The basic idea is that ... the external environment around us shows a free interaction among the elements. The interaction is appropriately described by 5-element theory. However, the environment that we see inside our bodies is very different.
What is the difference? The external environment is not "actively" regulated, whereas the internal physiological environment within us is "actively" regulated... by the endocrine system. The internal environment is regulated in order to maintain optimal levels of physiology. One does not see this expression in the external environment. Consequently, one should not use the same element theory to describe both environments. To do so is a fundamental error.
6-element theory describes the internal environment of a higher living being, where the internal interactions reach homeostasis through active and dependent regulation.
Chinese medical theory will evolve when it incorporates a different element model to distinguish this difference between the external and the internal environments.