Enter the Gate
In Zen, Sudden Enlightenment (Ta-Wu, Satori) is the single experience that resolves all of your doubts.
Thus, Hoshang saw that "the sun is round." It was not something explained to him. He just suddenly realized it, just as Dogen on his enlightenment realized that "my eyes are horizontal and my nose is vertical."
This sudden, explosive entry into the truth of life is not a matter of intellect.
If you have not had this experience, you should earnestly investigate yourself until you do have it.
Seek the help of an enlightened teacher. [If you can find one!]
Do not just run off your mouth about books you have read or ideas you've conceived as to the meaning of these books. For if you proceed in such a way, you will never "enter the gate" and will die without ever having clarified the Great Matter of Life and Death.