That means, going to the stream of water with one's cup covered up: wanting the water, and yet the cup is closed, filled with preconceived ideas. Where have the preconceived ideas come from? No idea can be called one's own! All ideas have been learned from one source or another, but in time one comes to think that they are one's own. For these ideas one will argue and dispute, although they do not satisfy fully. At the same time they are one's battleground, and all the time they will keep the cup covered up.