The Answer Lies Inside

Riddle: Looking for something? Open me. I'm sure that something inside of me lies. Of course, you can always find hope in me (Though despair must come first; and later, surprise), What's sought, though, depends on the seeker - One looks for bobbin; another, for beaker; Others, for nature; still others, for nurture - The quarry will vary from searcher to searcher. And yet (I suspect this will strike you as strange), My contents are set and will not ever change. If you cannot still guess what I mean, here's a clue: The answer - what I mean - lies inside of me, too. What am I?