Human beings are fundamentally weak of will, so although we want to do the virtuous things we know we should, we are all too soon enticed by laziness.
There is a story about “bird’s nest Zen master” Daolin, who was always sitting in Zen meditation atop a tree. The literati poet Bai Juyi, who had been appointed the administrator of that region, went to the master and asked him, "What kind of teaching is Buddhism?" Daolin answered by telling him exactly what Buddhism teaches: “To refrain from doing evil, to undertake the good, and to purify oneself is the teaching of Buddhism.” Bai Juyi countered by saying, “That’s something even a three year-old child knows.” To which Daolin replied, “What a three year old child knows is something that even an 80-year old gentlemen is incapable of putting into practice.”
How can we overcome this human weakness?
Our compatriots, the sangha, are crucial to this. We do not make a silent promise to ourselves simply within our hearts, but make a pledge by putting it into words in front of many of our compatriots. This is what it is to “make a vow.”
For the practice of faith it’s important to transform one’s heart, but not many people can do it all by themselves. Everyone in the sangha has complete faith in the buddha-nature of the person who makes a vow, and the sangha continues to watch over them affectionately even if they suffer a few failures. The motivation to unfailingly respond to the sangha’s encouragement and expectations are what transforms a person.
From Kaisozuikan 9, Kosei Publishing Co. 1997, pp.122-23