The best evidence for this belief comes from Lady Macbeth's inner monologue in Act I, Scene V, after receiving Macbeth's letter where he tells of his encounter with the witches and of their prophecy. Lady Macbeth is just as ambitious as his husband (probably more), and while mulling over their future prospects laments what she perceives to be a weakness on her husband's part: "yet do I fear thy nature; / It is too full o’ the milk of human kindness / To catch the nearest way". This human kindness, she believes, would prevent him from taking the necessary actions in order to be king.
