>>1 Okay, let me try to think of a satisfying answer.
Let's get an example, money. Money can come in many different forms, like bank notes and credit cards, but to some people, this type of money is completely worthless because they have no use for it (try giving credit cards to cavemen).
Both parties have to mutually understand the value of whats being given for it to have any "true" value.So if you replace the above example with the hearts of different humans, you'll get what I mean.