"Everything flows, out and in; everything has its tides; all things rise and fall; the pendulum-swing manifests in everything; the measure of the swing to the right is the measure of the swing to the left; rhythm compensates."
This Principle embodies the truth that in everything there is manifested a measured motion, to and fro; a flow and inflow; a swing backward and forward; a pendulum-like movement; a tide-like ebb and flow; a high-tide and low-tide; between the two poles which exist in accordance with the Principle of Polarity [...]
There is always an action and a reaction; an advance and a retreat a rising and a sinking. This is in the affairs of the Universe, suns, worlds, men, animals, mind, energy, and matter. This law is manifest in the creation and destruction of worlds; in the rise and fall of nations; in the life of all things; and finally in the mental states of Man [...]
The Kybalion, Chapter 2: "The Seven Hermetic Principles"