Chapter 17, Part 4D - Chuang Tzu

"From the point of view of function, if we regard a thing as useful because there is a certain usefulness to it, then among all the ten thousand things there are none that are not useful. If we regard a thing as useless because there is a certain uselessness to it, then among the ten thousand things there are none that are not useless. If we know that east and west are mutually opposed but that one cannot do without the other, then we can estimate the degree of function.
~ Burton Watson translation via Terebess Asia Online ~

Go here to read the introductory post to the chapters of the Book of Chuang Tzu.