Also sprach Zarathustra (Thus Spake Zarathustra)

The biggest philosophical influence. Themes include:

• Eternal recurrence ("everything repeats.")
• Fate versus free will.
• The possibility of escaping cyclical existence.

The series repeatedly quotes Nietzsche and builds much of its structure around these ideas.

The Strauss tone poem that opens the film is named after the book, and Kubrick chose it knowing what it was quoting. The borrowing goes past the music: Nietzsche's ape, man and overman is exactly the film's three-part shape, from the bone thrown at the Dawn of Man to the Star Child looking back at Earth. Kubrick and Clarke both discussed the film as being about the step after the human one. https://en.wikipedia.org/wiki/Also_sprach_Zarathustra_(Strauss)