"The Theurgist has to acquire a practical, infallible capacity to consciously project himself in the Astral Body. This capacity must be established as a habit that can be exercised voluntarily at any given moment and under any circumstance; it does not matter if one is alone or before witnesses. Otherwise, one is not a Theurgist." - Samael Aun Weor
Logos, Mantra, Theurgy is included in the book The Divine Science.