I feel as though the teachings of Lao Tzu are meant to be interpreted individually. I love seeing how different people perceive different aspects of the teachings. This statement to me means that the one who knows only does so from their perspective of reality. The one who speaks about what "should be" has no idea because it's impossible for him to speak of reality from each individual point of view. Just my take on it at least. Great responses! Thanks a bunch, everyone!