The Dagger of the Mind

The "dagger of the mind" speech is given by Macbeth right before he kills Duncan. He describes seeing a floating dagger before him, with the blade pointed toward Duncan and the handle toward him. Macbeth tries to reach out and grab the dagger, but he cannot touch it, stating that "...[his] eyes are made fools o' the other senses." He also observes blood on this imaginary dagger, a motif that will recur throughout the rest of the play. As he draws his real daggers, Macbeth fearfully and reluctantly follows through with the murder, beginning his slow descent into madness.
I watched a version of this section of Macbeth on YouTube where Ian McKellen gives the speech, and it certainly gave me more of an idea of what was actually going on than just reading the script. Macbeth is in the dark, grasping for a nonexistent dagger before pulling out a real one, all the while ranting about whether or not he should believe what he is seeing. The gravity of the scene is difficult to understand from just words on a page, but when watching a live rendition, one can clearly see Macbeth's transition into madness. As he is giving his monologue, his voice gets higher and faster until he is in a frenzy. It is in this state that Macbeth kills Duncan.

--Wald der Schartig