Kant on metaphysics

Right in the beginning of the book, Kant presents the question to all people whether a thing like metaphysics be at all possible trying to have people open their minds to what he has to say. I have trouble understanding him, but he says if a science like metaphysics is at all possible...there must be some sort of doubt surrounding the same subject, or at least that is what i gathered from what he says. Kant wants the reader to think about other various possibilities to go back and answer his original question.