Authentic Thaumaturgy
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Authentic Thaumaturgy is a set of rules for portraying magic in role-playing games, written by Isaac Bonewits, based on the magic system Bonewits uses in real life in his capacity as the Archdruid of Ár nDraíocht Féin. The first edition was published by Chaosium in 1978, and a substantially expanded edition was published by Steve Jackson Games in 1998.
In Bonewits' system, players use mathematical formulas to translate their characters' adherence to magical laws directly into physical effects (kinetic energy, heat, etc.).