Zhu Xi synthesized the thought of earlier Neo–Confucians, and his interpretation of the Four Books became the basis of the civil service examinations (see Section 1, above).  Zhu held that everything in existence has two aspects, Pattern and qì.  Pattern (lǐ, also translated “principle”) is the underlying structure of the universe.  Qi is the spatio–temporal stuff that constitutes concrete objects.  The complete Pattern is fully present in each entity in the universe; however, entities are individuated by having distinct allotments of qi (variously translated as “ether,” “psychophysical stuff,” “vital fluid,” “material force,” and others).  In addition to having spatio–temporal location, qi has qualitative distinctions, described metaphorically in terms of degrees of “clarity” and “turbidity.” The more “clear” the qi, the greater the extent to which the Pattern manifests.  This explains how speciation is possible.  For example, plants have more “clear” qi than do rocks, which is reflected in the more complex repertoire of interactions plants have with their environments.