In Buddhadasa Bhikkhu's words:
http://www.what-buddha-taught.net/Books ... o_Tree.htm
Even if it is only a very small Nibbana, merely a taste, it's exactly the same thing as true, lasting Nibbana; it differs only in duration. It doesn't last because we don't know how to protect ourselves from the disease and how to destroy it. Consequently, every now and again the disease penetrates and interrupts Nibbana.
If anyone has the boon of intelligence to the extent of knowing that, in truth, the mind is fundamentally empty, that it's already Nibbana, then the only thing that has to be watched out for is not to let it be infiltrated by new things. So, don't let them in. Drive them out! If we don't let them into our house, it can be empty all the time.