We are all aware that life is made up of two components, the physical (ke or ketai) and the spiritual (ku or kutai). They are two, but not two (shikishin funi) and cannot exist one without the other. The body or physical aspect becomes useless without the mind, or spiritual aspect, and the mind is helpless without the body.
The mind however, can continue to function without the body, when we sleep for example. We have all experienced dreams where we perform feats that would be totally impossible in the physical world, like being able to fly.
So we have two rather different components, maybe working in a way that is not necessarily harmonious, until chu (or chutai) takes control. Chu is the harmonisation of ke and ku. It controls each aspect, making sure one or the other does not dominate or drag us off course.
This is known as The Middle Way (chudo).
