You don't? I think this is a, as pooh said, a philosophical thing that cannot be answered by mechanics or logic. How do you know what you already know is actually the right answer? How do you know a bird is, in fact, a bird? Your parents probably told you so, and theirs before them, but how do they know? How do you know? The human mind is far from perfect. Math was a human invention. Science was a human invention. Religion was a human invention. We don't know jack shit. So there's no right answer, anywhere.
I don't even understand if I'm answering the question here. But then again... How do you know I am the one with the answer?