Sorry but this will be a monster of a thread. I'll try to break it up into short paragraphs so it can be scanned more easily.
I teach at a school of Chinese medicine. I'm a medical Anthropologist by training and I have specialized in the differences between the fundamental medicals models of bio-medicine and Chinese medicine. One of the first things to keep in mind is that the metaphysical models of one paradigm cannot be explained in terms of another paradigm. Ideas of one world-view can be outlined or discussed using another world-view's models, but there comes a point where deeper meaning is lost in the translation.
Qi has been subject to this kind of cross-cultural parsing error to the point of obscurity. No-one who understood what is implied by Qi would say they don't believe in it. Saying you don't believe in Qi is like saying you don't believe in "relationship" or "influence."
Qi is an old idea. The model I will try to explain is at least 2300 years old as it is the one clearly laid out in the Huangdi Neijing, the seminal work of Chinese medicine. While essentially a medical text it also lays out a metaphysical model that is basic to Chinese philosophy. The oldest character for Qi represented vapours rising. In time the radical for grain was placed below to show the steam that rises from cooking millet. What happens when you smell food cooking? Your body responds; stomach gurgles, mouth waters, etc. So something about the cooking food exerted an influence on you. Qi.
What is the nature of this influence? It requires that we introduce two other philosophical concepts that have been subjected to ideational mutilation, Yin and Yang. We often see the simplistic folk models of Yin and Yang that are the lists of things associated with the two concepts. (Yin is femine, cool, dark, receptive/ Yang is masculine, warm, light, creative). These lists are not strictly wrong, yet they obscure the reason the lists were made in the first place.
"Between heaven and earth are Qi and its laws; between Yin and Yang are Qi and its laws." This classic statement points to the most important aspect of Qi, its context. So Qi is the interplay of Yin and Yang. This brings us back to the nature of Yin and Yang.
Yin is the tendency of things in the universe to coalesce and slow down. Yin is the tendency to crystalization and struction. Yin is the tendancy for matter to fall down and shadows to form in opposition to light.
Yang is the tendency for things to change and become other things. The tendency to dissolve and the tendency to grow. The tendency for clouds to boil and the skin to tan.
All objects of sense experience are an interplay of Yin and Yang. This includes individual things as well as systems. This is why the concept of Taiji is the great terminus or the grand ultimate, because it includes all things of the universe. Yin and Yang are linked and one cannot be discussed without invoking the other. Stucture cannot be divorced from function.
They stand in mutual opposition.
They are rooted in each other.
They define each other's limits.
They transform into one another.
When we discuss the balance of Yin and Yang at any given moment or in any given state we are talking about Qi. It is important to understand that Yin and Yang are not absolute states of being, rather they are part of a moving frame that can applied in a variety of ways to any situation. There are many situations where the place of Yin and Yang is considered to be understood and relatively predictable so they are left out of the discussion. One of these places is in the context of the relationship between body and mind.
When we look at the basic idea of body and mind in Chinese philosophy we are talking about Jing (essence) and Shen (spirit or the summmation of mental-emotion aspects of the individual). The relative interaction of Jing and Shen is the normal context for the discussion of Qi in the martial arts. When someone says "lead Qi to your fingertips" the implication is to focus the mind on the fingertips to perceive the interaction between Jing and Shen at that point, hence Qi. However, you could also say "absorb the opponents force to use their strength against them." In this case Yin is found in the reception and relaxed connection of the defender (Song), and the Yang is found in the aggressor's force (Li). The resulting Qi is how the interaction plays out. If the defender is subtle and skilled the Qi will be the Qi of a throw perhaps. If the aggressor is more subtle and skilled the result may be the Qi of a successful hit. If the aggressor is very subtle they may be able to become more Yin that the defender and so invert the frame of Yin and Yang.
So to come back to the idea "Do you believe in Qi?" may be more usefully phrased "Do you believe in the interplay of structure and function? Do you believe in the interaction of order and chaos?"
I don't mean to take sides in the hugely long debate on that "other" thread. I don't really agree with either the pragmatic disbelievers or the advocates. No-one on that thread convinced me that they really understood the concept of Qi. Perhaps we can move towards a middle ground?