Rebbe Pinchas from Koritz explained this mishna homiletically: A tzaddik is called a 'builder' for he builds himself palaces in heaven with his Torah study and good deeds. Even so, he must not rest on his laurels but must continue to grow and serve Hashem, for complacency is the forerunner of descent. Thus the mishna may be read as follows.

Q. A builder (tzaddik): How much must he build and still be obligated to continue building?

A. As long as he thinks that he has completed his required spiritual growth (kol shehu = everything there is), for a tzaddik must continually strive to come closer to Hashem.