Re: ‘If we use the 5-element theory to explain the wubu, then we should be consistent. Fire rises, it doesn't go forward. Water descends, but doesn't naturally go backward.’
I know what you’re saying, but conceptual metaphors are often coherent in ways that are not immediately obvious. In English, for example, we relate to time in spatial terms. The future is ahead of us; the past is behind us. Yet, we sometimes use spatial references for time that appear inconsistent with that orientation, as when we say, “the week following next week.” The word “following,” in most contexts means “behind,” or “in back of,” so one would think it would be incoherent with spatial references to future time as being in front of us. George Lakoff analyses this in his book, _Metaphors We Live By_, and demonstrates that in fact there is coherence in these divergent images that make them work. It would be best for me not to attempt to repeat his argument, as I would probably mangle it.
Conceptual metaphors are also culturally bound, so that while we would say that a Frisbee that has landed between “me” and a large rock is “in front of the rock.” In some cultures, in the same situation, the Frisbee would be said to be “behind the rock.” In neither case does the rock in question have a native front-back orientation, but culturally it is assigned its orientation with respect to the observer.
I think we need to view the wubu in light of much earlier metaphorical vehicle of the five phases, and understand that the metaphors are very open ended. That is, the metaphors are consistently metaphors, so that any references to directionality, cardinal compass points, etc., are not to be taken literally, but to suggest a network of relationships. You’re right, in wuxing theory fire rises and water descends. But these actions can refer to operations that don’t physically rise or descend. Also, culturally, retreating may dovetail with descending (or receding) in Chinese thinking, and advancing with rising. In any case, in the first document of the Yang Forty (see Wile, pp. 65, 135), retreating is specifically correlated with water, and advancing is correlated with fire.
I think this sort of investigation into the conceptual metaphors working in taiji is worth pursuing.