Section 112(3) BSA: Paradox of Parol Evidence

Yaar, I'm really lost. Is it just me or does Section 112(3) BSA sound like it's saying two completely opposite things? The section states that no contract can be explained by oral evidence unless it's an agreement in writing, but it also says that a contract in writing may be explained or supplemented by oral evidence. Kya main hi confused hoon ya koi aur bhi is tarah se pata lagata hai? (Am I the only one or is anyone else feeling the same way?) It's like the BSA is saying 'no oral evidence, no way' and then suddenly 'oh, but wait, maybe there's an exception'. Help, please! Someone explain me that Section 112(3) magic in simple terms.