I return to Xunzi's (312-? B.C.E.) objection to Zhuangzi's (369-286 B.C.E.) philosophy: "Zhuangzi was obsessed with nature (tian=Heaven) but did not know the human." (Xunzi 21/22) As mentioned previously, Kjellberg offers several interpretations of what this might mean and settles on one as the most likely (in Essays on Skepticism, Relativism, and Ethics in the Zhuangzi), namely that Xunzi believes Zhuangzi is suggesting that nature is 'good' and thus that spontaneous (natural) living will lead to a 'good' life. We need only remember that it is Xunzi who famously said "human nature is bad" to understand why he found this disturbing. For my part, I suggested that Xunzi was essentially oblivious to what Zhuangzi was actually saying.
Before discussing how this was so, however, I would like to mention the hypothesis that Xunzi's Confucianism may have been a direct response to the philosophy of Zhuangzi. (Kjellberg references David Nivison in this regard). This should not surprise us since the 'Daoist' sensibility was already gaining ground as an alternative to Confucianism at the time of his writing. Thus, his negative assessment of human nature would have been a direct answer to Zhuangzi's 'naturalism'.
What then of Xunzi's misunderstanding of Zhuangzi? Might we begin with the simple fact that he was a moralist concerned with right and wrong, whereas Zhuangzi, though certainly acknowledging the validity of a relative morality in human culture, understood nature (reality) as amoral? It's hard to over-estimate the consequence of this difference when considering one's orientation in the world. I have several times quoted the Zen statement to the effect that he who is concerned with right and wrong is bound by right and wrong. That the question of moral outcomes so often immediately arises in these discussions is evidence of this bondage; and it tends to abort any possibility of understanding the transcendent sensibility of a perspective such as Zhuangzi's. (A parallel is seen in the rationalist's inability to grasp the possibility of another way of knowing.)
Did Zhuangzi then believe, like Mengzi (Mencius) (372-289 B.C.E.), that human nature is good? Not at all; he believed that it is what it is. Within the context of human society, we sometimes behave badly and sometimes well, but humanity, like everything else in the universe, is essentially amoral. We do not declare a mountain lion 'bad' because she takes domestic goats as well as deer, though for reasons of human concern we might deem her relatively so. Similarly, we can deem some behaviors as harmful to human society generally, without declaring individuals 'evil'.
What tends to be overlooked in objections such as Xunzi's is the transformational character of an approach like that of Zhuangzi who does not proclaim to the world at large the virtues of running wild in spontaneity, but rather suggests a way of spiritual integration from which spontaneity arises. You don't have the one without the other. Spontaneity is descriptive of a way of being, not a prescription for achieving it.
You can check out Scott's writings on Zhuangzi here.