One major philosophy presented in Vonnegut's novel is Christianity. The novel discusses topics within Christianity, especially in regards to fate and free will. Billy Pilgrim experiences and applies these principles.
The role of religion in the life of Billy Pilgrim is a key point in Slaughterhouse-Five. Toward the beginning of the novel the narrator states that Pilgrim started out in the Second World War as a chaplain's assistant "and had a meek faith in a loving Jesus which most soldiers found putrid." This understanding of the Christian Jesus is challenged after the war as Pilgrim comes in contact with the work of Kilgore Trout's novel The Gospel from Outer Space. Trout's fictional novel within Slaughterhouse-Five explores the journey of a visitor from outer space who studies Christianity to determine "why Christians found it so easy to be cruel." This "cruel Christianity" presents a direct contrast to Billy's loving Jesus. Taking some of Trout's novel to heart, the narrator and Billy Pilgrim look to create a new sense of Christianity and a more human-like Jesus.
The idea of the human-Jesus is a central piece in analyzing Pilgrim's eventual struggle with fate and free will. Establishing a Christian figure that is not initially divine in nature sets a completely different tone to the overall understanding of humanity's placement with God. In David Vanderwerken's piece "Kurt Vonnegut's Slaughterhouse-Five at Forty: Billy Pilgrim - Even More a Man of Our Time," Vanderwerken states that the narrator may be calling for a "humanly centered Christianity in which Jesus is a 'nobody' (94), a 'bum' (95), a man."
What Vonnegut suggests here is that Christ's divinity stands in the way of charity. If the "bum" is Everyman, then we are all adopted children of God; we are all Christs and should treat each other accordingly.[…] If Jesus is human, then He is imperfect and must necessarily be involved in direct or indirect evil. This Jesus participates fully in the human condition.
There is some question of Christ's divinity and how that plays a part in Christian principles and it is suggested that the voice in the novel desires a form of collectivism where humanity looks at one another as equal parts and equal heirs of God. This human-Jesus argument within the novel stands as an effort to make humanity, whom Trout may consider to be "bums" and "nobodies", have more importance. The narration's call for a more human-Jesus and Christianity is seen in the last part of the discussion on Trout's novel where God speaks from heaven stating, "From this moment on, He [God] will punish horribly anybody who torments a bum who has no connections!" Trout's novel attempts to make everybody somebody, as well as to emphasize the supposed cruelty of Medieval Christian thinking, and how it ought to be changed.
The desire for a "human" Jesus is not the only Biblical topic discussed in the novel. Another reference involves the story of Lot's wife disobeying and turning to look back at the destruction of Sodom and Gomorrah. The narrator of that chapter, possibly seen as Vonnegut himself, claims that he loves Lot's wife doing so "because it was so human." Amanda Wicks speaks on this incorporation of Lot's wife and the narrating voice stating, "Vonnegut naturally aligns himself with Lot's wife since both occupy the role of spectators shattered by the act of witnessing." The narrator's alignment with Lot's wife also creates a good pretext for the understanding of Billy Pilgrim's psyche throughout the destruction of Dresden.
Along with asking moral questions, Slaughterhouse-Five is also a novel that focuses on the philosophies of fate and free will. In the novel, Billy Pilgrim tries to determine what his role in life is and what the purpose of everything going on around him is as well. When abducted by the Tralfamadorians, Pilgrim asks them why he is chosen from among all the others. He questions the fate of the situation and what led up to that point. Billy Pilgrim considers his fate and actions to be a part of a larger network of actions, his future manipulated by one thing over another based on decision. All things that happen would happen for a reason. Indeed, Pilgrim's beginning mindset would suggest that he believed in free will, fate, whys, decisions and things happening for reasons. However, many of these thoughts are quickly challenged by the Tralfamadorians' ideology.
As Billy Pilgrim becomes "unstuck in time", he is faced with a new type of philosophy. When Pilgrim becomes acquainted with the Tralfamadorians, he learns a different viewpoint concerning fate and free will. While Christianity may state that fate and free will are matters of God's divine choice and human interaction, Tralfamadorianism would disagree. According to Tralfamadorian philosophy, things are and always will be, and there is nothing that can change them. When Billy asks why they had chosen him, the Tralfamadorians reply, "Why you? Why us for that matter? Why anything? Because this moment simply is." The mindset of the Tralfamadorian is not one in which fate and free will exist. Things happen because they were always destined to be happening. The narrator of the story explains that the Tralfamadorians see time all at once. This concept of time is best explained by the Tralfamadorians themselves, as they speak to Billy Pilgrim on the matter stating, "I am a Tralfamadorian, seeing all time as you might see a stretch of the Rocky Mountains. All time is all time. It does not change. It does not lend itself to warnings or explanations. It simply is." After this particular conversation on seeing time, Billy makes the statement that this philosophy does not seem to evoke any sense of free will. To this, the Tralfamadorian reply that free will is a concept that, out of the "visited thirty-one inhabited planets in the universe" and "studied reports on one hundred more", exists solely on Earth.
Using the Tralfamadorian passivity of fate, Billy Pilgrim learns to overlook death and the shock involved with death. Pilgrim claims the Tralfamadorian philosophy on death to be his most important lesson:
The most important thing I learned on Tralfamadore was that when a person dies he only appears to die. He is still very much alive in the past, so it is very silly for people to cry at his funeral. All moments, past, present, and future, always have existed, always will exist. ... When a Tralfamadorian sees a corpse, all he thinks is that the dead person is in bad condition in that particular moment, but that the same person is just fine in plenty of other moments. Now, when I myself hear that somebody is dead, I simply shrug and say what the Tralfamadorians say about dead people, which is "So it goes."
Billy Pilgrim continues throughout the novel to use the term "so it goes" as it relates to death. The ideas behind death, fate, time, and free will are drastically different when compared to those of Christianity.
Tralfamadorian philosophy did not appear ex nihilo, but draws on many strands of thought. The idea of all time existing at once (as the Tralfamadorians experience it) can be found in sources ranging from Pre-Socratic Greek philosophy (e.g. Parmenides' monism) to Neo-Classical Christian theology (e.g. Sir Thomas Browne's Religio Medici) to twentieth century popular science (e.g. repeated statements by Albert Einstein). Likewise, the idea of determinism was prevalent in twentieth century philosophy and Tralfamadorian passivity can be traced back to the Stoics.