This is the way of Zen, not to say things to their completion. This has to be understood; it is a very important methodology. Not to say everything means to give an opportunity to the listener to complete it.
All answers are incomplete. The master has only given you a direction... By the time you reach the limit, you will know what is going to remain. This way, if somebody is trying to understand Zen intellectually he will fail. It is not an answer to the question but something more than the answer. It is indicating the very reality....
The buddha nature is not something far away - your very consciousness is buddha nature. And your consciousness can witness these things which constitute the world. The world will end but the mirror will remain, mirroring nothing.
Osho Joshu: The Lion's Roar Chapter 5