I know this idea. In the modern psychology is theme of a "shadow" and reintegrated idea. But this theme is not only a thelemic. In the europian tradition is HGA one of three special mercuric angels. This theme is very complicated for this prezentation. And... in this tradition is NOT a devil daimón.The Shepherd of Hermas (c. 140–150 CE): This popular early Christian text (which narrowly missed making it into the New Testament canon) explicitly states, "There are two angels with a man—one of righteousness, and the other of iniquity." It describes how the "angel of iniquity" fills a person with anger, greed, and dark thoughts, while the "angel of righteousness" inspires purity and peace.
It later was adopted by Islamic theology that two angels sit on a person's shoulders at all times. The angel on the right shoulder records the person's good deeds, while the angel on the left shoulder records their sins.
The belief continued underground in Christianity to one degree or another.