What's done is done
Lady Macbeth:
How now, my lord, why do you keep alone,
Of sorriest fancies your companions making,
Using those thoughts which should indeed have died
With them they think on? Things without all remedy
Should be without regard: what's done, is done.
Lady Macbeth's soothing words are odd, to say the least, coming from a conspirator. She intends her blandishments to calm her husband, who's having more trouble than she forgetting that he murdered King Duncan. She means by "what's done, is done" exactly what we mean by it today—"there's no changing the past, so forget about it." Neither then nor now is the psychology of this advice very sophisticated, but the Lady isn't trying to be profound. She's merely trying to treat Macbeth's guilty hallucinations with the blandest possible palliative. When Lady Macbeth herself succumbs to guilty dreams, she will sing the same tune, but in a different key. Sleepwalking, as has become her wont, she mutters, as if to Macbeth, "What's done cannot be undone" (Act 5, scene 1, 68).