The Principle of Rhythm is the fifth of the seven Hermetic Principles, summed up in the axiom: "Everything flows, out and in; everything has its tides." It holds that all things swing between their poles in a measured rhythm, a rise answered by a fall and a fall by a rise.
A modern statement, an old name
These seven principles should be dated with care. They were set out in The Kybalion, published in Chicago in 1908 under the pseudonym "Three Initiates," a work generally attributed to the New Thought writer William Walker Atkinson. Although it invokes the name of Hermes Trismegistus and calls itself a Hermetic teaching, the seven-principle scheme is a product of the early twentieth-century New Thought movement, not of the ancient Hermetic writings such as the Corpus Hermeticum.
The principle is described here as part of that documented modern esoteric tradition, which has been widely influential in later occult and self-help literature.