The titular character in Shakespeare?s “Macbeth” sees in his mind?s eye a dagger.

Macbeth grasps the specter of the blade, and soon thereafter, a real dagger to murder his lord, King Duncan. But the “dagger of the mind,” as Shakespeare wrote, is also Macbeth?s conscience. And as the tragedy unfolds, it strikes at his sanity, as it strikes his queen, Lady Macbeth, his hand-washing-obsessed accomplice in regicide.