A good traveler leaves no track or trace. A good speech leaves no flaws. A good reckoner uses no counters. A well-shut door needs no bolts, and yet it cannot be opened. A well-tied knot needs no rope and yet none can untie it. Therefore the sage is always good in saving men and consequently no man is rejected. He is always good in saving things and consequently nothing is rejected. This is called following the light (of Nature) (1) Therefore the good man is the teacher of the bad, And the bad is the material from which the good may learn. He who does not value the teacher, Or greatly care for the material, Is greatly deluded although he may be learned. Such is the essential mystery.

(Lao Tzu)