Macbeth does not know he is to be made Thane of Cawdor so how do the witches know?
The witches can foresee the future because they are in contact with wicked supernatural powers. Apparently they cannot change the future. Their powers are very limited. Macbeth tries to change the future--the part he doesn't like. He doesn't want Banquo to be the sire of future kings of Scotland, so he tries to have him murdered. Macbeth has many bad traits, but the one thing that is admirable about him is that he has so much courage that he is willing to challenge Fate itself, regardless of the consequence. When he confronts the witches in Act 4, Scene 1 he says:
Though you untie the winds and let them fight
Against the churches, though the yeasty waves
Confound and swallow navigation up,(55)
Though bladed corn be lodged and trees blown down,
Though castles topple on their warders’ heads,
Though palaces and pyramids do slope
Their heads to their foundations, though the treasure
Of nature's germens tumble all together(60)
Even till destruction sicken, answer me
To what I ask you.
He realizes the consequences of challenging what is predestined to happen. Macbeth believes in these witches, and yet he won't accept their prophecy. In a letter to his wife he tells her:
“They met me in the day of success, and I have
learned by the perfectest report, they have more in them
than mortal knowledge. When I burned in desire to question
them further, they made themselves air, into which
they vanished. Whiles I stood rapt in the wonder of it, came(5)
missives from the King, who all-hailed me ‘Thane of
Cawdor’; by which title, before, these weird sisters saluted
me, and referred me to the coming on of time with ‘Hail,
King that shalt be!’
The witches have more in them than mortal knowledge. There is nothing so remarkable, therefore, that they should know that King Duncan was going to have the Thane of Cawdor executed and confer that title on Macbeth as a reward for his loyalty and heroism. As a matter of fact, they could have known about this for a long, long time. It was destined to happen because of a long train of cause and effect going all the way back to the beginning of the world. Knowledge is about the only thing these Weird Sisters possess. They are weak women. They cannot give Macbeth the kind of assistance he would really like to have. He would like to find some powerful being who could erase the future and rewrite it in his favor. Macbeth is a formidable warlord, but even he cannot change Fate, as hard as he tries.
The play really seems to be about a struggle between Macbeth and Fate. It raises the question of whether we can change our destinies. Do we have free choice? Or is everything that is ever going to happen to us already predetermined? Suppose we knew we were destined to be killed, say, in a car crash at a certain intersection on a certain date? Couldn't we arrange to stay in bed that day or get clear out of town? Or would some invisible power draw us to that scene at that precise time to meet with Fate?
The very talented and entertaining writer Cornell Woolrich, who also used the name William Irish, deals with that question in one of his best novels: Night Has a Thousand Eyes. Many of Woolrich's stories and novels have been turned into films. Night Has a Thousand Eyes (1948) stars Edward G. Robinson. It is an oldie but a goodie. The best-known film based on a Woolrich tale is Alfred Hitchcock's production of Rear Window (1954).
If you can look into the seeds of time,
And say which grain will grow and which will not,
Speak then to me... (I.iii)
The witches have power of prophecy. When they encounter Macbeth and Banquo, they make three prophecies. One that Macbeth will be named Thane of Cawdor, another that he will later be king, and the last that Banquo will not rule on his own in Scotland, but will be father to future kings.