Metaphor is often used in Shakespeare’s work.
This is a metaphor spoken by Lady Macbeth: “My keen knife see not the wound it makes, nor heaven peep through the blanket of the dark to cry, “Hold, hold!”
“My keen knife see not the wound it makes…” means that she is about to do something horrible and she is so ashamed that she doesn’t even want her knife to see. As knives are currently unable to see, the knife has been used as a metaphor. In this metaphor, the knife has been used as an extension of herself.