Macbeth has told his servant to go to bed and immediatedly sees an apparation. (When first performed, this night scene would have been mesmerizing, all performances were held in the blazing afternoon sun.) This takes place in Macbeth's imagination, there could not be a physical representation of the dagger. He must see it vividly and try to rationalize its appearance. He grabs for it but it is not material. What then is it? a result of an over taxed brain? The dagger is exactly like the one he is to use to kill Duncan. There are even drops of blood on the blade. And yet it is not material.
Unless one believes witchcraft is real, one must believe Macbeth has willed it into being by use of his imagination in order to bolster himself to kill Duncan which his timidity or reluctance will prevent. He must carry out the plan that he and Lady Macbeth have agreed upon. "It is the bloody business which informs thus to mine eyes."
He needs this to prod himself into action. Throughout this scene we feel his palpable reluctance to carry out the murder. "Thou sure and firm-set earth, hear not my steps...and take the present horror from the time...Words to the heat of deeds too cold breath gives" The bell sounds and he is propelled into action.
Macbeth needs this device to give him the impetus to cross this moral divide that will now impel him to further murders, which become easier with each succeeding one, Banquo, Lady Macduff.
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