SB 2.6.40-41

viśuddhaṁ kevalaṁ jñānaṁ
 pratyak samyag avasthitam
satyaṁ pūrṇam anādy-antaṁ
 nirguṇaṁ nityam advayam
ṛṣe vidanti munayaḥ
 praśāntātmendriyāśayāḥ
yadā tad evāsat-tarkais
 tirodhīyeta viplutam
Word for word: 
viśuddham — without any material tinge; kevalam — pure and perfect; jñānam — knowledge; pratyak — all-pervading; samyak — in fullness; avasthitam — situated; satyam — truth; pūrṇam — absolute; anādi — without any beginning; antam — and so also without any end; nirguṇam — devoid of material modes; nityam — eternal; advayam — without any rival; ṛṣe — O Nārada, O great sage; vidanti — they can only understand; munayaḥ — the great thinkers; praśānta — pacified; ātma — self; indriya — senses; āśayāḥ — sheltered; yadā — while; tat — that; eva — certainly; asat — untenable; tarkaiḥ — arguments; tiraḥ-dhīyeta — disappears; viplutam — distorted.
Translation: 
The Personality of Godhead is pure, being free from all contaminations of material tinges. He is the Absolute Truth and the embodiment of full and perfect knowledge. He is all-pervading, without beginning or end, and without rival. O Nārada, O great sage, the great thinkers can know Him when completely freed from all material hankerings and when sheltered under undisturbed conditions of the senses. Otherwise, by untenable arguments, all is distorted, and the Lord disappears from our sight.
Purport: 

Here is an estimation of the Lord apart from His transcendental activities in the temporary, material creations. Māyāvāda philosophy tries to designate the Lord as contaminated by a material body when He accepts forms of incarnation. This sort of interpolation is completely denied herein by the explanation that the Lord’s position is pure and unalloyed in all circumstances. According to Māyāvāda philosophy, the spirit soul, when covered by nescience, is designated as jīva, but when freed from such ignorance or nescience he merges in the impersonal existence of the Absolute Truth. But here it is said that the Lord is eternally the symbol of full and perfect knowledge. This is His speciality: perpetual freedom from all material contaminations. This distinguishes the Lord from the individual, common living entities who have the aptitude for being subordinated by nescience and thus becoming materially designated. In the Vedas it is said that the Lord is vijñānam ānandam, full of bliss and knowledge. The conditioned souls are never to be compared to Him because such individual souls have the tendency to become contaminated. Although after liberation the living entity can become one with the same quality of existence as the Lord, his very tendency to become contaminated, which the Lord never has, makes the individual living entity different from the Lord. In the Vedas it is said, śuddham apāpa-viddham: the individual ātmā becomes polluted by sin, but the Lord is never contaminated by sins. The Lord is compared to the powerful sun. The sun is never contaminated by anything infectious because it is so powerful. On the contrary, infected things are sterilized by the rays of the sun. Similarly, the Lord is never contaminated by sins; on the contrary, the sinful living entities become sterilized by contact with the Lord. This means that the Lord is also all-pervading like the sun, and as such, the word pratyak is used in this verse. Nothing is excluded from the expansions of the Lord’s potency. The Lord is within everything, and He is all-covering also, without being disturbed by the activities of the individual souls. He is therefore infinite, and the living entities are infinitesimal. In the Vedas it is said that only the Lord alone exists, and all others’ existences depend on Him. He is the generating reservoir for everyone’s existential capacity; He is the Supreme Truth of all other categorical truths. He is the source of everyone’s opulence, and therefore no one can equal Him in opulence. Being full of all opulences, namely wealth, fame, strength, beauty, knowledge and renunciation, certainly He is the Supreme Person. And because He is a person, He has many personal qualities, although He is transcendental to the material modes. We have already discussed the statement itthaṁ-bhūta-guṇo hariḥ (Bhāg. 1.7.10). His transcendental qualities are so attractive that even the liberated souls (ātmārāmas) are also attracted by them. Although possessed of all personal qualities, He is nevertheless omnipotent. Therefore, personally He has nothing to do, for everything is being carried out by His omnipotent energies. This is confirmed by the Vedic mantras: parāsya śaktir vividhaiva śrūyate svābhāvikī jñāna-bala-kriyā ca. This suggests His specific spiritual form, which can never be experienced by the material senses. He can be seen only when the senses are purified by devotional service (yam evaiṣa vṛṇute tena labhyaḥ). As such, there are basic differences between the Lord and the living entities, in so many respects. No one can be compared to the Lord, as the Vedas declare (ekam evādvitīyaṁ brahma, dvaitād vai bhayaṁ bhavati). The Lord has no competitor, and He has nothing to fear from any other being, nor can anyone be equal to Him. Although He is the root of all other beings, there are basic differences between Him and other beings. Otherwise there would have been no necessity for the statement in the previous verse that no one can know Him one-hundred-percent as He is (na yaṁ vidanti tattvena). That no one can fully understand Him is explained also in this verse, but the qualification for understanding to some degree is mentioned here. Only the praśāntas, or the unalloyed devotees of the Lord, can know Him to a greater extent. The reason is that the devotees have no demands in their lives but to be obedient servants of the Lord, while all others, namely the empiric philosophers, the mystics and the fruitive workers, all basically have some demand, and as such, they cannot be pacified. The fruitive worker wants reward for his work, the mystic wants some perfection of life, and the empiric philosopher wants to merge in the existence of the Lord. Somehow or other, as long as there is a demand for sense satisfaction, there is no chance for pacification; on the contrary, by unnecessary dry speculative arguments, the whole matter becomes distorted, and thus the Lord moves still further away from our understanding. The dry speculators, however, because of their following the principles of austerity and penance, can have knowledge of the impersonal features of the Lord to some extent, but there is no chance of their understanding His ultimate form as Govinda because only the amalātmanas, or the completely sinless persons, can accept pure devotional service to the Lord, as confirmed in the Bhagavad-gītā (7.28):

yeṣāṁ tv anta-gataṁ pāpaṁ
 janānāṁ puṇya-karmaṇām
te dvandva-moha-nirmuktā
 bhajante māṁ dṛḍha-vratāḥ