Designing a contractual magic system?
 

I'm thinking about designing a pure contractual magic system in which it all based upon shared magical essence in which everything from clerics to wizards are following the same rules but they are all doing it in different ways because they have different social agreements.

What GURPS books help with designing a magic system? Most of my GURPS experience is on the tech/SF side, so I really don't know where to start.

Re: Designing a contractual magic system?
 

Thaumatology is the guide to designing magic systems.

Re: Designing a contractual magic system?
 

What do you mean by "contractual magic"? Signing contracts with demons is the first thing that leaps to my mind.

If you have some details about how you want the system to feel and how it works in-universe, people might be able to generate some suggestions. "Same rules but different ways" and "different social agreements" seems pretty broad to me. You could describe GURPS Magic that way, for instance, where those social agreements or stylings are the only difference between wizards and clerics. But it doesn't sound like that's what you have in mind.

Re: Designing a contractual magic system?
 

I don't know if this helps; but GURPS Spirits described three kinds of spiritual magic users: there were priests, who were defined as servants of godlike spirits; there were shamans, who were defined as interacting with spirits on a more or less equal footing; and there were sorcerers, who had spirits serving them. In order to avoid unfortunate implications, my own approach to this has been to categorize spirits into three broad classes: gods, spirits, and essences. Essences are like spirits in most ways, except that they aren't sapient; and many of them aren't even sentient. They're like the flora and fauna of the spirit world. Priests serve gods; shamans deal with spirits; and sorcerers control essences.

If you want, you could break it down a little bit further, splitting sorcerers into sorcerers and wizards, where sorcerers deal with spiritual fauna (sentience, but no sapience; spiritual pets that the sorcerers train and command) while wizards harvest and use spiritual flora (no sentience; in practice, there's little difference between this and “manipulating impersonal magical energies”).

This could be thought of as a framework for a “contractual” magical system, with wizards, sorcerers, shamans, and priests representing four different kinds of “contracts” — though in the case of wizards, it's more of a lack of contracts.

You might even throw “psychics” into the mix, where a psychic operates on the basis of “human spirits”: astral projection is the power to separate your spirit from you're body; possession is the ability to force your spirit into someone else's body; telepathy and mind control would be the powers to directly communicate with and manipulate other people's spirits.

In fact, you might consider making psychic powers the basis for this magic system: a psychic develops his own spirit's ability to manipulate fire and calls it pyrikinesis; a wizard or sorcerer gathers and directs fire-aspected essences and thus performed pyromancy; a shaman bargains with fire elementals; and a priest serves a god of fire. But whether it's a god, a spirit, a “spiritual fauna”, or even a “spiritual flora”, its ability to manipulate flames is exactly the same as the pyrokinetic's ability to do so; the only difference is that the psychic manipulated the flames directly; all of the others get something or someone else to do so, either by praying to them (in the case of gods), bargaining with them (in the case of spirits), commanding them (in the case of “spiritual fauna”), or simply using them (in the case of “spiritual flora”).

Though in a sense, all five types of “magic users” in this system would technically be psychics: wizards would need to develop their own spirits to be able to detect and interact with “spiritual flora”; sorcerers would need to develop their own spirits to be able to summon, bind, banish, and ward against “spiritual fauna”; shamans would need to develop their spirits to be able to sense and communicate with spirits; and while priests wouldn't have to develop any special spiritual abilities of their own, their gods would likely require them to develop their spirits in other ways, such as developing appropriate codes of conduct in order to turn themselves into more effective channels for their gods' powers.

Re: Designing a contractual magic system?
 

The Romans were fond of contractual religion, let alone magic - they had a tendency to draft contracts with their gods and then use various sorts of augury to determine whether the gods accepted or not. There was also the concept of do ut des - "I give that you might" - whereby they expected a solid quid pro quo for their sacrifices. As also noted, contracts are very much a thing amongst shamen, who deal with spiritual entities as peers.

Sounds a lot like the RuneQuest trichotomy - in a universe in which the distinctions between gods, spirits and mortals can get distinctly fuzzy. Also, in context, a demon is simply a more powerful, sapient and hostile spirit. I've enjoyed adding the aspect that, as well as binding and enslaving spirits, sorcerers can also consume them for raw power, which makes them especially hated by shamen who find their friends and business partners being effectively burned up as fuel, and even the gods are displeased as their servants occasionally get destroyed. Meanwhile, the boundary between a shaman with a powerful and demanding patron spirit and a priest can also be quite slender.

Re: Designing a contractual magic system?
 

The distinctions don't have to be fuzzy, any more than the distinction between a dog and a cat or a dog and a man is fuzzy. But the original poster did ask for essentially a unified system wherein “everything from clerics to wizards are following the same rules but they are all doing it in different ways because they have different social agreements”.

This is an attempt to address that, with the added caveat that what you're contracting with should say something about what kind of contract you form, and vice versa: a shaman attempting to bargain with a god will likely get punished for his hubris; and attempting to bargain with the spiritual equivalent of a dog will just be ineffective. A sorcerer who attempts to treat the sorts of spirits that a shaman deals with in the way he deals with the “dog-like” spirits that are his usual purview is engaging in a form of slavery; attempting to do that to a god is just plain stupid. It's entirely appropriate for a priest to serve a god; it is not appropriate to serve anything less than a god.

So priests serve gods, shamans bargain with sapient spirits, and sorcerers command non-sapient spirits. There are exceptions to these three pairings; but those exceptions are foolish or evil or both. The evil ones are suitable for villains but not heroes; the foolish ones are only suitable for the village idiot.

And yes, within each of these broad categories, you can have different types, such as human, demonic, fae, alien, etc. You can blur the boundaries among them if you wish; or you can make the boundaries sharp and distinct.

Re: Designing a contractual magic system?
 

There's an implication here of a cosmic hierarchy, which calls to mind a Celestial Bureaucracy that maintains the records and paperwork that keep that hierarchy in place. A natural adventure would, of course, be infiltrating said bureaucracy so you can alter the records to your benefit... Likewise, all these bargains, supplications, and commands must follow the correct forms. Law (Spirits) is a prerequisite skill for any magic worker. (I don't think that's what you're aiming for, but it struck me as a neat way to build off of the idea.)

Re: Designing a contractual magic system?
 

It wasn't (what I was aiming for); but it is (a neat way to build off of the idea). I'm not sure I'd go with one Celestial Bureaucracy, myself; though I'd we were doing something kind of like Yrth but with this cosmology and magic system, I'd be fine with the equivalent of Megalos featuring a religion built around a single Divine Hierarchy. Other nations, not so much. Or rather, other religions, not so much. You might even go with the orc lands not worshipping any gods, having only shamans and sorcerers among them.

Re: Designing a contractual magic system?
 

There are different kinds of contract

1. You want something done. You want someone dead, or healed, you want your vermin infestation evicted, you want your forest fire put out, you want your enemy spied upon, you want to find a deposit of gold or an aquifer, you want to be transported to another country in a hurry or to have the clock turned back so you can make a different choice, so you get in touch with something that can do it and make it worth their while. It might want blood, or a musical performance, a cow, a gourmet meal, a memory, a box of aromatic powder, a four leaf clover, the heart of a ferocious beast, or a promise of your firstborn child. For each task a new deal with a new reaction roll.

2. You want power. You want to be given one or more advantages, and in return you need to trade something that will probably be bigger, like a body part, your ability to sing and speak, a vow of servitude, all of your memories, your post mortem soul, your youth, your beauty, your sense of humour. Usually you take on a disadvantage in exchange for your advantage.