And for good measure, here is Srila Prabhupada's purport for Isopanisad, verse 15:
In Bhagavad-gita, the Lord explains His personal rays (brahmajyoti), the dazzling effulgence of His personal form, in this way:
brahmano hi pratisthaham
amrtasyavyayasya ca
sasvatasya ca dharmasya
sukhasyaikantikasya ca
"And I am the basis of the impersonal Brahman, which is the constitutional position of ultimate happiness, and which is immortal, imperishable and eternal." (Bg. 14.27)
Brahman, Paramatma and Bhagavan are three aspects of the same Absolute Truth. Brahman is the aspect most easily perceived by the beginner. Paramatma, the Supersoul, is realized by those who have further progressed, and Bhagavan realization is the ultimate realization of the Absolute Truth. This is confirmed in Bhagavad-gita, where the Lord says that He is the ultimate concept of the Absolute Truth, the source of the brahmajyoti as well as the all-pervading Paramatma. In Bhagavad-gita Krishna says that He is the ultimate reservoir of the brahmajyoti, the impersonal conception of the Absolute Truth, and that there is no need to explain His unlimited potencies.
atha va bahunaitena
kim jnatena tavarjuna
vistabhyaham idam krtsnam
ekamsena 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." (Bg. 10.42) Thus by His one plenary expansion, the all-pervading Paramatma, the Lord maintains the complete material cosmic creation. He also maintains all manifestations in the spiritual world as well; therefore in the sruti mantra of Sri Isopanisad, the Lord is addressed as pusan, the ultimate maintainer.
The personality of Godhead, Sri Krishna, is always in transcendental bliss (anandamayo 'bhyasat). When He was present at Vrndavana in India 5,000 years ago, He always remained in transcendental bliss, even from the beginning of His childhood pastimes. The killing of varieties of demons--such as Agha, Baka, Putana and Pralamba--were but pleasure excursions for Him. In the village of Vrndavana 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 that was performed by the Lord at Vrndavana was performed for the pleasure of His associates there. The Lord created these pastimes to attract the dry speculators and the acrobats of the so-called hatha-yoga system who had come to find the Absolute Truth.
Of the childhood play between the Lord and His playmates, the cowherd boys, Sukadeva Gosvami said in Srimad-Bhagavatam:
ittham satam brahma-sukhanubhutya
dasyam gatanam para-daivatena
mayasritanam nara-darakena
sakam vijahruh krta-punya-punjah
"The Personality of Godhead, who is perceived as the impersonal, blissful Brahman, who is worshiped as the Supreme Lord by the devotees, and who is considered an ordinary human being by the mundane, played with the cowherd boys, who had attained their position after accumulating many pious activities." (SB. 10.12.11)
Thus the Lord is always engaged in transcendental loving activities with His spiritual associates in the various relationships of santa (neutrality), dasya (servitorship), sakhya (friendship), vatsalya (paternal affection), and madhurya (conjugal love).
Since it is said that the Lord never leaves Vrndavana-dhama, one may ask how He manages the affairs of the creation. This is answered in Bhagavad-gita (Bg. 13.14): The Lord pervades the entire material creation by His plenary part known as the purusa incarnation. 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 Paramatma or Supersoul. Every living entity is known as atma, soul, and the principal atma who controls them all is Paramatma, the Supersoul.
This system of God realization is a great science. The materialists can only analyze and meditate on the twenty-four factors of the material creation, for they have very little information of the purusa, the Lord. The impersonal transcendentalists are simply bewildered by the glaring effulgence of the brahmajyoti. If one wants to see the Absolute Truth in full, he has to penetrate beyond the twenty-four material elements and the glaring effulgence as well. Sri Isopanisad points towards this direction, praying for the removal of the hiranmaya-patra, the dazzling covering. Unless this covering is removed so one can perceive the personality of Godhead as He is, factual realization of the Absolute Truth can never be attained.
The Paramatma feature of the Personality of Godhead is one of three plenary expansions, collectively called visnu-tattva. The visnu-tattva within the universe (one of the three principal deities--Brahma, Vishnu and Shiva) is known as the Ksirodakasayi Vishnu. He is the all-pervading Paramatma in each and every individual living entity. The Garbhodakasayi Vishnu is the collective Supersoul within all living entities. Beyond these two is the Karanodakasayi Vishnu lying in the causal ocean. He is the creator of all universes. The yoga system teaches the serious student to meet the visnu-tattvas after overcoming the twenty-four material elements of the cosmic creation. The culture of empiric philosophy helps one realize the impersonal brahmajyoti, which is the glaring effulgence of the transcendental body of Lord Sri Krishna. This is confirmed in Bhagavad-gita (Bg. 14.27) as well as Brahma-samhita (5.40):
yasya prabha prabhavato jagad-anda-koti-
kotisv asesa-vasudhadi vibhuti-bhinnam
tad brahma niskalam anantam asesa-bhutam
govindam adi-purusam tam aham bhajami
"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 brahmajyoti. This brahmajyoti is but the personal rays of the Supreme Personality of Godhead, whom I worship." This mantra from Brahma-samhita is spoken from the platform of factual realization of the Absolute Truth, and the sruti mantra of Sri Isopanisad confirms this mantra as a process of realization. It is a simple prayer to the Lord to remove the brahmajyoti so that one can see His real face.
Perfect knowledge means knowing Krishna as the root of Brahman. The root of Brahman is Lord Sri Krishna, and in scriptures such as Srimad-Bhagavatam the science of Krishna is perfectly elaborated. In Srimad-Bhagavatam, the author, Srila Vyasadeva, has established that the Supreme Truth is described as Brahman, Paramatma or Bhagavan according to one's realization of Him. Srila Vyasadeva never states that the Supreme Truth is a jiva, an ordinary living entity. The living entity should never be considered the all-powerful Supreme Truth. If he were, there would be no need for the living entity to pray to the Lord to remove His dazzling cover so that the living entity can see His real form.
The conclusion is that in the absence of spiritually potent manifestations of the Supreme Truth, the impersonal Brahman is realized. Similarly, when one realizes the material potencies of the Lord, having little or no information of the spiritual potency, he attains Paramatma realization. Thus both Brahman and Paramatma realization of the Absolute Truth are partial realizations. However, when one realizes the Supreme Personality of Godhead, Sri Krishna, in full potency after the removal of the hiranmaya-patra, he realizes vasudevah sarvam iti: Lord Sri Krishna, known as Vasudeva, is everything--Brahman, Paramatma and Bhagavan. He is Bhagavan, the root, and Brahman and Paramatma are His branches.
In Bhagavad-gita there is a comparative analysis of the three types of transcendentalists, the worshipers of the impersonal Brahman (jnanis), the worshipers of the Paramatma feature (yogis), and the devotees of Lord Sri Krishna (bhaktas). It is stated in Bhagavad-gita (Bg. 6.46-47) that amongst all types of transcendentalists, he who is a jnani, who has cultivated Vedic knowledge, is supreme. Yet the yogis are still greater than the jnanis and far superior to fruitive workers as well. And amongst all yogis, he who constantly serves the Lord with all his energies is the topmost. In summary, a philosopher is better than a laboring man, and a mystic is superior to a philosopher. And of all the mystic yogis, he who follows bhakti-yoga, constantly engaged in the service of the Lord, is the highest. Sri Isopanisad directs us toward this perfection.