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.
The juxtaposition of predestination with the exercise of free will is as old as human thought itself. In the pagan world, before the rise of Western civilization and Christianity, the idea of predestination was accepted as truth. Pagan gods were supreme and decided the fates of humans, who had no effect on their own destiny. The belief in predestination was still commonly held throughout much of the medieval world. It was believed that an all-embracing plan was based in an aspect of God called Providence, and that the carrying out of Providence's decrees was delegated to a force called Destiny.
Sometime around 500 A.D., the Roman writer Boethius published a tract called The Consolation of Philosophy, a document that was instrumental in bringing about changes in philosophy in the Middle Ages. Boethius raised important questions: If things are predestined, humans do not have to worry about their own actions because they can blame their behavior on predestination. But if humans have a choice in whatever they do, then how can God truly have foreknowledge? Ultimately, Boethius acknowledged that God's foreknowledge and humans' free will are mutually exclusive: They have nothing to do with one another.
More than seven hundred years later, Thomas Aquinas corroborated Boethius' theory, but Aquinas' approach was somewhat different. Aquinas' explanation depended on the understanding that God exists and functions outside of time.
God's being is measured not temporally, in terms of humans' notion of time, but by eternity, which overlaps the whole of time. The things that happen to humans at different times are, to God, "present time." Consequently, an event that is likely to happen is not future, but present. In short, God does not have foreknowledge as humans define it, but rather a knowledge of a never-changing present.
Vonnegut takes a clearly secular position concerning the dichotomy of predestination and free will. Although he includes many biblical allusions and offers a number of references to Christianity in Slaughterhouse-Five, he rejects Christianity as a truth unto itself, but he does ascribe to the principles of Christianity's philosophy. While most people choose sides in a conflict, Vonnegut's concept of our world affords him no earthly position of judgment. For example, he refuses to say if there is a right or a wrong side in waging modern warfare. Neither the Americans, nor the Japanese, nor even the Germans are more to blame for war's destruction.
Vonnegut assigns no fault, nor does he ask for punishment. Likewise, he never rewards his characters for their heroism, namely, because calling someone a hero means judging that person's actions as good, something Vonnegut will not do. The character who comes closest to being a hero is Edgar Derby, who stands up to the American Nazi Howard W. Campbell, Jr. But remember what absurdly happens to Derby shortly after the war ends: He is executed for stealing a teapot. Vonnegut never judges Derby, neither as a hero who deserves to be celebrated, nor as a thief who deserves to die. There are no heroes, there are no villains. Even Vonnegut's commentary on the assassinations of Robert Kennedy and Martin Luther King, Jr., two of America's most respected and beloved leaders, is the same as his comment on all death: So it goes. Vonnegut offers nothing further.
Vonnegut further emphasizes this notion of So it goes with the introduction of the Tralfamadorians' fourth-dimensional perception, which is similar to Aquinas' reconciliation of the dichotomy of predestination and free will. If one substitutes "Tralfamadorians" for "God" in Aquinas' thinking on the matter, the message rings the same. Things that occur at different times to humans are all in the present to Tralfamadorians, just as Aquinas argued that God perceives everything simultaneously, at once, and not in the future. In short, Tralfamadorians do not have foreknowledge as defined in human terms, but rather a knowledge of a never-changing present.
Billy Pilgrim, kidnapped by Tralfamadorians, is the only human — albeit Montana Wildhack, who is a special case — privy to Tralfamadorian philosophy. Aware that events in his life are predestined, Billy's time traveling remains different from that of his captors'. While Tralfamadorians see all events at once, Billy must be satisfied with his ability to travel from event to event without being able to experience two or more of these events at the same time after all, the Tralfamadorians are amazed that Billy perceives time and events only in a three-dimensional view. Montana Wildhack's case is special if only because we never see her except in the Tralfamadore zoo. She is obviously aware of Billy's being unstuck in time, yet the narrator never mentions if she, too, is unstuck. Nevertheless, Billy and Montana are unique because both have traveled in time for Montana, if only to the Tralfamadore zoo — and both are aware of their lives being predestined despite having only a three-dimensional vision of time.