Matter and energy are two opposites. To energize a piece of matter requires a constant flow.

Existence and nothingness are two opposites. To keep the world in existence demands a constant renewal.

Heaven and earth are two opposites. To infuse the earthly with spirituality requires a constant effort.

Woman and man are two opposites. And so courtship never ends.

Likkutei Sichot, vol. 5, page 174.