Sri Krishn has preached in Bhagwad Geeta:
pravṛttiṁ ca nivṛttiṁ ca kāryākārye bhayābhaye
bandhaṁ mokṣaṁ ca yā vetti buddhiḥ sā pārtha sāttvikī
“That intellect is immaculate, O Parth, which is aware of the essence, of the way of inclination as also of renunciation, of worthy and unworthy action, of fear and fearlessness, and of bondage and liberation.”
In other words, the righteous, morally good intellect is that which is aware of the distinction between the way that leads to God and the way to recurrent birth and death.
yayā dharmamadharmaṁ ca kāryaṁ cākāryameva ca
ayathāvatprajānāti buddhiḥ sā pārtha rājasī
‘‘That intellect is of the nature of passion and moral blindness, O Parth, by which one cannot even know the righteous and the unrighteous as well as what is worthy or unworthy of being done.”
He says further:
adharmaṁ dharmamiti yā manyate tamasāvṛtā
sarvārthānviparītāṁśca buddhiḥ sā pārtha tāmasī
“That intellect is of the nature of ignorance, O Parth, which is enveloped in darkness and which apprehends the sinful as virtuous and views everything in a distorted way.”
Intellect is graded into three kinds. The intellect which is well aware of the action that has to be engaged in and the action that has to be shunned, as well as of that which is fit or unfit to be done, is characterized by moral excellence.
The intellect which has only a dim perception of the righteous and the unrighteous action, and which does not know the truth, is dominated by passion. The perverse intellect that deems the sinful as virtuous, the destructible as eternal, and the inauspicious as auspicious, is shrouded in the gloom of ignorance.
Sri Krishn also explains of how one who is immaculate achieves realization of the Supreme Being, which represents the culmination of knowledge?
buddhyā viśuddhayā yukto dhṛtyātmānaṁ niyamya ca
śabdādīnviṣayāṁstyaktvā rāgadveṣau vyudasya ca
viviktasevī laghvāśī yatavākkāyamānasaḥ
dhyānayogaparo nityaṁ vairāgyaṁ samupāśritaḥ
ahaṁkāraṁ balaṁ darpaṁ kāmaṁ krodhaṁ parigraham
vimucya nirmamaḥ śānto brahmabhūyāya kalpate
“Blessed with a pure intellect, firmly in command of the Self, with objects of sensual gratification like sound forsaken , with both fondness and revulsion destroyed,dwelling in seclusion, eating frugally, subdued in mind, speech and body, incessantly given to the yog of meditation, firmly resigned,giving up conceit,arrogance of power,yearning,ill humour,and acquisitiveness, devoid of attachment,and in possession of a mind at repose,a man is worthy of becoming one with God.”
As expounded by Swami Adgadanand Paramhans,most revered Gurudev.