The key to high level martial skill is in the understanding of yin and yang, and the nature of qi.
Tai chi teachers and students will easily acknowledge the importance of being "centred". But how many of them really understand what that means. In tai chi, the word centre does not mean the same thing that it does when we talk about donuts. It is not a space or a creamy filling.
The centre is the thing in the middle. Sure, that makes so much sense that it is a bit of a joke. But what is it in the middle of?
The centre of gravity is not necessarily the geographical centre of an object, nor is it the exact geographical centre of the human body. In tai chi the word centre refers also to the mental and emotional centre, as well as the centre of the person's position in the universe. One must relate the centre to so many different frames of reference that it might be impossible to define. That is, unless we can find a common denominator for all of those different frames of reference - a unified field, if you will.
When tai chi theory becomes impossibly complex, it serves us well to return to the absolute basics of tai chi theory. For us, that is the understanding of yin and yang. The centre, therefore is between yin and yang. Look at it this way, and you may find that you can make sense of some of the most challenging concept in philosophy and science, such as the nature of our relationship to space/time, and the true nature of qi and energy.