I would want to answer this question by refering to the illusory dagger that Macbeth sees. Let us remember that this soliloquy begins by Macbeth commenting on this dagger that seems so real, with its handle pointing towards his hand. However, when he tries to grasp it, he is not able to. As he contemplates the dagger and the way that it seems to be leading him to Duncan's bedchamber, the dagger changes and becomes bloody and Macbeth comments upon this as follows:
There's no such thing:
It is the bloody business which informs
Thus to mine eyes.
Macbeth thus comments that the dagger has turned bloody because of the murderous thoughts that plague him at this particular time.
What is interesting and ironic about this however is the way that Macbeth seems to recognise that the apparition of the dagger is not real. Macbeth does not show similar ability to distinguish between appearance and reality in other sections of the play, and most crucially in his thoughts about the witches' prophecies. Also, even though he identifies that the dagger is not real, he still lets it impact his actions, just as the prophecy impacts his actions. Discerning between appearance and reality may be one thing, but Macbeth shows that he still is greatly impacted even by what is illusory.