The Hamlet Project

This one baffles me. If I weren’t attempting to read closely, I’d probably sail on by a mystery like this. Sandwiched as it is between “How say you then?” and “But you’ll be secret?” you can sort of elide the thought to just continue the line of “Can you keep a secret?” but I don’t see how it connects when I pull it out. Is it a sort of call back to what he just experienced? Are the ghost’s words so unfathomable that the general heart of mankind should not even be able to consider it? Or that the secret could be revealed?
Is “How say you then?” to the ghost, then? Then, the next line perhaps actually questions the heart of man, given the ghost’s position in the earth. As we’ll shortly see, the ghost might have more direct access to the heart of man than the living standing upon it. Or Hamlet could be questioning his own decision, checking in his own man’s heart before confirming the secretness of his friends.
The general heart of man contains quite a lot. I find I can really only truly know the heart of one.