General overall or individual reference

Hi. Please help. I think both are correct? Anyway, are both correct? Let's say someone is trying to say something (make a statement) about the heart of men (quite philosophical and a difficult thing to do, I think). I also think some words that could be used after the verb "is" or "are" are "good," "unsteady."

The heart (or hearts?) of men is (or are?) ...