From 30 Philosophers, Chapter 4, Laozi, Touchstone 12: Unknowable Dao.

The “unknowable Dao” is the skeptical belief that the true nature of reality is unknowable. The Unknowable Dao is a mysterious universal reality that transcends both language and thought; it encapsulates the ultimate reality and its inherent natural order. It’s the idea that the Dao cannot be fully comprehended nor expressed in words. It resonates in modern times. What we perceive as reality is merely a shadow of the full scope of what reality actually is. Consider, for instance, the concept of visible and non-visible light. You perceive visible light and may think it’s the whole spectrum, but science tells us it’s not.