折疑論 5.51

5.51

視之不足見（道德經云。視之不見。名曰夷。夷者無色之謂。其道故無色可見）。

Looking at it, it cannot be seen. (The Daodejing says: "Looking at it, it is not seen; it is called the formless (yi)." Yi refers to being without color; its Way is therefore devoid of color to be seen.)