Only when the body is disorganised do you become conscious of that working.
Normally, the motion of your blood, the building up by assimilation of your muscles and nerves, the life of your cells, the protective action of some of the living cells in your body--the "devourers," as they are called--go on without your knowledge, without your thought, without your giving one moment's conscious attention to them.

In the Perfect Man, the consciousness of all this is ever present, but in us, imperfect, it is not; we are not yet sufficiently vitalised and unfolded to carry on the whole of our consciousness, with full awareness of all its activities.

We are only able to manage a very small part of it, and so have let go the consciousness that keeps at work the physical body, to concentrate ourselves in a higher world, and utilise the nervous mechanism as the apparatus of our thinking.
That law obtains, then, all through.

If you want to organise and build up your astral body, you can only do it from the mental plane.

You must raise your thought to a higher power by concentration, by regular meditation, by deliberately working on the consciousness, before you can raise it to that power from which it shall be able to organise your astral body, as it has already organised your physical body.