"I only walk on the sword's blade"
Once there was a sick man, who went for help to many doctors, but they only spread their arms helplessly—they were not able to help him. At last he found a person that we usually call a wise old man. This man was a herbalist. He said: "I can help you. Not far away from here in the forest, in the mountains, grows a medicinal plant. If you only look for it according to my instructions—patiently and carefully—you will surely find it and you will be healthy again". So this man, feeling very happy, set off to look for a plant. At the beginning he was alert, patiently and carefully looked around. But as time passed he became less careful—he noticed a lot of interesting bushes and a lot of beautiful rocks and stones and slowly, slowly he forgot about the instructions of the herbalist and finally forgot why he had come to the forest.
This situation is very similar to people practicing meditation. The Big Question weakens after some time. The beginner's mind changes into the expert mind and our practice reaches a dead point. When I was finishing a driving course I thanked my instructor and told him that I hoped I would not bring shame on him by causing an accident soon. He answered: "Don't worry! Statistically, accidents caused by beginners happen very rarely. Most of the accidents are caused by people who have been drivers for several years, because they lose their attention and vigilance and think they know what's what. But you should be careful anyway." In one of the kong-ans, one Master comes to another Master and asks: "In the Mu kong-an there are ten sicknesses. How can you stay healthy?" The second Master's answer is: "I only walk on the sword's blade". This kind of vigilance! If you walk on the sword's blade you must be very careful. If you are not careful you may hurt yourself. So the thing is—the more you practice the more careful you must be.
(from a dharma talk in 2008)