These impersonalist, they are thinking falsely that they have become Nārāyaṇa or liberated. Actually, it is not so? Why? Aviśuddha-buddhayaḥ: their intelligence is not yet purified. Because there is material attachment, that means the intelligence is still materially affected. Otherwise, how they should be materially inclined and come to the material platform? The answer is aviśuddha-buddhayaḥ. The intelligence is not yet purified.
Why it is not purified? That is also explained: āruhya kṛcchreṇa paraṁ padam (SB 10.2.32). By severe austerities, penances, they follow very strictly the rules and regulation of renouncement. That is called kṛccha sādhana, difficult procedure for self-realization. But despite all these endeavors, because their intelligence is not purified, they fall down. Āruhya kṛcchreṇa paraṁ padam.
They realize, actually, the impersonal Brahman, nirbheda-brahmānusandhana. Āruhya kṛcchreṇa paraṁ patanty adhaḥ. Again they fall down from that platform. Why? Anādṛta-yuṣmad-aṅghrayaḥ: because they could not adore the lotus feet of Kṛṣṇa. On account of their impersonal impression, they could not appreciate the transcendental, sac-cid-ananda vigraha (Bs. 5.1) of Kṛṣṇa, and could not surrender there. Kṛṣṇa therefore says, bahūnāṁ janmanām ante jñānavān māṁ prapadyate (BG 7.19). After many, many births of such struggle, when actually one becomes wise, jñānavān, he surrenders unto the lotus feet of Kṛṣṇa. That is the perfection of knowledge.
So impersonal Brahman realization, nirbheda-brahmānu-sandhana, is good attempt, but above that if one cannot make progress . . . brahmeti paramātmeti bhagavān iti śabdyate (SB 1.2.11). The Absolute Truth is realized as impersonal Brahman, then Paramātmā, then the Supreme Personality of Godhead. So unless a wise jñānavān, jñānī, does not reach to the platform of understanding the personality, the Supreme Personality of Godhead, his intelligence is still not very much purified.