Things you haven't seen or heard with your own eyes and ears may be true — but they may not be.
Literal
Own eyes-ears [with-で] seen-heard not things [topic-は], true maybe [but-が], true not maybe-not.
目耳で見聞きしていない 'have not seen or heard with your own eyes and ears' — an emphatic direct-evidence standard. 本当かもしれないが、本当ではないかもしれない balances two possibilities with elegant symmetry. A philosophical point about secondhand information.