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SB 10.87.24

ka iha nu veda batāvara-janma-layo ’gra-saraṁ
 yata udagād ṛṣir yam anu deva-gaṇā ubhaye
tarhi na san na cāsad ubhayaṁ na ca kāla-javaḥ
 kim api na tatra śāstram avakṛṣya śayīta yadā
Synonyms: 
kaḥ — who; iha — in this world; nu — indeed; veda — knows; bata — ah; avara — recent; janma — whose birth; layaḥ — and annihilation; agra-saram — who came first; yataḥ — from whom; udagāt — arose; ṛṣiḥ — the learned sage, Brahmā; yam anu — following whom (Brahmā); deva-gaṇāḥ — the groups of demigods; ubhaye — both (those who control the senses and those who live in the regions above the heavenly planets); tarhi — at that time; na — no; sat — gross matter; na — no; ca — also; asat — subtle matter; ubhayam — that which is comprised of both (namely, the material bodies); na ca — nor; kāla — of time; javaḥ — the flow; kim api na — none at all; tatra — there; śāstram — authoritative scripture; avakṛṣya — withdrawing; śayīta — (the Supreme Lord) lies down; yadā — when.
Translation: 
Everyone in this world has recently been born and will soon die. So how can anyone here know Him who existed prior to everything else and who gave rise to the first learned sage, Brahmā, and all subsequent demigods, both lesser and greater? When He lies down and withdraws everything within Himself, nothing else remains — no gross or subtle matter or bodies composed of these, no force of time or revealed scripture.
Purport: 

Here the śrutis express the difficulty of knowing the Supreme. Devotional service, or bhakti-yoga, as described in these prayers of the personified Vedas, is the surest and easiest path to knowledge of the Lord and to liberation. In comparison, the philosophic search for knowledge, known as jñāna-yoga, is very difficult, favored though it is by those who are disgusted with material life but still unwilling to surrender to the Lord. As long as the finite soul remains envious of the Lord’s supremacy, the Lord does not reveal Himself. As He states in Bhagavad-gītā (7.25):

nāhaṁ prakāśaḥ sarvasya
 yoga-māyā-samāvṛtaḥ
mūdho ’yaṁ nābhijānāti
 loko mām ajam avyayam

“I am never manifest to the foolish and unintelligent. For them I am covered by My internal potency, and therefore they do not know that I am unborn and infallible.” And in the words of Lord Brahmā,

panthās tu koṭi-śata-vatsara-sampragamyo
 vāyor athāpi manaso muni-puṅgavānām
so ’py asti yat-prapada-sīmny avicintya-tattve
 govindam ādi-puruṣaṁ tam ahaṁ bhajāmi

“I worship Govinda, the primeval Lord, only the tip of the toe of whose lotus feet is approached by the yogīs, who aspire after the transcendental and betake themselves to prāṇāyāma by drilling the respiration; or by the jñānīs, who search out the undifferentiated Brahman by the process of elimination of the mundane, extending over thousands of millions of years.” (Brahma-saṁhitā 5.34)

Brahmā, the first-born living being in this universe, is also the foremost sage. He is born from Lord Nārāyaṇa, and from him appear the hosts of demigods, including both the controllers of earthly activities and the rulers of heaven. All these powerful and intelligent beings are relatively recent productions of the Lord’s creative energy. As the first speaker of the Vedas, Lord Brahmā should know their purport at least as well as any other authority, but even he knows the Personality of Godhead only to a limited extent. As Śrīmad-Bhāgavatam states (1.3.35), veda-guhyāni hṛt-pateḥ: “The Lord of the heart hides Himself deep within the confidential recesses of the Vedic sound.” If Brahmā and the demigods born from him cannot easily know the Supreme Lord, how then can mere mortals expect success in their independent pursuit of knowledge?

As long as this creation lasts, living beings face many obstacles on the path of knowledge. Because of identifying themselves with their material coverings, consisting of body, mind and ego, they acquire all sorts of prejudices and misconceptions. Even if they have the divine scripture to guide them and the opportunity to execute the prescribed methods of karma, jñāna and yoga, the conditioned souls have but little power for gaining knowledge of the Absolute. And when the time of annihilation comes, the Vedic scriptures and their regulative injunctions become unmanifest, leaving the dormant jīvas completely in darkness. Therefore we should abandon our futile endeavors for knowledge without devotion and simply surrender ourselves to the Supreme Lord’s mercy, heeding the advice of Lord Brahmā:

jñāne prayāsam udapāsya namanta eva
 jīvanti san-mukharitāṁ bhavadīya-vārtām
sthāne sthitāḥ śruti-gataṁ tanu-vāṅ-manobhiḥ
 ye prāyaśo jita jito ’py asi tais tri-lokyām

“Those who, even while remaining situated in their established social positions, throw away the process of speculative knowledge and with their body, words and mind offer all respects to descriptions of Your personality and activities, dedicating their lives to these narrations, which are vibrated by You personally and by Your pure devotees, certainly conquer Your Lordship, although You are otherwise unconquerable by anyone within the three worlds.” (Bhāg. 10.14.3)

In this regard, the Taittirīya Upaniṣad (2.4.1) refers to the Supreme as yato vāco nivartante aprāpya manasā saha, “where words cease, and where the mind cannot reach.” The Īśopaniṣad (4) states:

anejad ekaṁ manaso javīyo
 naitad devā āpnuvan pūrvam arśat
tad dhāvato ’nyān atyeti tiṣṭhat
 tasmin apo mātariśvā dadhāti

“Although fixed in His abode, the Personality of Godhead is more swift than the mind and can overcome all others running. The powerful demigods cannot approach Him. Although in one place, He controls those who supply the air and rain. He surpasses all in excellence.” And in the Ṛg Veda (3.54.5) we find this mantra:

ko ’ddhā veda ka iha pravocat
 kuta āyātāḥ kuta iyaṁ visṛṣṭiḥ
arvāg devā visarjanenā-
 thā ko veda yata ā babhūva

“Who in this world actually knows, and who can explain, whence this creation has come? The demigods, after all, are younger than the creation. Who, then, can tell whence this world has come into being?”

Śrīla Śrīdhara Svāmī thus prays:

kvāhaṁ buddhy-ādi-saṁruddhaḥ
 kva ca bhūman mahas tava
dīna-bandho dayā-sindho
 bhaktiṁ me nṛ-hare diśa

“What am I, a being entrapped by the material coverings of worldly intelligence and so on? And what are Your glories by comparison, O almighty one? O friend of the fallen, O ocean of mercy, Lord Nṛhari, please bless me with Your devotional service.”