In the Muṇḍaka Upaniṣad (3.1.9) the measurement of the atomic spirit soul is further explained:
eṣo 'ṇur ātmā cetasā veditavyo
yasmin prāṇaḥ pañcadhā saṁviveśa
prāṇaiś cittaṁ sarvam otaṁ prajānāṁ
yasmin viśuddhe vibhavaty eṣa ātmā
"The soul is atomic in size and can be perceived by perfect intelligence. This atomic soul is floating in the five kinds of air (prāṇa, apāna, vyāna, samāna and udāna), is situated within the heart, and spreads its influence all over the body of the embodied living entities. When the soul is purified from the contamination of the five kinds of material air, its spiritual influence is exhibited."
(Bhagavad-Gita----2:17-----purport).

The influence of the atomic soul can be spread all over a particular body. According to the Muṇḍaka Upaniṣad, this atomic soul is situated in the heart of every living entity, and because the measurement of the atomic soul is beyond the power of appreciation of the material scientists, some of them assert foolishly that there is no soul. The individual atomic soul is definitely there in the heart along with the Supersoul, and thus all the energies of bodily movement are emanating from this part of the body.
(Bhagavad-Gita----2:17-----purport).

The Vedas, like the Muṇḍaka Upaniṣad, as well as the Śvetāśvatara Upaniṣad, compare the soul and the Supersoul to two friendly birds sitting on the same tree. One of the birds (the individual atomic soul) is eating the fruit of the tree, and the other bird (Kṛṣṇa) is simply watching His friend. Of these two birds—although they are the same in quality—one is captivated by the fruits of the material tree, while the other is simply witnessing the activities of His friend. Kṛṣṇa is the witnessing bird, and Arjuna is the eating bird.
(Bhagavad-Gita----2:22-----purport).

The jīva soul is struggling very hard on the tree of the material body, but as soon as he agrees to accept the other bird as the supreme spiritual master—as Arjuna agreed to do by voluntary surrender unto Kṛṣṇa for instruction—the subordinate bird immediately becomes free from all lamentations. Both the Muṇḍaka Upaniṣad (3.1.2) and Śvetāśvatara Upaniṣad (4.7) confirm this:
samāne vṛkṣe puruṣo nimagno
'nīśayā śocati muhyamānaḥ
juṣṭaṁ yadā paśyaty anyam īśam
asya mahimānam iti vīta-śokaḥ
"Although the two birds are in the same tree, the eating bird is fully engrossed with anxiety and moroseness as the enjoyer of the fruits of the tree. But if in some way or other he turns his face to his friend who is the Lord and knows His glories—at once the suffering bird becomes free from all anxieties."
(Bhagavad-Gita----2:22-----purport).

In the Kena Upaniṣad it is stated that the Supreme Brahman is the rest for everything, and Kṛṣṇa has already explained that everything is resting on Him. TheMuṇḍaka Upaniṣad confirms that the Supreme Lord, in whom everything is resting, can be realized only by those who engage constantly in thinking of Him. This constant thinking of Kṛṣṇa is smaraṇam, one of the methods of devotional service. It is only by devotional service to Kṛṣṇa that one can understand his position and get rid of this material body.
(Bhagavad-Gita----10:12-13-----purport).

Similarly, if the living entity is put into the body of a demigod, he must act according to his body. This is the law of nature. But in all circumstances, the Supersoul is with the individual soul. That is explained in the Vedas (Muṇḍaka Upaniṣad 3.1.1) as follows: dvā suparṇā sayujā sakhāyaḥ. The Supreme Lord is so kind upon the living entity that He always accompanies the individual soul and in all circumstances is present as the Supersoul, or Paramātmā.
(Bhagavad-Gita----13:21-----purport).

The Supreme Personality impregnates that total substance, and thus innumerable universes become possible. This total material substance, the mahat-tattva, is described as Brahman in the Vedic literature (Muṇḍaka Upaniṣad 1.1.9): tasmād etad brahma nāma-rūpam annaṁ ca jāyate. The Supreme Person impregnates that Brahman with the seeds of the living entities.
(Bhagavad-Gita----14:3-----purport).

The Vaikuṇṭha planets and the Goloka Vṛndāvana planet are all self-illuminating, and the rays scattered by those sva-dhāma of the Lord constitute the existence of the brahmajyoti. As further confirmed in the Vedas like the Muṇḍaka (2.2.10), Kaṭha (2.2.15) and Śvetāśvatara Upaniṣads (6.14):
na tatra sūryo bhāti na candra-tārakaṁ
nemā vidyuto bhānti kuto 'yam agniḥ
tam eva bhāntam anu bhāti sarvaṁ
tasya bhāsā sarvam idaṁ vibhāti
In the sva-dhāma of the Lord there is no need of sun, moon or stars for illumination. Nor is there need of electricity, so what to speak of ignited lamps? On the other hand, it is because those planets are self-illuminating that all effulgence has become possible, and whatever there is that is dazzling is due to the reflection of that sva-dhāma.
(Srimad Bhagavatam----2:4:14-----purport).

Yasmin vijñāte sarvam evaṁ vijñātaṁ bhavati (Muṇḍaka Upaniṣad 1.3). Anyone who is a devotee of the Lord knows about the Lord to some extent, and devotional service to the Lord makes him able to know everything by the grace of the Lord. Although a devotee may apparently express himself to be ignorant, he is full of knowledge in every intricate matter.
(Srimad Bhagavatam----3:7:8-----purport).

Yasmin vijñāte sarvam evaṁ vijñātaṁ bhavati (Muṇḍaka Upaniṣad 1.3). This Vedic hymn declares emphatically that the devotee of the Lord knows everything material and spiritual in relationship with the Lord. Devotees are not simply emotional, as is ill conceived by certain less intelligent men. Their direction is practical. They know everything that is and all the details of the Lord's domination over the different creations.
(Srimad Bhagavatam----3:7:26-----purport).

Dhruva Mahārāja's understanding of the Supreme Personality of Godhead is complete. In the Vedas it is said, yasmin vijñāte sarvam evaṁ vijñātaṁ bhavati (MuṇḍakaUpaniṣad 1.3): knowledge received through the transcendental, causeless mercy of the Lord is so perfect that by that knowledge the devotee becomes acquainted with all the different manifestations of the Lord.
(Srimad Bhagavatam----4:9:14-----purport).

There is a Vedic version, yasmin vijñāte sarvam evaṁ vijñātaṁ bhavati (Muṇḍaka Upaniṣad 1.3), which means that by knowing the Supreme Personality of Godhead, everything becomes known to the devotee. Similarly, by going to the planet of the Supreme Personality of Godhead, one can know all the other planetary systems on the path to Vaikuṇṭha. We should remember that Dhruva Mahārāja's body was different from our bodies.
(Srimad Bhagavatam----4:12:34-----purport).

"To learn transcendental subject matter, one must approach the spiritual master. In doing so, he should carry fuel to burn in sacrifice. The symptom of such a spiritual master is that he is expert in understanding the Vedic conclusion, and therefore he constantly engages in the service of the Supreme Personality of Godhead." (Muṇḍaka Upaniṣad1.2.12)
(Srimad Bhagavatam----4:28:65-----purport).

From the Upaniṣads (Muṇḍaka Upaniṣad 3.2.3) we learn:
nāyam ātmā pravacanena labhyo
na medhayā na bahunā śrutena
yam evaiṣa vṛṇute tena labhyas
tasyaiṣa ātmā vivṛṇute tanūṁ svām
Our relationship with the Supreme Lord is never advanced by simple study of the Vedas. There are many Māyāvādī sannyāsīs fully engaged in studying the Vedas, Vedānta-sūtra and Upaniṣads, but unfortunately they cannot grasp the real essence of knowledge.
(Srimad Bhagavatam----4:31:11-----purport).

The Vedas therefore enjoin, yasmin vijñāte sarvam evaṁ vijñātaṁ bhavati (Muṇḍaka Upaniṣad 1.3): if one simply understands the original cause, Kṛṣṇa, the cause of all causes, then naturally everything else is understood, although it may be presented in different varieties. By understanding the original cause of different varieties, one can understand everything. If we understand Kṛṣṇa, the original cause of everything, we do not need to separately study the subsidiary varieties. Therefore from the very beginning it is said: satyaṁ paraṁ dhīmahi.
(Srimad Bhagavatam----5:12:8-----purport).

In the human form of life, one should understand the Supreme Personality of Godhead, for then one will understand everything else. Yasmin vijñāte sarvam evaṁ vijñātaṁ bhavati (Muṇḍaka Upaniṣad 1.3). According to this Vedic injunction, simply by understanding Kṛṣṇa one understands Brahman, Paramātmā, prakṛti, the illusory energy, the spiritual energy and everything else. Everything will be revealed. prakṛti, the material nature, is. working under the direction of the Supreme Lord, and we living entities are being carried away by various phases of prakṛti.
(Srimad Bhagavatam----6:16:53-54-----purport).

The educators, scholars and big political leaders worshiped by millions of people cannot understand the goal of life and take to Kṛṣṇa consciousness, for they have not accepted a bona fide spiritual master and the Vedas. Therefore in the Muṇḍaka Upaniṣad (3.2.3) it is said, nāyam ātmā pravacanena labhyo na medhayā na bahunā śrutena: one cannot become self-realized simply by having an academic education, by presenting lectures in an erudite way (pravacanena labhyaḥ), or by being an intelligent scientist who discovers many wonderful things.
(Srimad Bhagavatam----7:5:32-----purport).

"The Lord is obtained only by one whom He Himself chooses. To such a person He manifests His own form." (Muṇḍaka Upaniṣad 3.2.3)
(Srimad Bhagavatam----7:5:32-----purport).

This analysis can be considered by a person who is fully under the shelter of the lotus feet of the Supreme Lord. This is confirmed by the Vedic injunction yasmin vijñāte sarvam evaṁ vijñātaṁ bhavati (Muṇḍaka Upaniṣad 1.3). If one can understand the Supreme Personality of Godhead, one can automatically understand everything else. Because of not taking shelter of the Lord's lotus feet, even great scholars, scientists, philosophers and religionists are always bewildered.
(Srimad Bhagavatam----7:7:26-----purport).

They come especially to deliver the fallen souls of this age. pāpī tāpī yata chila, hari-nāme uddhārila. Their weapon is saṅkīrtana, hari-nāma. Thus one should accept the sword of knowledge from Kṛṣṇa and be strong with the mercy of Balarāma. We are therefore worshiping Kṛṣṇa-Balarāma in Vṛndāvana. In the Muṇḍaka Upaniṣad (3.2.4) it is said:
nāyam ātmā bala-hīnena labhyo
na ca pramādāt tapaso vāpy aliṅgāt
etair upāyair yatate yas tu vidvāṁs
tasyaiṣa ātmā viśate brahma-dhāma
One cannot attain the goal of life without the mercy of Balarāma. Śrī Narottama dāsa Ṭhākura therefore says, nitāiyera karuṇā habe, vraje rādhā-kṛṣṇa pābe: when one receives the mercy of Balarāma, Nityānanda, one can attain the lotus feet of Rādhā and Kṛṣṇa very easily.
(Srimad Bhagavatam----7:15:45-----purport).

The Vedic mantras say: yasmin vijñāte sarvam evaṁ vijñātaṁ bhavati (Muṇḍaka Upaniṣad 1.3). When the devotee sees the Supreme Personality of Godhead by his meditation, or when he sees the Lord personally, face to face, he becomes aware of everything within this universe. Indeed, nothing is unknown to him. Everything within this material world is fully manifested to a devotee who has seen the Supreme Personality of Godhead.
(Srimad Bhagavatam----8:6:9-----purport).

Thus Mahārāja Ambarīṣa, being a devotee of the Personality of Godhead, was in full awareness of Paramātmā, Brahman, māyā, the material world, the spiritual world, and how things are going on everywhere. Everything was known to him. Yasmin vijñāte sarvam evaṁ vijñātaṁ bhavati (Muṇḍaka Upaniṣad 1.3). Because the devotee knows Vāsudeva, he knows everything within the creation of Vāsudeva (vāsudevaḥ sarvam iti sa mahātmā sudurlabhaḥ). Such a devotee does not give much value to the highest standard of happiness within this material world.
(Srimad Bhagavatam----9:5:25-----purport).

The spiritual master and disciple do not need to understand anything more than Kṛṣṇa because simply by understanding Kṛṣṇa and talking about Kṛṣṇa, one becomes a perfectly learned person (yasmin vijñāte sarvam evaṁ vijñātaṁ bhavati, Muṇḍaka Upaniṣad 1.3).
(Srimad Bhagavatam----10:1:4-----purport).

Those who are interested in ātma-tattva worship the Supreme Personality of Godhead (yasmin vijñāte sarvam evaṁ vijñātaṁ bhavati) (Muṇḍaka Upaniṣad 1.3). However, as explained in the next verse of this chapter, those who cannot understand ātma-tattva (apaśyatām ātma-tattvam) worship Yogamāyā in her different features.
(Srimad Bhagavatam----10:2:10-----purport).

Mere śāstra jñāna, or knowledge in the Vedas, does not help anyone understand the Personality of Godhead. Only one who is favored or shown mercy by the Lord can understand Him. This is also explained in the Upaniṣads (Muṇḍaka Upaniṣad 3.2.3):
nāyam ātmā pravacanena labhyo
na medhasā na bahunā śrutena
yam evaiṣa vṛṇute tena labhyas
tasyaiṣa ātmā vivṛṇute tanuṁ svām
"The Supreme Lord is not obtained by expert explanations, by vast intelligence, or even by much hearing. He is obtained only by one whom He Himself chooses. To such a person, He manifests His own form."
(Srimad Bhagavatam----10:13:54-----purport).

"The Supreme Personality of Godhead is Mahāprabhu, who disseminates transcendental enlightenment. Just to be in touch with Him is to be in contact with the indestructible brahmajyoti."
(3) From the Muṇḍaka Upaniṣad (3.1.3):
yadā paśyaḥ paśyate rukma-varṇaṁ
kartāram īśaṁ puruṣaṁ brahma-yonim
"One who sees that golden-colored Personality of Godhead, the Supreme Lord, the supreme actor, who is the source of the Supreme Brahman, is liberated."
(Sri Caitanya Caritamrta-----1:2:22-----purport).

In this Age of Kali the practical system of religion for everyone is the chanting of the name of Godhead. This was introduced in this age by Lord Caitanya. Bhakti-yoga actually begins with the chanting of the holy name, as confirmed by Madhvācārya in his commentary on the Muṇḍaka Upaniṣad.
(Sri Caitanya Caritamrta-----1:3:40-----purport).

In all the Vedas it is seen that to cross the ocean of nescience there is no alternative to the chanting of the holy name.” Similarly, Śrī Madhvācārya, while commenting upon the Muṇḍaka Upaniṣad, has quoted the following verse from the Nārāyaṇa-saṁhitā:
dvāparīyair janair viṣṇuḥ pañcarātrais tu kevalaiḥ
kalau tu nāma-mātreṇa pūjyate bhagavān hariḥ
"In Dvāpara-yuga one could satisfy Kṛṣṇa or Viṣṇu only by worshiping Him gorgeously according to the pāñcarātrikī system, but in the Age of Kali one can satisfy and worship the Supreme Personality of Godhead Hari simply by chanting the holy name."
(Sri Caitanya Caritamrta-----1:7:76-----purport).

The Supreme Personality of Godhead is by nature the master of all potencies. By nature, the living entities, being infinitesimal, are always under the influence of the Lord's potencies. According to the Muṇḍaka Upaniṣad (3.1.1–2):
dvā suparṇā sayujā sakhāyā
samānaṁ vṛkṣaṁ pariṣasvajāte
tayor anyaḥ pippalaṁ svādv atty
anaśnann anyo ’bhicākaśīti
samāne vṛkṣe puruṣo nimagno
’nīśayā śocati muhyamānaḥ
juṣṭaṁ yadā paśyaty anyam īśam
asya mahimānam eti vīta-śokaḥ
The Muṇḍaka Upaniṣad completely distinguishes the Lord from the living entities. The living entity is subjected to the reactions of fruitive activity, whereas the Lord simply witnesses such activity and bestows the results. According to the living entity's desires, he is wandering from one body to another and from one planet to another, under the direction of the Supreme Personality of Godhead, Paramātmā.
(Sri Caitanya Caritamrta-----2:6:162-----purport).

"The Supreme Absolute Truth is that from which everything is born." Similarly, in the Muṇḍaka Upaniṣad (1.1.7) it is stated, yathorṇa-nābhiḥ sṛjate gṛhṇate ca: "(The Lord creates and destroys the cosmic manifestation) as a spider creates a web and draws it back within itself." All of these sūtras indicate the transformation of the Lord's energy. It is not that the Lord undergoes direct transformation, which is called pariṇāma-vāda. However, being very anxious to protect Śrīla Vyāsadeva from criticism, Śaṅkarācārya became a pseudo gentleman and put forward his theory of illusion (vivarta-vāda). Śaṅkarācārya concocted this meaning of pariṇāma-vāda, and by word jugglery he endeavored very hard to establish pariṇāma-vāda as vivarta-vāda.
(Sri Caitanya Caritamrta-----2:6:172-----purport).

They are always doubting and engaging in mental concoctions. These talks cannot be understood by such whimsical people. Transcendental topics remain far, far away from those engaged in mundane arguments. In this regard, the Vedic mantras in the Kaṭha Upaniṣad (1.2.9) state, naiṣā tarkeṇa matir āpaneyā proktānyenaiva su-jñānāya preṣṭha. According to the MuṇḍakaUpaniṣad (3.2.3):
nāyam ātmā pravacanena labhyo
na medhayā na bahunā śrutena
yam evaiṣa vṛṇute tena labhyas
tasyaiṣa ātmā vivṛṇute tanūṁ svām
And according to the Brahma-sūtra (2.1.11), tarkāpratiṣṭhānāt.
All Vedic literatures declare that transcendental subjects cannot be understood simply by argument or logic. Spiritual matters are far above experimental knowledge.
(Sri Caitanya Caritamrta-----2:8:310-----purport).

The Supreme Personality of Godhead Kṛṣṇa and His pastimes cannot be understood by blunt material senses. One has to purify the senses by rendering transcendental loving service unto the Lord. When the Lord is pleased and reveals Himself, one can understand the transcendental form, name, qualities and pastimes of the Lord. This is confirmed in the Kaṭha Upaniṣad (2.23) and Muṇḍaka Upaniṣad (3.2.3): yam evaiṣa vṛṇute tena labhy
(Sri Caitanya Caritamrta-----2:9:126-----purport).

Even though Śrī Caitanya Mahāprabhu is the Supreme Personality of Godhead, He placed Himself in the position of a gopī. He also accepted the King's son directly as the son of Mahārāja Nanda, Vrajendra-nandana Hari. This is perfect vision according to the direction of the Vedic culture, as confirmed in Śrīmad Bhagavad-gītā: paṇḍitāḥ sama-darśinaḥ (B.G.5:18). Such acceptance of the Absolute Truth according to Vaiṣṇava philosophy is explained in both the Muṇḍaka Upaniṣad (3.2.3) and the Kaṭha Upaniṣad (1.2.23) in the following words:
nāyam ātmā pravacanena labhyo
na medhayā na bahunā śrutena
yam evaiṣa vṛṇute tena labhyas
tasyaiṣa ātmā vivṛṇute tanūṁ svām
"The Supreme Lord is not obtained by expert explanations, by vast intelligence, nor even by much hearing. He is obtained only by one whom He Himself chooses. To such a person He manifests His own form."
(Sri Caitanya Caritamrta-----2:12:61-----purport).

Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura gives the following commentary on the words bhāgavata vicāra. As confirmed in the Muṇḍaka Upaniṣad (1.1.4–5), there are two kinds of educational systems:
dve vidye veditavya iti, ha sma yad brahma-vido vadanti—parā caivāparā ca. tatrāparā ṛg-vedo yajur-vedaḥ sāma-vedo ‘tharva-vedaḥ śikṣā kalpo vyākaraṇaṁ niruktaṁ chando jyotiṣam iti. atha parā yayā tad akṣaram adhigamyate.
"There are two kinds of educational systems. One deals with transcendental knowledge (parā vidyā) and the other with material knowledge (aparā vidyā). All the Vedas—the Ṛg Veda, Yajur Veda, Sāma Veda and Atharva Veda, along with their corollaries, known as śikṣā, kalpa, vyākaraṇa, nirukta, chanda and jyotiṣa—belong to the inferior system of material knowledge (aparā vidyā). By parā vidyā one can understand the akṣara—Brahman or the Absolute Truth."
(Sri Caitanya Caritamrta-----2:19:17-----purport).

In the Muṇḍaka Upaniṣad it is stated that there are two birds sitting on the same tree, and one of these birds is eating the fruit of this tree while the other bird is simply witnessing his activities. Only when the bird eating the fruit looks at the other bird does he become free from all anxieties. This is the position of the infinitesimal living entity. As long as he is forgetful of the Supreme Personality of Godhead, who witnesses all his activities, he is subjected to the threefold miseries. But when he looks to the Supreme Lord and becomes the Supreme Lord's devotee, he becomes free from all anxieties and material miseries.
(Teachings of Lord Caitanya).

It is further recommended in the Muṇḍaka Upaniṣad, tad-vijñānārthaṁ sa gurum evābhigacchet: (MU 1.2.12) "To understand that transcendental science, one must approach a bona fide spiritual master." Thus this method of submissively receiving transcendental confidential knowledge is not merely based on mental speculation. In this regard, Śrī Caitanya Mahāprabhu told Rūpa Gosvāmī:
brahmāṇḍa bhramite kona bhāgyavān jīva
guru-kṛṣṇa-prasāde pāya bhakti-latā-bīja
"In the course of traversing the universal creation of Brahmā, some fortunate soul may receive the seed of bhakti-latā, the creeper of devotional service. This is all by the grace of guru and Kṛṣṇa." (CC.2:19:151) The material world is a place of confinement for the living entities who are by nature ānandamaya, pleasure-seeking.
(Nectar of Instruction).