You can treat Tai-Chi with different angles ---
        here are the stages from a beginner to the great master level

• Tai-Chi, a Chinese culture

     Just like every ancient civilization has its unique culture, China has a great treasure of culture phenomena, which influence the way of people's  life. Tai-Chi, with a root of thousand years in China, the idea of Tai Chi, the attitude towards life from TaiChi, influence Chinese  as a whole, even when they do not notice.
 
• Tai-Chi, a physical practice --  positive relax

    Moving Meditation to balance body and mind. The slow movements of Tai Chi are graceful, relaxed, balanced, and Meditative.  This form of exercise unites the mind and the body. A "positive relax"  is not like common relax, like a sleep, lying down for a while on grassland, which both your body and spirit relaxed. Instead  it have you to do something, spirit goes up, while body relax much more than the common state. The key lies in the interaction of mind and body.
 
• Tai-Chi, an performance art

    You are the artist as well and the works. You are required to feel the sensitive and subtle meaning with your body's movement, improve every small things that is not perfect. At this stage, you are required to find the "right" position, find where is balanced or where is not, try to correct them, from big unbalance to very trivial unbalance, and finally you can achieve "lightness". Never use force, use mind.  The name of the art works can be called "Flowing Clouds over Meandering River".
 
• Tai-Chi, a healing power, long life and health

    What is "vital energy"? or "inner gas"? What is the relationship with the vital energy and the mind? Can you control it and use it to improve your health and prevent disease? Some people "believe" there is "vital energy" or not. But through TaiChi, you can FEEL and USE it. What is the balance INSIDE your body? you will understand more and more about these, and the more you FEEL, the more you can adjust and control. No, TaiChi does not require you to believe first. You practice, you improve, you feel, you use it. The theory of vital energy and the special route inside the body for the flowing of the energy, is the basis of Acupuncture, which is accepted in western modern medical science.
 
• Tai-Chi, a way to understand life, a philosophy

    The Philosophy of Taoism is not a pure theory. Many people, in the ancient past or now, realize the theory. Yin and Yang are not abstract theoretical terms. They mean something inside your body. Through understanding yourself,  understand others. Through understanding people, understand changes in life. Through understanding changes in life, understand how you should do.
 
• Tai-Chi, a martial art.

    Yes, in fact, Tai-Chi is called "inner martial art". it is powerful. In history, the master of TaiChi are all great knights in some period of their lives, like their nick name, as "Yang the Invincible" and so on. When the body and soul are fully combined, and with some other complementary training methods for fighting, it will generate great power. -- But, it is TOTALLY different with the force we see in boxing. The force from Tai-Chi is "mildest and strongest".
 
• Tai-Chi, a Tao

    I do not want to say too much for this stage. It means a deep understanding of nature and even fate. This stage sounds not so "scientific", and I am a Ph.D. student in Electrical Engineer Dept., UCLA. If we have chance, we can talk about it as a legend and fairy tale :) --- But they are real.
 
To sum up: the road is like this:
a culture ->positive relax -> balance and calm-> lightness -> binding of mind and body -> find and improve inner energy -> peace in mind, and clear in mind -> agility generated from peace -> empety and the things behind empety -> understand the real self, deep peace and joy in heart -> integration with nature -> ......... more understanding, more peace, more inner joy .......

now I hope you know why it is a realization of a phylosophy, though at first you are encouraged to think of it as a way to relax and a way to improve health.