A spirit, also called an Aetheric projection, is a mind that has imprinted itself into the Aether and continues to exist independently of its body or other physical substrate. This is possible because the field responds closely to cognition, though a mind can leave such an imprint only under certain circumstances.
An esper who is astrally projecting may forget the way back to their body or become severed from it. Certain high-energy excitations of the Aetheric Field can also create a projection.
A spirit retains all of its cognitive and psi abilities, but its capacity to change and learn is limited to the context it can hold in its mind. Both thinking and acting consume energy. Without an external supply of energy, a spirit will eventually fade.