That all opposites—such as mass and energy, subject and object, life and death—are so much each other that they are perfectly inseparable, still strikes most of us as hard to believe. But this is only because we accept as real the boundary line between the opposites. It is, recall, the boundaries themselves which create the seeming existence of separate opposites. To put it plainly, to say that "ultimate reality is a unity of opposites" is actually to say that in ultimate reality there are no boundaries. Anywhere.
Ken Wilber, No Boundary: Eastern and Western Approaches to Personal Growth
boundaries, nonduality, opposites, polarities, spirituality, unity