Opposites - 對立 - Duìlì
Before emptiness, there must be fullness.
Before shrinking, there must be expansion.
Before falling, there must be ascent.
To destroy something, lead it to the extreme.
To preserve, keep to the middle course.
~written by Deng Ming-Dao
Although we speak of opposites, they are not truly antagonistic. All opposites exist as parts of the same entity. Like a two-headed serpent, opposites make up two parts of the same whole. They define each other, as black defines white. They alternate, as war alternates with peace.
When any phenomenon reaches the extreme, it will change towards its opposite, just as the darkest night gives way to dawn, and the coldest winter is followed by a spring thaw. Therefore, any time anyone wishes to destroy a thing, simply lead it to the extreme or crush it at the onset. The easiest times to fall a tree are as a supple sapling or a dessicated old growth, too tall to support its weight.
This principle holds true if one wishes to nurture something. You can prevent its destruction by bringing it close, but not over, its apex. Take a branch from an old tree and grraft it. The wisdom lies on the middle ground. Followers of Tao change a situation as it reaches the apex. By joining efforts to a new situation, just budding, we attain perpetuity.