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[anagogy]

About the lower astral beings, that was the first thing I suspected. I don't understand why they would want to nourish themselves on our energy. Can't they generate their own energy themselves?

Well, it is not like they will die if they don't do it.  It is just that this is their nature.  It is somehow satisfying, to their consciousness, to do this.  They are service to self thoughtforms, so their consciousness is enhanced by service to self actions.  It strengthens their thoughtform, and raises their consciousness, by increasing their polarity.  This makes them stronger.  This is their manner of evolution.