Put simply, it can be found in Zhuangzi's section "On Ordering Things" (qi wu lun). Here Zhuangzi says that our knowledge about nature and about the truth and falsity of opinion is entirely based on criteria that we choose for ourselves.
如果我們簡單的說，這套理論就表現在他的《齊物論》裡面。他認為我們對萬物的知識，輿論的是非，都是自己選定的標準。