The corruption of nature is a theme that surfaces and resurfaces in the same act. When Duncan greets Macbeth, for example, he states that he has “begun to plant thee and will labor / to make thee full of growing" (I iv 28-29). Following the metaphor of the future as lying in the “seeds of time,” Macbeth is compared to a plant that Duncan will look after (I iii 56). By murdering Duncan, then, Macbeth perverts nature by severing himself effectively from the very "root" that feeds him. For this reason, perhaps, the thought of murdering Duncan causes Macbeth's heart to "knock at [his] ribs / Against the use of nature" (I iii 135-36). Just as the Weird Sisters pervert the normal course of nature by telling their prophecy, Macbeth upsets the course of nature by his regicide.