『不易流行』（ふえきりゅうこう）/ Inseparable Relationship Between the Transient and the Immutable

新しみを求めてたえず変化する流行性にこそ、永遠に変わることのない不易の本質があり、不易と流行とは根元において一つである。

The transient (=ever-changing) and the immutable (=eternal) are inseparable, because as you constantly pursue new things, you will end up discovering truths of the universe that do not change over time. Thus, the eternal and the changeable derive from the same root.

Note: An interchange between the transient and the immutable is central to the soul of haiku.