This knowledge of how man's manifold constitution could be most quickly and most completely regenerated to the point of spiritual illumination constituted the secret, or esoteric, doctrine of antiquity. Certain apparently physical organs and centers are in reality the veils or sheaths of spiritual centers. What these were and how they could be unfolded was never revealed to the unregenerate, for the philosophers realized that once he understands the complete working of any system, a man may accomplish a prescribed end without being qualified to manipulate and control the effects which he has produced. For this reason long periods of probation were imposed, so that the knowledge of how to become as the gods might remain the sole possession of the worthy.