Principia Magica

Magic, even in games, is governed by certain rules. These are reflected in game rules and structures, but in-world they are understood and studied (by wizards, obviously).

1. Perception before product. Will-workers of whatever kind are able to make magic only because they can, in some way, perceive it. This is why detect magic is on every spell list. Divination is the root of magic, because without a way to perceive power you cannot exercise your will to shape it.

2. Bodily sovereignty. The body is a temple, which is why you can't create water inside someone's lungs or something. Sentient creatures - anything with any mental stats - have a conception of the self that is the foundation of all of their experiences throughout their existence, and so is extremely strong against modification. This is why spells like polymorph require so much power, and why they are temporary and limited (e.g., when the "mouse" is killed the subconscious takes over and restores the body). Spells that affect the interior of a person directly require huge amounts of power to override the "mind-body barrier," as Mordenkainen calls it.

3. Reduction of power. No spell or effect creates more power than it consumes, even when this appears to be the case. Magical powers draw on sources from the Astral, either directly or divinely, and as such have a certain amount of energy to work with. This is to say: a spell cannot grant you a spell slot of a greater level, or even an equal level. Spells like wish, which have huge costs in power, time, and materials, still only give you the ability to cast "any 8th-level spell," as the excess power is drawn off and wasted. Mages have spent tons of time trying to discover a perpetual magic device or wishing for more wishes, but it cannot be done - even by deity-level powers.

4. Material over magical. Without material there is no magic - there has to be a mind to create and perceive magic, even if that mind is alien or deific - but no such requirement exists for magic. We can also see this principle in an antimagic field: material objects and creatures continue to exist, even while magical effects do not.

5. Equilibrium. Magical energy, like water, seeks a stable state. That's why a fireball flashes into, then out of existence, rather than persisting. The world wants to return to a specific order, and anything that deviates requires power: summoned creatures, magical effects, spells. We can see this in action with concentration effects - presence of mind is necessary to keep them going. Even 'permanent' effects like mythals draw power from somewhere in order to produce their ongoing effects.

There are further principles that aren't laws, but explain magical properties:

1. Sympathy. Like affects like. This is at play with poppets, scrying, blood magic - any kind of thaumaturgy. An image of a bird is like a bird, and so an image of a bird can be used to scry on that bird.

2. Similarity. Like acts like like. This is the foundation of many material component choices. A lightning bolt acts like static electricity; a message acts like a telegraph; a paper bird behaves like a messenger pigeon. A component or tool that resembles a spell effect is more powerful than one that doesn't, and the more it resembles it the more powerful it is.

3. Correspondence. Magic occurs, essentially, because actions on the Material Plane affect structures or processes in the Astral, which then have effects on the Material Plane again. The large affects the small, the material affects the spiritual, the imaginary affects the real.

4. Resistance. Some objects, people, or places are less or more conductive to the communication of magic. These things are 'closer' or 'father' from the Astral, typically, and this explains why some materials or objects annul or resist magic, or why certain people are especially vulnerable or resilient against magic.

5. Backlash. If a spell is flubbed, badly, it can have negative effects. Unpredictable astral structures created by pouring energy into a poorly-constructed ritual or process can have unpredictable effects on the Material Plane - thus, wild magic surges or similar events.