Is this a dagger that I see before me, the handle toward my hand?

To remember this quote use the following mnemonic.

Is this a dagger that I see before me? Because I'm not sure.

"Is this a dagger that I see before me, the handle toward my hand?" Act 2, scene 1, line 33. 

By asking this question, it becomes evident that Macbeth is uncertain about the reality of the dagger. However the vision of the dagger reveals that his violent ambition is strongly urging him to murder Duncan. This internal struggle deeply torments him.