Magic Systems

I often find myself thinking about magic systems in games. I've seen many. Just a couple:

• D&D: Spells are divided into levels, with certain spells available at each level depending on your class. You can cast a number of spells of each level per day of game time, again based on your character's level.
• GURPS: Spells are divided into colleges, which are really only important as organisational tools. You learn each spell individually in the same way as skills, using weaker spells as prerequisites for more powerful spells. Casting spells costs Fatigue Points, which are based on your character's Strength.
• Shadowrun: There is a spell list. You can learn any spell you like. When you cast a spell, you have to roll (the exact roll depends on which spell you're casting) to determine the effects of "drain."
• Ars Magica: There are five "verbs" and ten "nouns," with varying ratings in each. To cast a spell, you roll a number of dice equal to the verb + noun.
• Mage: There are nine spheres that govern all possible magical effects. The higher your rating in a sphere, the more control you have over that realm. Spheres can be combined for more powerful effects. Roll your Arete (magical awareness) to cast spells.
• Talislanta: there are twelve "modes," which cover different potential actions (such as Attack, Defend, Heal, Move, Illusion, &c.). Roll your rating in the appropriate mode to cast a spell.

There's a lot of variation there. I've even seen a book (Authentic Thaumaturgy) written by a man with a degree in Magic describing how to use "real world" magic systems as a basis for gaming magic.