Body and the mind
For a person to function normally, the body and the mind must work together seamlessly. In taijiquan, this state of being is called the harmony of the body and the mind. It is also called the union of the body and the mind.
In terms of yin yang theory in taijiquan, body and mind are the opposite. They don't always harmonize in many activities. The body mind connection becomes more and more distant as humans age. The practice of taijiquan, is said to help rebuild the body and mind connection for many people.
Source: https://practicalmethod.com/?p=40929