The Lord is called adhokṣaja, or the person who is beyond perception by the blunt, limited potency of our senses. One cannot perceive the transcendental name or form of the Lord by mental speculation. The mundane Ph.D.'s are completely unable to speculate on the Supreme with their limited senses. Such attempts by the puffed up Ph.D's are compared to the philosophy of the frog in the well. A frog in a well was informed of the gigantic Pacific Ocean, and he began to puff himself up in order to understand or measure the length and breadth of the Pacific Ocean. Ultimately the frog burst and died. The title Ph.D. can also be interpreted as Plough Department, a title meant for the tillers in the paddy field. The attempt of the tillers in the paddy field to understand the cosmic manifestation and the cause behind such wonderful work can be compared to the endeavor of the frog in the well to calculate the measurement of the Pacific Ocean.
(Srimad Bhagavatam-----3:6:10-----purport).

The froggish philosophers may go on with their mental speculations on the subject matter of the virāṭ, the gigantic form exhibited by the yogamāyā internal potency of the Supreme Personality of Godhead, but factually no one can measure such a vast exhibition. In Bhagavad-gītā (11.16), Arjuna, the recognized devotee of the Lord, says:
aneka-bāhūdara-vaktra-netraṁ
paśyāmi tvāṁ sarvato 'nanta-rūpam
nāntaṁ na madhyaṁ na punas tavādiṁ
paśyāmi viśveśvara viśva-rūpa
"O my Lord, O gigantic viśva-rūpa form, O master of the universe, I see innumerable hands, bodies, mouths and eyes in all directions, and they are all unlimited. I cannot find the end of this manifestation, nor do I see the middle, nor the beginning."
(Srimad Bhagavatam-----3:6:35-----purport).

There are some froggish philosophers who want to know the Supreme Soul by means of philosophy and mental speculation. And when the devotees, who are to some extent in knowledge of the Supreme Lord, admit that the glories of the Lord are inestimable or inconceivable, the froggish philosophers adversely criticize them. These philosophers, like the frog in the well who tried to estimate the measurement of the Pacific Ocean, like to take trouble over fruitless mental speculation instead of taking instructions from devotees like the original poet, namely, Brahmā. Lord Brahmā underwent a severe type of meditation for one thousand celestial years, yet he said that the glories of the Lord are inconceivable. Therefore what can the froggish philosophers hope to gain from their mental speculations?
(Srimad Bhagavatam-----3:6:38-----purport).

The pure devotees of the Lord take shelter of the parā prakṛti, the internal potency of the Lord called Lakṣmīdevī, Sītādevī, Śrīmatī Rādhārāṇī or Śrīmatī Rukmiṇīdevī, and thus they become actual mahātmās, or great souls. Mahātmā are not fond of indulging in mental speculations, but they actually take to the devotional service of the Lord, without the slightest deviation. Devotional service is manifested by the primary process of hearing and chanting about the activities of the Lord. This transcendental method practiced by the mahātmās gives them sufficient knowledge of the Lord because if the Lord can at all be known to some extent, it is only through the means of devotional service and no other way. One may go on speculating and waste the valuable time of his human life, but that will not help anyone to enter into the precincts of the Lord. The mahātmās, however, are not concerned with knowing the Lord by mental speculation because they enjoy hearing about His glorious activities in His transcendental dealings with His devotees or with the demons. The devotees take pleasure in both and are happy in this life and the life after.
(Srimad Bhagavatam-----3:6:38-----purport).

The word tantram is significant herein. Sometimes tantram is misunderstood to be the black spiritual science of materialistic persons engaged in sense gratification, but here tantram means the science of devotional service compiled by Śrīla Nārada Muni. One can take advantage of such regulative explanations of the path of devotional service and make progressive advancement in the devotional service of the Lord. Sāṅkhya philosophy is the basic principle of acquiring knowledge, as will be explained by the sage Maitreya. The Sāṅkhya philosophy enunciated by Kapiladeva, the son of Devahūti, is the real source of knowledge about the Supreme Truth. Knowledge not based on the Sāṅkhya philosophy is mental speculationand can yield no tangible profit.
(Srimad Bhagavatam-----3:7:30-----purport).

Lord Brahmā was perplexed about his creation, the lotus and the world, even though he tried to understand them for one millennium, which is beyond calculation in the solar years of human beings. No one, therefore, can know the mystery of the creation and cosmic manifestation simply by mental speculation. The human being is so limited in his capacity that without the help of the Supreme he can hardly understand the mystery of the will of the Lord in terms of creation, continuance and destruction.
(Srimad Bhagavatam-----3:8:17-----purport).

The Supreme Lord can be experienced only through the process of devotional service and not by one's personal endeavor in mental speculation. The age of Brahmā is calculated in terms of divya years, which are distinct from the solar years of human beings. The divya years are calculated in Bhagavad-gītā (8.17): sahasra-yuga-paryantam ahar yad brahmaṇo viduḥ. Brahmā's one day is equal to one thousand times the aggregate of the four yugas (calculated to be 4,300,000 years). On that basis, Brahmā meditated for one hundred years before he could understand the supreme cause of all causes, and then he wrote the Brahma-saṁhitā, which is approved and recognized by Lord Caitanya and in which he sings, govindam ādi-puruṣaṁ tam ahaṁ bhajāmi. One has to wait for the mercy of the Lord before one can either render service unto Him or know Him as He is.
(Srimad Bhagavatam-----3:8:22-----purport).

Regarding the personal and impersonal features of the Supreme Absolute Truth, the personal forms exhibited by the Lord in His different plenary expansions are all for the benediction of all the universes. The personal form of the Lord is also worshiped in meditation as Supersoul, Paramātmā, but the impersonal brahma-jyotir is not worshiped. Persons who are addicted to the impersonal feature of the Lord, whether in meditation or otherwise, are all pilgrims to hell because, as stated in Bhagavad-gītā (12.5), impersonalists simply waste their time in mundane mental speculation because they are addicted more to false arguments than to reality. Therefore, the association of the impersonalists is condemned herewith by Brahmā.
(Srimad Bhagavatam-----3:9:4-----purport).

The Personality of Godhead can be experienced only by His causeless mercy, not by mental speculation or with the help of the material senses. Material senses cannot approach the transcendental understanding of the Supreme Personality of Godhead. He can be appreciated only by submissive devotional service when He reveals Himself before the devotee. Only by love of Godhead can one know God, and not otherwise. The Personality of Godhead cannot be seen with the material eyes, but He can be seen from within by spiritual eyes opened by the ointment of love of Godhead. As long as one's spiritual eyes are closed due to the dirty covering of matter, one cannot see the Lord. But when the dirt is removed by the process of devotional service, one can see the Lord, without a doubt. Brahmā's personal endeavor to see the root of the lotus pipe failed, but when the Lord was satisfied by his penance and devotion, He revealed Himself from within with no external endeavor.
(Srimad Bhagavatam-----3:9:37-----purport).

A conditioned soul is under the influence of mental speculation. However great one may be in the estimation of mundane education and learning, he cannot be free from the influence of psychic activities. Therefore it is very difficult to give up lust and the desires for low activities until one is in the line of devotional service to the Lord. When one is frustrated in lust and low desires, anger is generated from the mind and expressed from between the eyebrows. Ordinary men are therefore advised to concentrate the mind by focusing on the place between the eyebrows, whereas the devotees of the Lord are already practiced to place the Supreme Personality of Godhead on the seat of their minds. The theory of becoming desireless is untenable because the mind cannot be made desireless.
(Srimad Bhagavatam-----3:12:26-----purport).

Viśvaṁ samastam is very significant here. There are the material world and the spiritual world. The sages pray: "Both worlds are bewildered by Your different energies. Those who are in the spiritual world are absorbed in Your loving service, forgetting themselves and You also, and those in the material world are absorbed in material sense gratification and therefore also forget You. No one can know You, because You are unlimited. It is best not to try to know You by unnecessary mental speculation. Rather, kindly bless us so that we can worship You with causeless devotional service."
(Srimad Bhagavatam-----3:13:45-----purport).