Based on the novel, is the ocean of Solaris is a sentient being or not? Is it possible to know whether the ocean is self-aware?

Is the Rheya that Kelvin meets on the research station a real person in any sense? To what extent is she the Rheya that Kelvin knew on Earth? To what extent is she Kelvin?

Kelvin claims to love the imitation Rheya after she tries to kill herself. Does Kelvin really love the version of Rheya he meets on Solaris Station?

Kelvin attempts to determine his own sanity by comparing his own mathematical calculations to external data. Is this a good test of external reality? Is there any way to test whether what you see and hear is objective reality or whether it is a projection of your own imagination?

Could there be a form of life so alien to...