A common metaphor for the heart of the sage is that, in its stillness, it mirrors the world like unagitated water. I have had difficulty understanding what this might really mean. Ames (Yuan Dao) goes a long way in clarifying it.
He begins by discussing the implications of xin, the classical Chinese understanding of self, an understanding quite different than the insular, divided, self-entity concept of the West. Xin, he tells us, is best translated 'heart-mind'. No true distinction is made between the emotions and intelligence, reason and will, and thus no internal war is envisioned as causing one's sense of disharmony in the world.
Rather, the source of this disharmony is relational; the self is a complex of relationships, and the way to establishing harmony is by way of bringing equilibrium to these relationships. The source of disharmony is thus largely external. But Ames also makes the point that in Chinese thought, external (nei) and internal (wai) are correlative categories, which is to say, they are united on a continuum, and not independent of each other. This also speaks to the relational character of self, which though particular in itself, cannot be removed from its relational context with the world. Either/or is not an option which comes readily to the Daoist mind.
At this point I would like to interject that, if there is a significant divergence between the classical Chinese understanding of self and that in the West, although it behooves us to understand how it is different, both as a means to better understanding the Daoist message and as a means to allowing it to inform our own projects, we must, as Jung pointed out, begin where we are. To attempt to impose a philosophical Daoist perspective on our own cultural realities would not only fail to address our own particular needs, but would itself violate the spirit of Daoism itself. Thus, since there is typically a war within the Western self, that may very well be the first place to begin the project of realizing harmony. The first relationship which needs to be addressed in the West is one's relationship with oneself.
Harmony, in the Daoist vision, is therefore established through one's relation with the world. And one essential facet of this harmony is disallowing the external to cause internal agitation. Zhuangzi tells us to not allow things or events to enter our Numinous Reservoir; we are to preserve our inner-most awareness like a still and reflective pool. This then is one central attribute of the mirror-like heart-mind: In reflecting the world, the heart-mind does not allow it to agitate its stillness. Mirroring is thus a form of non-reactive acceptance.
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