At the end of act 1, scene 7 of William Shakespeare's Macbeth, Macbeth resolves to murder King Duncan and take the throne of Scotland for himself in order to fulfill the prophecy made to him by the three witches in act 1, scene 3.
Macbeth's last lines in act 1, scene 7—"Away, and mock the time with fairest show: / False face must hide what the false heart doth know"—seem to be addressed to Lady Macbeth but are, in fact, addressed to himself.
Macbeth has no need to remind Lady Macbeth to be duplicitous, since she said essentially the same thing to Macbeth earlier in the play, only in different words:
LADY MACBETH. To beguile the time,
Look like the time; bear welcome in your eye,
Your hand, your tongue; look like the innocent flower,
But be the serpent under't.
(act 1, scene 5, lines 68–71)
"Look like the innocent flower, / But be the serpent under't" and "False face must hide what the false heart doth know" both mean for Macbeth and Lady Macbeth to appear to the world as if they have no evil intentions towards Duncan but to remain steadfast in their decision to kill him.
Macbeth has occasion to put his own words into practice almost immediately. In the next scene, Macbeth is on his way to Duncan's chambers to kill him when he encounters Banquo and Banquo's son, Fleance, on midnight watch. This must have been unnerving for Macbeth, who's still not entirely convinced that he's doing the right thing by killing Duncan, which is evidenced by his "Is this a dagger which I see before me" soliloquy later in the scene (lines 41–69).
Banquo also reminds Macbeth about the prophecies of the witches—one prophecy saying that Macbeth "shalt be King hereafter!"—which Macbeth intends to fulfill within the next few minutes.
BANQUO. I dreamt last night of the three weird sisters:
To you they have show'd some truth.
(act 2, scene 1, lines 24–25)
Macbeth responds to Banquo with a lie, saying, "I think not of them," which is the first time since he decided to kill Duncan that Macbeth acts like an innocent flower and hides his false heart with a false face.
This statement by Macbeth shows that, as act 1 ends, he has fully embraced the plot to kill Duncan.
As a result, he is being "false" to his king in his heart. In other words, he knows he is going to betray him by killing him. He knows, too, that Duncan trusts him as one of his closest relatives and allies, but that trust is misplaced: Macbeth has turned on him. For this very reason, Macbeth must put on a false face or be phony towards his king. He must pretend to be the loyal subject who always would put Duncan's welfare ahead of his own. He can't afford in any way to tip Duncan off as to his intentions.
Regicide, or killing one's king, was one of the very worst crimes in Renaissance England. To murder anyone was wrong, but a...
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king was anointed by God to be the leader, making him a sacred figure. Further, in this period, to offer hospitality was a serious, even sacred trust. By hosting the king, the Macbeths are promising to protect him and keep him safe. To kill a king and to kill him in one's own home are deep violations of the moral code that the Macbeths have been raised with.
"False face must hide what the false heart doth know" is the very last line of the very first act in Macbeth.
It is uttered by Macbeth, who has finally been convinced by Lady Macbeth to engage in a plot to murder to Scottish King, Duncan, in order for Macbeth to eventually become the ruler of Scotland. In the scene, Duncan and everyone else are enjoying a welcome feast at Macbeth's castle, where they are visiting. Macbeth and Lady Macbeth steal away for a moment to talk. When Macbeth had first heard the three witches / weird sisters' prophecy that he would become Thane of Cawdor and then King of Scotland, he immediately felt greedy for those positions. His ambitious feeling began to well up over his good sense. However, his moral code fought against it and he tried to convince himself that evil deeds would not be worth it. He attempted to battle his evil ambitions. When he goes home to his wife, though, her resolve is absolutely steelier than his. She is 100 percent sure that murdering Duncan is the quickest and best path to making her husband King and herself the queen. She manipulates, insults, and coerces him into eventually agreeing with her. She uses their sexual passion and his fear of being "unmanly" and cowardly to make him agree to kill Duncan.
When King Duncan arrives at Macbeth's castle for a visit, he is jolly and optimistic as usual. He doesn't know that his seemingly gracious hosts are plotting his death. In the last scene (scene 7) of the first act, Macbeth shows that he is finally convinced to commit an evil deed by saying this line. He agrees with Lady Macbeth that they must outwardly hide (false face must hide) their secret evil plans (which are from their false heart). Although they know what they plan to do, the success of the plan relies on them acting like they are in grief and anger when Duncan is discovered to be murdered.
This quote falls nicely into the "appearance vs reality" theme where Macbeth knows what he will do to further his ambition and become King, but he can not show this to the world or he will be called on it.
"False face must hide"--put on an act and pretend to be something you are not (a mask of sorts)
"what a false heart doth know"--his heart knows the truth of who he is and what he is planning
Macbeth and Lady Macbeth have just gone over the plan to kill Duncan. Now, they have to go about business as usual without letting anyone know about their plan. The quote means that Macbeth will put on a 'false face' or pretend to look happy and normal to cover for his 'false heart' or heart that is betraying his king.