Perhaps you have heard the laws of physics. Such laws are theorems that are accepted as being valid. At sea level water will freeze at 32F and boil at 212F. At 1000 feet water will boil at a lower temperature. Magic on the other hand is more pretend. Magical theorems and laws are at the whims of the designers.

Oblivion magical spells were reduced if armor was worn, Not sure of the theorem behind this. Skyrim changed this into a theorem. Hand gestures were needed to cast spells. So one hand to be free of a weapon. Start of a theorem. Skyrim doesn't allow spell casting with two handed weapons. Oblivion did. No logic behind the difference.

In some games TES spell school of Illusion would be called mind magic. A basic law of magic must be that casting spells requires an alert mind. Otherwise a spell caster could kill him or her self while sleeping. When in deep sleep, the body usually can't move sort of a protection thingy. Mine magic works on many but not all nastys. Theorem behind the reason for this could the nasty has a stronger mind or immune to mystical attacks.

What magical theorems exist in other games where magic is practiced?