In Macbeth Act I Scene 5, Lady Macbeth says the following:
Glamis thou art, and Cawdor; and shalt be
What thou art promised: yet do I fear thy nature;
It is too full o' the milk of human kindness
To catch the nearest way: thou wouldst be great;
In line 2, what does the pronoun 'what' refer to? Why doesn't she just use an appropriate noun?
The other answers have explained the meaning of the line—that Macbeth shall be king, as he was promised by the witches—but there is more to say about the choice of wording.
The difficulty here arises because Lady Macbeth is expressing herself evasively. Why doesn’t she come straight out and say, “thou shalt be king as thou art promised”? The reason is that she is already contemplating the murder of Duncan to put her husband on the throne of Scotland, an act which she knows is against law and society and religion, and this discomfort reveals itself in her choice of words.
There’s more evasive language later in the same speech:
Yet do I fear thy nature.
It is too full o’ the milk of human kindness
To catch the nearest way.
By “the nearest way” she means “the murder of Duncan”, which is the nearest (most direct) way for Macbeth to become king.
Thou’ldst have, great Glamis,
That which cries, “Thus thou must do, if thou have it;”
And that which rather thou dost fear to do
Than wishest should be undone.
These four lines are particularly difficult as all substantive nouns have been removed, and the syntax is unclear as well. “That which cries” is the crown of Scotland, and by “Thus” and “that which thou dost fear to do” she means “murder”. The phrase “if thou have it” could mean “if thou have courage”, or maybe “if thou wouldst have the crown”? There seems to be a missing or implied “thou must do” before “that which rather”.
The evasive language, contorted syntax, and missing words, indicate Lady Macbeth’s discomfort with the idea of murder. She knows that it is wrong, and so she cannot speak of it plainly or coherently.
Coleridge pointed out that although Lady Macbeth is speaking about her husband in this passage, she is at the same time talking about her own character and desires. She too would rather become queen “holily”.
Macbeth is described by Lady Macbeth so as, at the same time to reveal her own character. Could he have every thing he wanted, he would rather have it innocently;—ignorant, as alas! how many of us are, that he who wishes a temporal end for itself, does in truth will the means
Samuel Taylor Coleridge (1849). Notes and Lectures Upon Shakespeare and Some of the Old Poets and Dramatists, volume I, p. 246. London: William Pickering.
What Lady Macbeth means with "what thou art promised" is the kingdom that Macbeth was supposedly "promised" in the witches' prophecy.
All hail, Macbeth! hail to thee, thane of Glamis!
All hail, Macbeth, hail to thee, thane of Cawdor!
All hail, Macbeth, thou shalt be king hereafter!
So he is Thane of Glamis, became Thane of Cawdor and the remaining thing for him to become now is the very king of Scotland.
Now why doesn't she just say "Glamis thou art, and Cawdor; and shalt be king" right away? Beyond being ultimately a question of taste and poetic wording, there might be a bit more to it than that. It emphasises that Lady Macbeth doesn't see the prophecy as merely a foretelling of the future, but even more so a "promise". Macbeth is promised the position of king by fate and thus it is his right to take it for himself because it is his anyway. She (and Macbeth, be it by persuasion or his own will) uses the supposed inevitability of the prophecy as absolution for actively working towards it by any means necessary.
And it is this classic misinterpretation, impatience or overambition (depending how you want to see it) that ultimately makes the prophecy self-fulfilling and is at the very core of the tragedy's unfortunate events.
The words "Glamis thou art, and Cawdor; and shalt be / What thou art promised" can be explained as follows:
The verb "be" in "and shalt be" here takes a subject complement, and this subject complement is on the next line: "What thou art promised" (i.e. what you have been promised).
Update:
Note that "shall" (as in "shalt be") can mean "will" ("you will be king") but also "will inevitably or assuredly, be bound to, must" (see A Shakespeare Glossary by C. T. Onions, revised by R. D. Eagleson, 1986). When the witches say "thou shalt be king hereafter" and Lady Macbeth mimics these words, there is more going on than a simple prediction: the words also have overtones of unavoidable destiny.
From Lady Macbeth's point of view, Macbeth's kingship is something that requires effort, and even an effort to overcome whatever humaneness exists in Macbeth's character (see the related question What kind of language features appear in Lady Macbeth's line “too full o' the milk of human kindness”?). Whether the witches use "shall" in the sense of "will" or "will inevitably or assuredly" is ambiguous and is interpreted differently by different characters. Macbeth's initial reaction (Act I, scene 3) in incredulity: "and to be king / Stands not within the prospect of belief", whereas Lady Macbeth begins to think of means to turn that prediction into reality.
This is a much more interesting question than the o/p perhaps understood.
There is legitimate scholarly opinion to the effect that Shakespeare did not include any Witches in his play. The modern notion of 'witches' is a misunderstanding; or perhaps may be classed rather as an exaggeration.
The text which modern academics ascribe to Shakespeare speaks of 'weird sisters', not of witches. There is much ambiguity in this play, as in many of his plays, due to corruption of the text during the process of printing it, under the exceedingly primitive conditions so long ago. The printed text contains much textual corruption.
But, above all, it is now thought that some scenes which appear in modern editions of the play were added by another author, perhaps in the years between Shakespeare's death and the First Folio printing. The references to witches are thought to have been added by another hand.
Calling them witches certainly attributes supernatural powers to them, such as could account for their having an ability, with some credibility, to foretell the future. But although Shakespeare is still thought to have written the scene in which Lady Macbeth appears to urge her husband to murder King Duncan, some part of the ambiguity in her words can now be accounted for by the fact that she may be referring to a rather different version of Act 1 Scene 3 than is currently included in modern printings of this text. Shakespeare may have written her dialogue without any witches in mind, and they may have been added by another author so as to strengthen Lady Macbeth's position in this scene, among others, where she needs to call on a supernatural prophecy that Macbeth 'wilt be King hereafter'.
In the 16th century, playhouse audiences loved magic -- which most common people still firmly believed in. So adding witches to this play certainly strengthened its appeal for audiences. By adding supernatural elements, Shakespeare had improved other plays -- such as where he uses the ghost of Hamlet's father to include a ghost in that play.
Yet he seems to have only had three 'weird sisters' in mind in Macbeth. Someone unknown made the play a better box office attraction, by inserting just one extra scene with a reference to witches -- throughout the rest of the play the term was not originally used -- to give a better justification for Lady Macbeth's claims that fate has prophesied that Macbeth will become king.
Her words, perhaps made ambiguous by simple textual corruption, but maybe because they originally referred to a quite different scene to the one now included in the play, have very little meaning. But the reason seems not to lie with Shakespeare but with the unknown hand who "collaborated" with him on the script - perhaps long after his death.
Indeed the Folio text of 1623 is thought to be a revision of the original play, probably adapted by Thomas Middleton (and unquestionably using Middleton’s material), and is very short by Shakespeare’s standards, suggesting abridgement.
