Chapter 6, Part 1 - Chuang Tzu

He who knows what it is that Heaven does, and knows what it is that man does, has reached the peak.

Knowing what it is that Heaven does, he lives with Heaven. Knowing what it is that man does, he uses the knowledge of what he knows to help out the knowledge of what he doesn't know, and lives out the years that Heaven gave him without being cut off midway - this is the perfection of knowledge.

However, there is a difficulty. Knowledge must wait for something before it can be applicable, and that which it waits for is never certain. How, then, can I know that what I call Heaven is not really man, and what I call man is not really Heaven? There must first be a True Man' before there can be true knowledge.
~ Burton Watson translation via Terebess Asia Online ~

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