This is kind of a catch-all category for a theme, but there are a lot of big head-scratching nail-biting philosophical questions posed in Ulysses. Joyce is an author whose main concern is with how it feels to exist as a human being, with what it means to be alive. The characters in the book, Stephen Dedalus in particular, don't just struggle with personal problems, they are tormented by philosophical questions about the nature of truth. Yet the remarkable thing about Ulysses (and one of the things that makes it so hard) is that Joyce refuses to separate the abstract questions of philosophy from daily life. To an extent, there are parts of the book where the characters get more lost in thought than others, but in general they are both thinking and living at the same time. At no point do we get lost in universal questions about human nature without also getting drenched in the particular details of what it means to be, say, Leopold Bloom. In one moment, Bloom can simultaneously be speculating on how something came from nothing and also thinking that he has to pee badly.
The fact that the character's innermost thoughts are still expressed in language – however fragmented – suggests that one's relation to the world and to other people in the world is always mediated by the words that one uses.
There is no such thing as an impersonal theory in the novel, but especially in Stephen's case, subjectivity lends credence to his theories rather than undermining them. Complete objectivity is revealed as an impossible dream.