[Johnny]

I quote Dr Peter Della Santina

It is because emptiness reveals and expresses the relativity of
all phenomena that it becomes the key to understanding nonduality.
We can see how recognition of the relativity – and subsequent
transcendence – of opposites is tantamount to the perception
of nonduality, or non-differentiation.
At this point we come to the central Mahayana doctrine of
the nonduality, or non-differentiation, of samsara and nirvana.
is is indicated in the Heart Sutra when Avalokiteshvara says
that form is not different from emptiness and emptiness is not
different from form. e other aggregates, too, are not different
from emptiness, and emptiness is not different from the aggregates.
us samsara and nirvana, the aggregates and emptiness,
phenomena and the unconditioned, the conditioned and the
transcendental are all alternatives that are relative to each other:
they have no independent existence. Indeed, because they are
relative to each other, they are, each of them, ultimately unreal
and empty. Hence the duality of samsara and nirvana is dissolved
in the vision of emptiness. Emptiness is the way out of all
extremes, even the extremes of samsara and nirvana. From this and other teachers I have always come to understand how the relationship between dualism (Samsara), The Practice of the way out (Noble eightfold path) and Nondualism (Nirvana) is understood in a dualistic fashion.

http://www.buddhanet.net/pdf_file/tr...ightenment.pdf