The most significant theme in Slaughterhouse-Five concerns the dichotomy of predestination and free will. Over and over again, Vonnegut proclaims that there is no such thing as free will. Humankind is the slave of predestination, meaning that all human actions are prescribed before they occur. A person who chooses to do something is not really choosing at all the choice is already made. This complicated issue can be confusing, but grasping the history of the arguments and Vonnegut's take on them will help us better understand and enjoy the novel.