Solaris by Stanislaw Lem

When Kris Kelvin arrives at the planet Solaris to study the ocean that covers its surface he is forced to confront a painful, hitherto unconscious memory embodied in the physical likeness of a long-dead lover. Others suffer from the same affliction and speculation rises among scientists that the Solaris ocean may be a massive brain that creates incarnate memories, but its purpose in doing so remains a mystery . . .

Solaris raises a question that has been at the heart of human experience and literature for centuries: can we truly understand the universe around us without first understanding what lies within?