Muley's primary purpose in Grapes of Wrath is to provide a contrast to the Joad family and to keep the veil up that would otherwise have fallen with the Joads' departure over life in Oklahoma. Muley is stubborn (hence, the name Mule-y) and pessimistic, both of which drive him to negative destructive action as opposed to positive upbuilding action. He is also bound by these traits to the inability to admit a mistake, which, if he could have done so, would have sent him to California in pursuit of his wife and children. In addition, Muley's presence in Oklahoma gives a view into the lives of those who chose to remain, The landowners are driven to such cruelty that Muley, for one, hides and sleeps in secret run down places and set his strength and energy to making the landowners miserable with difficulties. This creates the feeling in Muley that he might be going insane, which metaphorically brings up the the question of whether extreme pessimism can even be a sane, rational choice when faced with adversity.