Tai chi is kind of a moving meditation in one sense and according to the chinese medicine theory, there is "chi (like the 'air' we breathe in but not exactly the same stuff)" in our Meridians. So does tai chi chuan believes in, the "chi" is important.
Click to read the Chinese original version
My translation:
"Concentrate on your 'chi' and don't let it be desultory or your movements will be unorganized. The 'chi' should be taken into your backbones and be unobstructive. To breathe in means to close and to store; to breathe out means to open and to explode. If you can lift your opponent by breathing in, then you can also knock him out by breathing out. What we are talking about here is to use your 'mind power' but not 'strength force'. "