My reasoning was pretty simple: if you sit in a bath things can leave your body and also have ample opportunity to re-enter your body. Your comment on relative concentrations is helpful and makes me reconsider this notion. As a naive and simple-minded person I think of entering a bath whilst covered in mud. Yes, the relative concentrations of mud on my skin's surface and mud in the water would clearly suggest I am "clean," but will I really feel clean stepping out of a tub that looks more opaque than clear? Maybe it is no surprise I have such trouble understanding the world with my simple-minded approaches.