The sixth chapter of the Zhuangzi begins with a discussion of the relationship between the Heavenly and the Human. Zhuangzi quotes and deconstructs a maxim, very likely espoused by his Confucian contemporary Xunzi (fl. 298-236 BCE), to the effect that there is a distinct difference between them. The Human is seen as exceptional in that it is capable of acting outside of Dao. Zhuangzi suggests that no such distinction can be made given the limits of our ability to understand. It may be that all that Humanity does and knows is Dao. The roots of our knowing and doing disappear in mystery; we know neither how we know nor why we act as we do. This may be tantamount to equating Heaven and Humanity, but Zhuangzi is careful not to say so definitively; he prefers ambiguity.
Typically, Guo Xiang, in his development of Zhuangzi's philosophy, is not so tentative; all doing and all knowing he tells us, is "self-so", that is, spontaneously arising, and this is Dao. "'Heaven' is just another way of saying 'what is so of itself, the self-so'. For doing cannot be done by someone 'doing' doing. Doing is spontaneously doing; it is self-so. Knowing does not know by someone 'doing' knowing. Knowing is spontaneously knowing, it is self-so. As self-so knowing, knowing is not a result of knowing, is unknown, is itself a kind of nonknowing. Being always in this sense a nonknowing, what we call knowing emerges from nonknowing." (Zhuangzi; Ziporyn) How do we know? We don't know how we know; our knowing is rooted in what we cannot know; it is rooted in mystery.
What Zhuangzi implies, Guo articulates. Whereas Zhuangzi is content to let not-knowing extend into the apparent exceptionalism of the Human (as in some sense transcendent of Nature in the exercise of its doing and knowing), Guo knowingly insists that Human doing and knowing is not other than the Heavenly, that is, Dao manifest.
Personally, I tend toward Guo's position, but in either case the implication is the same: all that we are and do is rooted in mystery, and our 'completion' is to be had in surrendering into that mystery. When Yan Hui found that which "moves" him, that is the unknowable within him, he realized that not only was he not in charge, but that he had no 'me' at all. He lost his 'me' in his identification with mystery. To lose oneself in this way is to become all things; it is to lose one’s cage and thereby to freely roam in the Totality.
You can check out Scott's writings on Zhuangzi here.