido viduḥ
na tatra sūryo bhāti na candra-tārakaṁ
nemā vidyuto bhānti kuto 'yam agniḥ
tam eva bhāntam anu bhāti sarvaṁ
tasya bhāsā sarvam idaṁ vibhāti
brahmaivedam amṛtaṁ purastād brahma
paścād brahma dakṣiṇataś cottareṇa
adhaś cordhvaṁ ca prasṛtaṁ brahmai-
vedaṁ viśvam idaṁ variṣṭham
"In the spiritual realm, beyond the material covering, is the unlimited Brahman effulgence, which is free from material contamination. That effulgent white light is understood by transcendentalists to be the light of all lights. In that realm there is no need of sunshine, moonshine, fire or electricity for illumination. Indeed, whatever illumination appears in the material world is only a reflection of that supreme illumination. That Brahman is in front and in back, in the north, south, east and west, and also overhead and below. In other words, that supreme Brahman effulgence spreads throughout both the material and spiritual skies."
Perfect knowledge means knowing Kṛṣṇa as the root of this Brahman effulgence. This knowledge can be gained from such scriptures as Śrīmad-Bhāgavatam, which perfectly elaborates the science of Kṛṣṇa. In Śrīmad-Bhāgavatam, the author, Śrīla Vyāsadeva, has established that one will describe the Supreme Truth as Brahman, Paramātmā or Bhagavān according to one's realization of Him. Śrīla Vyāsadeva never states that the Supreme Truth is a jīva, an ordinary living entity. The living entity should never be considered the all-powerful Supreme Truth. If he were the Supreme, he would not need to pray to the Lord to remove His dazzling cover so that the living entity could see His real face.
The conclusion is that one who has no knowledge of the potencies of the Supreme Truth will realize the impersonal Brahman. Similarly, when one realizes the material potencies of the Lord but has little or no information of the spiritual potencies, he attains Paramātmā realization. Thus both Brahman and Paramātmā realization of the Absolute Truth are partial realizations. However, when one realizes the Supreme Personality of Godhead, Śrī Kṛṣṇa, in full potency after the removal of the hiraṇmaya-pātra, one realizes vāsudevaḥ sarvam iti: [Bg. 7.19] Lord Śrī Kṛṣṇa, who is known as Vāsudeva, is everything—Brahman, Paramātmā and Bhagavān. He is Bhagavān, the root, and Brahman and Paramātmā are His branches.
In the Bhagavad-gītā (6.46-47) there is a comparative analysis of the three types of transcendentalists—the worshipers of the impersonal Brahman (jñānīs), the worshipers of the Paramātmā feature (yogīs) and the devotees of Lord Śrī Kṛṣṇa (bhaktas). It is stated there that the jñānīs, those who have cultivated Vedic knowledge, are better than ordinary fruitive workers, that the yogīs are still greater than the jñānīs, and that among all yogīs, those who constantly serve the Lord with all their energies are the topmost. In summary, a philosopher is better than a laboring man, a mystic is superior to a philosopher, and of all the mystic yogīs, he who follows bhakti-yoga, constantly engaging in the service of the Lord, is the highest. Śrī Īśopaniṣad directs us toward this perfection.