Meanings of words are unclear
1. The Witches' chant "Fair is foul, and foul is fair" suggests things that appear good are actually evil
2. They use language to trick Macbeth and convince him of a false reality - they tell him that "none of woman born" will harm him, which gives him the false confidence to fight to protect his reign
3. Other characters speak using paradoxes, e.g. Macbeth says, "Nothing is/But what is not". These paradoxes create uncertainty - they show that nobody can tell what is real.