I have thee not and yet I see thee still.
Art thou not, fatal vision, sensible
To feeling as to sight? or art thou but
A dagger of the mind, a false creation
Proceeding from the heat-oppressed brain?
I see thee yet in form as palpable
As that which now I draw....
* * * * *
Mine eyes are made the fools o' the other senses.
Or else worth all the rest: I see thee still;
And on thy blade and dudgeon gouts of blood
Which was not so before.--There's no such thing."

What is all this but an illustration of Hamlet's assertion:

"There is nothing either good or bad
But thinking makes it so."

Just too as Hamlet swings on his mental balance, so that it is still a
debated question among academic critics whether his madness was feigned
or real, so here Shakespeare shows us how Macbeth loses his foothold on
reality and falls into the void.

The lyrical effusion that follows is not very successful, and probably
on that account Macbeth breaks off abruptly:

"Whiles I threat he lives,
Words to the heat of deeds too cold breath gives,"

which is, of course, precisely Hamlet's complaint: