It is essential to Descartes' argument that mind and body are separate and that mind and body are both clear ideas and distinct. Thus, Descartes proves that the mind can exist independently of the body. Ultimately, it is the same clarity and distinction that serves to distinguish the mind from the body that unites them into one clear and distinct idea linked to another. Descartes argues that the mind, an immaterial thing, interacts causally with the body, a material thing. . Because of his belief that dualistic interactionism is unique to human beings, it makes sense that Descartes would have difficulty drawing an analogy to illustrate how the mind and body interact. Descartes compares the mind housed inside the body to a pilot in a ship: when he feels pain or hunger, if his mind and body are not inextricably united, he would simply be aware of it.