To anyone seeking to make a case against Zhuangzi's mysticism I would certainly recommend Xunzi as a source of contemporary ammunition. His point of view is in many respects diametrically opposed to that of Zhuangzi. Indeed, as mentioned previously, some have thought his teaching to be a direct answer to Zhuangzi. This may be going too far, for his was a response to numerous philosophies emerging from the Warring States Period of which Zhuangzi was but one. Nevertheless, there are at least two occasions when he seems to be parodying Zhuangzi's story of the mighty bird Peng's flight and the silly dove's response. And then there are many pithy statements which seem to address Zhuangzi directly. Here's a sampling:
"If you guide it [the mind] with reason, nourish it with clarity, and do not allow external objects to unbalance it, then it will be capable of determining right and wrong and of resolving doubts." (Xunzi, 21; Watson) Xunzi was a rationalist and it would be hard to find a more definitive rebuttal of Zhuangzi’s dismissal of reason as a means to resolving life's fundamental issues.
"He who tries to travel two roads at once will arrive nowhere." (Xunzi, 1; Watson) Zhuangzi was an advocate of "Walking Two Roads", a metaphor with broad implications but which primarily speaks to the sage's ability to adapt to the opinions of others while understanding the relative character of every point of view. If Xunzi was in fact addressing this, he missed the point, however. His concern is that the would-be sage be doggedly single-minded in her self-cultivation: "Achievement consists in never giving up. . . they keep their minds on one thing." Elsewhere (21) he says: "There are not two Ways in the world; the sage is never of two minds."
"Learning should never cease. . . A piece of wood as straight as a plumb line may be bent in a circle as true as any drawn with a compass . . . The bending process has made it this way." (1) As a rationalist with a melancholy view of human nature, Xunzi takes the exact opposite view of Zhuangzi and Daoism generally, namely that returning to "the uncarved block", that is, the most natural, is the way to grow.
"[W]hat lacks completeness and purity does not deserve to be called beautiful." (1) Zhuangzi tells us that completeness, were it possible, would simply be the beginning of dissolution. A woman of great beauty, moreover, would still set deer to flight, for ultimately beauty resides in the eye of the beholder. Zhuangzi uses the grotesque as exemplars of sagacity to make this point.
There are many more such examples and, of course, statements of the more general Confucian emphasis on benevolence, righteousness and the necessity of ritual to which Zhuangzi was opposed. In being so radically different than Zhuangzi, Xunzi offers us a great way to take another look at Zhuangzi's thought and to ask if, and if so how, it might benefit from his critique.
You can check out Scott's writings on Zhuangzi here.