In the Upanisads it is clearly said that the Supreme Personality of Godhead can never be known simply by working very hard and taxing the good brain, nor can He be known simply by mental speculation and jugglery of words
SB Canto 2
O Nārada, although the potencies of the Lord are unknowable and immeasurable, still, because we are all surrendered souls, we know how He acts through yogamāyā potencies. And, similarly, the potencies of the Lord are also known to the all-powerful Śiva, the great king of the atheist family, namely Prahlāda Mahārāja, Svāyambhuva Manu, his wife Śatarūpā, his sons and daughters like Priyavrata, Uttānapāda, Ākūti, Devahūti and Prasūti, Prācīnabarhi, Ṛbhu, Aṅga the father of Vena, Mahārāja Dhruva, Ikṣvāku, Aila, Mucukunda, Mahārāja Janaka, Gādhi, Raghu, Ambarīṣa, Sagara, Gaya, Nāhuṣa, Māndhātā, Alarka, Śatadhanve, Anu, Rantideva, Bhīṣma, Bali, Amūrttaraya, Dilīpa, Saubhari, Utaṅka, Śibi, Devala, Pippalāda, Sārasvata, Uddhava, Parāśara, Bhūriṣeṇa, Vibhīṣaṇa, Hanumān, Śukadeva Gosvāmī, Arjuna, Ārṣṭiṣeṇa, Vidura, Śrutadeva, etc.
All the great devotees of the Lord, as mentioned above, who flourished in the past or present, and all the devotees of the Lord who will come in the future, are aware of the different potencies of the Lord along with the potency of His name, quality, pastimes, entourage, personality, etc. And how do they know? Certainly it is not by mental speculation, nor by any attempt by dint of limited instruments of knowledge. By the limited instruments of knowledge (either the senses or the material instruments like microscopes and telescopes) one cannot even fully know the Lord's material potencies, which are manifested before our eyes. For example there are many millions and billions of planets far, far beyond the scientist's calculation. But these are only the manifestations of the Lord's material energy. What can the scientist hope to know of the spiritual potency of the Lord by such material efforts? Mental speculations, by adding some dozens of "if's" and "maybe's," cannot aid the advancement of knowledge—on the contrary, such mental speculations will only end in despair by dismissing the case abruptly and declaring the nonexistence of God. The sane person, therefore, ceases to speculate on subjects beyond the jurisdiction of his tiny brain, and as a matter of course he tries to learn to surrender unto the Supreme Lord, who alone can lead one to the platform of real knowledge. In the Upaniṣads it is clearly said that the Supreme Personality of Godhead can never be known simply by working very hard and taxing the good brain, nor can He be known simply by mental speculation and jugglery of words. The Lord is knowable only by one who is a surrendered soul. Herein Brahmājī, the greatest of all material living beings, acknowledges this truth. Therefore, the fruitless spoiling of energy by pursuing the path of experimental knowledge must be given up. One should gain knowledge by surrendering unto the Lord and by acknowledging the authority of the persons mentioned herein. The Lord is unlimited and, by the grace of the yogamāyā, helps the surrendered soul to know Him proportionately with the advance of one's surrender.