In act 2 sc.1, lines 33ff, Macbeth sees the vision of an air-drawn dagger, which must be a product of his imagination:
" Is this a dagger which I see before me,
The handle toward my hand? Come, let me clutch
Macbeth tries to grasp the handle of the phantom-dagger, but fails. He wonders why the object is visible and yet not physically tractable:
" I have thee not, and yet I see thee still.
Art thou not, fatal vision, sensible
To feeling as to sight?"
The dagger is indeed a 'fatal vision', a hallucinatory dagger that leads Macbeth to King Duncan's bed-chamber. If the real dagger is fatal for Duncan, this 'dagger of the mind' is going to be no less fatal for Macbeth. Macbeth's self-addressed question is also self-explanatory:
" ..........................or art thou but
A dagger of the mind, a false creation,
Proceeding from the heat-oppressed brain?"
In the vision of the dagger, the supernatural is once again at work. It may be a vision sent by the witches; but, more likely, it is called up by the witchcraft of Macbeth's own over-heated imagination.
The supernatural hallucination further gains in visual sensationalism when Macbeth discovers patches of blood on the blade & the handle of the dagger. This surely relates to the killing of Duncan in the mind of the killer ahead of the actual killing.
Macbeth soon discovers the delusion, and dismisses it with a comtemptuous brushing aside:
" There's no such thing:
It is the bloody business which informs
Thus to mine eyes".
The visionary dagger gone, Macbeth returns to his real business with the help of the real dagger.