In Macbeth, Act 5, Scene 1, Lady Macbeth is sleepwalking and obsessively trying to wash imaginary bloodstains from her hands. While she once confidently told her husband, “A little water clears us of this deed,” her guilt later makes her realize that no physical cleaning agent can remove the metaphorical spot of her sins.

When the doctor observes Lady MacBeth in Act 5, Scene 1, he concludes that her sickness is mental, not physical, saying “More needs she the divine than the physician”. He indicates that only spiritual help, not a physical wash, can remedy her guilty conscience.

Lady Macbeth cries, “Out, damned spot!” and realizes that the stain is permanent, highlighting that the guilt is in her mind’s eye rather than on her skin. The play emphasizes that the blood is a symbol of irreversible moral guilt, meaning there is no solution to cleanse her hands.

The central tragedy is that no one can tell her how to cleanse her hands, because she is dealing with the psychological consequences of her actions.

William Shakespeare’s MacBeth – Act V Scene 1 – AI Overview

Who can say, I have made my heart clean, I am pure from my sin?  Proverbs 20:9

Come now, and let us reason together, saith the LORD: though your sins be as scarlet, they shall be as white as snow; though they be red like crimson, they shall be as wool. Isaiah 1:18