A monk asked Nansen, "Is there any dharma that has not been preached to the people?" Nansen answered, "There is." "What is the truth that has not been taught?" asked the monk. Nansen said, "It is not mind; it is not Buddha; it is not things."
If this is what has not been preached, then nothing has been preached. That is, in fact, what Nansen means. The truth cannot be taught; we must learn it for ourselves.