From The Heart of the Buddha's Teaching
by Thich Nhat Hanh:

Nirvana, the Third Dharma Seal, is the ground of being, the substance of all that is. A wave does not have to die in order to become water. Water is the substance of the wave. The wave is already water. We are also like that. We carry in us the ground of interbeing, nirvana, the world of no-birth and no-death, no permanence and no impermanence, no self and no nonself.

Nirvana is the complete silencing of concepts. The notions of impermanence and nonself were offered by the Buddha as instruments of practice, not as doctrines to worship, fight or die for. "My dear friends," the Buddha said," the Dharma I offer you is only a raft to help you to cross over to the other shore." The raft is not to be held onto as an object of worship. It is an instrument for crossing over to the shore of well-being. If you are caught in the Dharma, it is no longer the Dharma.

Impermanence and nonself belong to the world of Phenomena, like the waves. Nirvana is the ground of all that is. The waves do not exist outside the water. If you know how to touch the waves, you touch the water at the same time. Nirvana does not exist seperate of impermanence and nonself. If you know how to use the tools of impermanence and nonself to touch reality, you touch nirvana in the here and the now.

Nirvana is the extinction of all notions. Birth is a notion. Death is a notion. Being is a notion. Non being is a notion. In our daily lives, we have to deal with these relative realities. But if we touch life more deeply, reality will reveal itsef in a different way.

Nirvana means extinction, above all the extinction of ideas- the ideas of birth and death, existence and non existence, coming and going, self and other, one and many. All these ideas cause us to suffer. We are afraid of death because ignorance gives us an illusory idea about what death is. We are disturbed by ideas of existence and nonexistence because we have not understood the true nature of impermanence and nonself.

We worry about our own future, but we fail to worry about the future of the other because we think that our happiness has nothing to do with the happiness of the other. This idea of self and other gives rise to immeasurable suffering.

In order to extinguish these ideas, we have to practice. Nirvana is a fan that helps us extinguish the fire of all our ideas, including ideas of permanence and self. That fan is our practice of looking deeply every day.