Knowledge of mental development as applied in t'ai chi is often kept secret or considered too difficult to communicate or in many cases simply lost in the process of transmission.
'Without Yi there is no ch'i.'
The Yi is the mental force which moves the awareness towards an object but in the case of t'ai chi it is more than that. Together with the mind contact with the object there is the intention to apply a force. It is as if there is the mental act of visualising or imagining the energy without the actual tension in the body that would come with an ordinary intention to move an object. There is the idea without the action.
In the realm of western cultural belief systems the imagination is considered to be unreal. Of course, you might think, if it is imaginary it is unreal by definition. This highlights our problem. Without the conviction that the force is created by the mind the ch'i will not follow. A child does not have this problem. Their imagination has real power because they are able to suspend their judgement quite easily. We must learn to turn off the inhibitions which prevent us from becoming equally absorbed with our imagined and visualised perceptions.