How we cite our quotes: (Chapter.Paragraph) Note that chapters aren't numbered, so need to be numbered manually, 1 to 14.
Quote #4
"A normal man," he said. "What is a normal man? A man who has never committed a disgraceful act? Maybe, but has he never had uncontrollable thoughts?" (6.83)
Human beings aren't entirely in control of their heads; thoughts scurry out. Which means, on Solaris, that "a normal man" isn't just a normal man; he's also that odd thought, over there, hiding under the desk or just out of sight beyond the viewfinder. If your thoughts aren't yours, then who are you? A bug-eyed alien, as it turns out.
Quote #5
"How do you know I'm really the same old Ratface who landed here two years ago?" (6.110)
Snow is saying he could be a visitor himself. It's interesting that he refers to himself here by his nickname, Ratface. Maybe Ratface and Snow aren't the same person; maybe Ratface came on the station and eventually was replaced by, or turned into Snow. Think about a comic book for a second: Is the Superman in the first panel the same as the Superman in the second panel? You are you now, but is the you tomorrow still you? Even without an alien ocean to intervene, Superman replaces himself. Maybe you do, too.
Quote #6
The sight of the two identical dresses filled me with a horror which exceeded anything I had felt hitherto. (7.56)
One identical dead ex-wife in your room is disturbing, but two is downright horrifying. The key here is probably that the two dresses show that Rheya is replaceable or replicable; she's not one person, but a hollow form. As mentioned before, it's the uncanny valley thing again: people who are almost but not quite people are the scariest people in the world.