Martial arts are typically just a series of methods and principles designed to assist you in combat.

Tai chi is a little different.
The entire system is concerned with aligning your body and mind with tao; of moving in accord with the way of things.
It is a spiritual discipline.
The self defence component is a side effect of following tao.

Tai chi has more in common with zen, Japanese archery, calligraphy, poetry, oriental flower arranging or Japanese tea ceremony than it does boxing.