Authentic ThaumaturgyWritten by Philip Emmons Isaac Bonewits, Authentic Thaumaturgy is an RPG magic system based on the magic system Bonewits uses in real life in his capacity as the Archdruid of ADF - Ár nDraíocht Féin (“Our Own Druidism” in Irish Gaelic). 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.). Though this may require the use of a calculator, the rich effects achievable by this very flexible system are well worth the effort.
Bonewits also wrote Real Magic, about his real life magic system.