Copied from my Two Children post - the Temple:

There are four spiritual sheaths of humanity.  Man is said to consist of the physical body, the etheric body, the astral body and the Ego.  It is the power of the etheric body that promotes physical growth, through cellular activity.  More complex is the astral body.  It is the task of the astral body to render the etheric-invested physical body susceptible to feelings, and to make consciousness possible.  The spiritual activity of the astral body is expressed through the nervous system.  It is the emotional life, brought to expression through the astral body that galvanises humans into motion.

The astral body consists of a twofold nature, sometimes described as dark and light.  One spiritual purpose of human life is the transformation of the astral body in such a way as to ensure that the lower darkness is rejected in favour of the higher light.  Only the fully transformed astral body, which contains no element of darkness, may be rightly called ‘the body of the stars’.  The human overlord of this threefold organism of physical, etheric and astral is the Ego which gives direction and moral purpose to the three lower bodies.

The Ego is the Eternal Spirit of Man.  It is the Ego that re-embodies in new physical, etheric and astral bodies in progressing earth lives.  The Ego is the seat of control over the the Will.
As said above; it is the Ego which gives direction and moral purpose - This purpose, once filled with love and understanding that the Creation is Absolute, can be very powerful, regenerative and transform the Ego into its pure state while in the earth realm.