“Gainst nature still!”
The witches are nature itself (in form of a tree.)
Macbeth disturbs the very balance of the natural world with his attempts to control his fate.
He’s never alone, however, as the earth itself may be watching his every move.
“Thou sure and firm-set earth, Hear not my steps, which way they walk, for fear Thy very stones prate of my whereabout”
