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After arranging Uriah's death to take Bathsheba, David hears Nathan's story of a rich man who steals a poor man's only lamb and angrily condemns the thief to death. Nathan replies, 'Thou art the man.' David's moral judgment worked perfectly on a disguised case and failed completely on his own, which shows that self-deceit is a blind spot about oneself and not an absence of moral sense.
People rarely deceive themselves by openly reasoning that a wrong is right. Instead they form a settled habit of turning their attention away from their own conduct in a particular direction, so the fault is never examined and never judged.
Self-love makes us partial judges in our own cause, in the way a bribed judge is partial. We examine others' actions with full rigor. Our own get a lenient reading that stresses mitigating circumstances and softens the terms we use for them.
Many duties, such as fairness in dealings, generosity, and restraint, have no precisely fixed boundaries. People use this indeterminacy as cover and stretch the latitude in their own favor while keeping a clear conscience. The places where rules are ambiguous are where self-deceit hides most easily.
Self-deceit usually covers one specific area of conduct in a person who is otherwise upright or even strict. Real diligence in some duties becomes a kind of compensation that reassures the person about their general character. That reassurance makes the one unexamined fault harder to see.
The remedy is deliberate self-examination done by changing the vantage point. Ask how you would judge the same action if someone else did it, or how an impartial or even unfriendly observer would describe it. This recreates Nathan's trick: it strips out self-partiality so that ordinary moral sense can finally reach your own case.