Solaris — Stanislaw Lem (1961)

Polish author Stanislaw Lem's Solaris is the most philosophically ambitious SF novel ever written: a story about a planet-sized ocean that may be conscious but is utterly incomprehensible to human science. Lem uses first-contact fiction to argue that humanity is fundamentally incapable of understanding genuinely alien intelligence — a challenge to the optimism underlying most Anglo-American SF and to the scientific method itself.