What is the difference between "Zehirut" - caution regarding ones actions, and "Nekiyut" -complete taintlessness?
The "Zahir" - one who possesses Zehirut - is cautious about his actions, being careful not to stumble over those transgressions which are well known and common to all. While he is careful regarding common and well known sins, when it comes to those borderline transgressions that are not so apparent, he tends to be careless. His heart permits him to be lenient, and he is reluctant to distance himself from them.
Yet the "Naki" - one who has internalized the quality of Nekiyut - has developed the capacity of pure discernment, and therefore distances himself from anything even slightly resembling sin. He is not satisfied with merely separating himself from sins which are clear and well known.

The heart of the Zahir is still effected by bad habits, and he has yet to completely purify himself of lust. Therefore, his heart induces him to be lenient regarding those actions which are not clearly recognizable as sins. And though he does his best to conquer his impulses and restrain his desires, he still has a long road ahead of him. So long as his heart has not been purged of all its cravings, they continue to effect him, persuading him to be lenient in cases of uncertainty and to avoid thorough investigation.

Pathways in Personality Development (52)

Rabbi Zalman Baruch Melamed

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10 - 10. Self Criticism and Taintlessness

11 - 11. Clarifying "Nekiyut"

12 - 12. Greed and the Prohibition Against Theft

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Man must become extremely proficient in the art of Zehirut. He must first shake himself clean of the more common sins, and then accustom himself in worship and zeal, to the point where he becomes filled with love and desire for his Creator. The effect of such behavior will be to distance him from material concerns, and bring him to focus his thoughts on perfecting himself spiritually to the point where he is capable of attaining complete taintlessness. He will then clean his heart of all physical lust, and fill it with a longing for God. His perception will take on a pure and clear nature, and he will distance himself from all sins, even the smallest amongst them. These sorts of people are called the, "Pure-minded of Jerusalem."

King David himself was praiseworthy in this regard, and rejoiced in the possession of this trait, saying, "I will wash my hands in cleanliness and I will go around Your alter, O God." It no doubt takes a lot of effort to secure the perfection of this quality, for, while it is easy to guard oneself from recognizable and well known sins whose evil is apparent, the precision needed in order to arrive at Nekiyut is extremely difficult to attain. This is due to the fact that there are many prohibitions which man transgresses without even sensing that he has done so. Our Sages allude to this when they say, "Most are guilty of theft, a minority are guilty of sexual immorality... yet all are guilty of gossip." Because of the inconspicuous nature of the sin all stumble over it, for they are simply not aware of it.

True, the road to Nekiyut is a long and difficult one, yet not impossible. One must aspire to attain complete perfection of this quality. King David, who worked on himself, carefully cleansing himself of all sin, would set out to battle with complete confidence. He would request, "Let me chase my enemies and overtake them; and let me not return until I have destroyed them." The Kings Yehoshafat, Asa, and Hezekiah made no such request for they were not as taintless as King David. David himself declares, "Who will go up to the mount of God, and who will stand in His holy place? The clean of hand and the pure of heart."

Yes, the quality of Nekiyut is a great and difficult quality to attain, yet possible, and one who has reached this level has reached a high level indeed, for he has entered a great and mighty battle and come out victorious.