Coming back here to edit now that I've finished the main article, I note with irony that "quick" may be a relative term here. Still, no cut tags today. You get the whole thing! Also, I seem to have touched on a lot of the areas that I was intending to touch on in regard to the philosophical underpinnings, so I am no longer sure that a separate article is necessary. I guess I can answer questions about anything I skipped over, or go back to the article if there are enough questions to justify it.
For the most part, GURPS can handle magic without going outside of the realm of existing skills and relatively non-cinematic advantages. The exceptions are pretty well handled in GURPS Low Tech Companion 1: Philosophers and Kings, with one notable loss from the perspective of those of us who approach matters with an animist bent. Those who conceive the universe in purely materialistic terms don't even have to worry about that little bit.
Let's start with advantages and disadvantages, then skills, and then discuss the matter of spirits.
GURPS advantages that can be used to represent any number of realistic magical elements include, obviously, the set of Luck advantages, including Luck itself in its various levels, Daredevil, and Serendipity. My own experience seems to indicate that there is nothing so blatant as Super Luck in the real world—though I am open to the possibility—but Luck, Extraordinary Luck, and perhaps even Ridiculous Luck do seem to affect some people. As well, Unluckiness seems to follow some people, and I am open to the idea that some rare people may even be Cursed. Higher Purpose and Visualization might be considered to fit into this cluster, as well.
Similarly, the Empathy collection of advantages—Animal Empathy, Empathy, Plant Empathy, Sensitive, and so on—would represent the basis of a number of demonstrated "psychic" abilities.
Channeling and Medium, as well as Spirit Empathy, should be available if spirits are assumed. See the discussion below. Some would say that they represent something real even in a setting where "spirits" are not literal entities.
Charisma is a real thing. To some degree, it is a trained ability—for example, see The Charisma Myth by Olivia Fox Cabane and other books that discuss "charisma"—but also represents the force of personality that magical training can build in a person.
Is Common Sense a psychic power? It certainly is a mental one, which is, after all, what "psychic" means.
Danger Sense does seem to be a real thing, demonstrated by non-human animals, and seeming to exist in some humans. Sure, you can explain it away as the result of a variety of sensory impressions being taken as a gestalt, but to a large degree that is exactly what "magic" refers to. Not exclusively, but that philosophical discussion is mostly beyond the scope of this article.
Eidetic Memory certainly covers some of the Art of Memory methods, which have been considered to be a part of magic for longer than materialism has separated the world into magic and science as opposed pairs. Photographic Memory is of a similar character.
The Basic Set doesn't mark Gunslinger, Trained by a Master, or Weapon Master as outside the realm of reality, but that's a question each Referee will have to resolve for themself. Obviously, Heroic Archer is in this cluster of advantages as well. If they do exist, then they would represent a certain sort of magic that is associated with "martial arts", speaking broadly.
I don't know if Illuminated corresponds with anything in the real world. Maybe it does. Certainly, there's nothing about it that would immediately disqualify it. Probably limit to certain sorts of settings, but still within the realm of realistic magic, I guess.
Indomitable seems like it is magical in nature, being specifically connected with the idea of influence. Unfazeable may be similar.
Intuition, of course, is clearly magical. It codifies into rules the ability to derive helpful information from seeming ignorance. It may also represent the sort of thing that goes by terms like "remote viewing", at least if connected to a skill like Intelligence Analysis and a Technique. Let's quickly write up something that we might use. We'll start with the idea that such a feat is of "Impossible" difficulty, but that it can be trained.
Default: Intelligence Analysis-10
Prerequisite: Intelligence Analysis, Intuition, cannot exceed Intelligence Analysis
By making a Remote Viewing roll, a character can get an impression about a distant target. The character may always take Extra Time in any such attempt, with a basic time of 10 seconds per attempt. The Margin of Success indicates how many interesting elements that the character successfully Views, with each element being determined by the Referee. For example, with a Margin of Success of 3, a character could receive a result of "An open plain, with a tree, and no animals or men present" (and, of course, they or their handler would already know what the target was, so they'd have to apply this description to what they already know). Note that some targets specifically include interesting but uninformative elements in an area they wish to protect from Remote Viewers. Referees should not take up more than one, or two if the false-interest objects are specifically designed (with extra cost at some level that I haven't worked out yet; probably 10 times?) for the purpose, of the "interesting item" slots with false-interest items. Failures at Remote Viewing will introduce false elements. Referees may mix false and true elements in a failed Remote Viewing session in order to particularly mislead Remote Viewers, but should never provide true elements that the characters don't already know about. For example, a failed attempt to Remote View a submarine base might include elements relating to a submarine, but then also false elements that are misleading. If the characters know little or nothing about the target for certain (Remote Viewing Mars, for example) then everything might be a false element on a failure.
Is Remote Viewing possible in reality? Certainly, there is quite a bit of experimental evidence. Some people do dispute the evidence, and in practice the ability has apparently been shown to be of limited practical use for Intelligence purposes (or is it just that Intelligence agencies have chosen to bury it deeper from scrutiny? Who knows what is really going on in that hall of mirrors. As it happens, some public entities, companies and individuals, do in fact consult Remote Viewers as part of their overall decision process, but this is another thing that should be left up to individual Referees for their particular settings). For some arguments about the method, along with some other science-based arguments toward "magic" generally, see Real Magic by Dean Radin, for example. I realize that some people disparage Radin and some others (Daniel Bem, Rupert Sheldrake, etc) on an ad hominem basis, but I can't help those people. Their studies speak for themselves, and it's fun to watch materialists move the goalposts whenever one of them seems to meet the predetermined criteria for showing an effect.
Even so, a glance at books like Naturalistic Occultism by IAO131 can show that there are a plethora of effects that have been traditionally codified as "magic" which can be described by such disciplines as modern neuroscience, and measured with devices such as CAT Scans and so on. If Remote Viewing is off the table, still a number of other abilities described here are not even controversial.
Okay, that was a little more in-depth into the underpinnings of the "realistic magic" approach than I really wanted to get into for this article. Still, it's useful, I hope.
Continuing on, it may be possible for the Referee (or players!) to come up with other Techniques that rely on mixing Intuition with other skills. Be careful to limit their effectiveness. Note that, for example, the Remote Viewing Technique above requires that the player spend as much as 12 points in addition to the cost of Intuition to have even marginal effectiveness (1 point in a Hard Skill for IQ-2, 11 points in the Technique to have it at Skill, or more likely 2 points in the Skill for IQ-1 and 10 points in the Technique to give Skill-1, either way giving a final Success Chance before other modifiers of 8). It's probably also worth looking at mixing other Advantages such as Danger Sense or even Luck with various Skills to represent other apparent abilities, such as maybe the alleged Remote Influence described by some soi-disant "psychic spies". That's getting outside of what I would consider strictly likely from my own experiences, however.
Oracle seems to be reasonable, and may be a way to represent those people who seem to be able to provide useful advice from divination techniques. However, as we'll discuss when we get to Skills, this may not be strictly necessary to represent these. From my own experience, I do think that some people are able to go beyond the limits described in Skills, but I think that the question remains an open one. Referees should probably allow a Technique that combines Oracle with Fortune-Telling if they want to allow the Advantage. I would, but I'm not going to define the Technique right now.
Alternately, a Referee could go so far as to allow the Precognition Advantage. There is some experimental evidence in its favor and the Advantage is sufficiently pricy for a fairly limited game effect. Up to you. I might not go this far, myself, but I go back and forth on it. In such a setting, the Destiny Advantages and Disadvantages are probably worth including. Similarly, the Psychometry Advantage isn't outside the realm of possibility, even if its experimental evidence is even less conclusive than that for Precognition. It is a little overly-reliable as written in game terms, though, so it's another one that is questionable.
Some theorists, such as Timothy Leary, have seriously proposed that Racial Memory might be a real thing, with a discernible, even possibly materialist, mechanism. There is some slight evidence that some sort of environmental information might be transmitted through DNA, with such things as the effects of trauma or PTSD possibly having an effect on the specific genetic sequences passed to the following generation. I dunno, maybe? This seems less well-documented than, say, Bem's precognition results. At the least, though, it isn't very outlandish in effect. Racial Memory (Active) might be pushing things, though, even at 40 points.
Rapier Wit. I mean, it's not a crazy effect, but it seems like it's probably cultural in nature. I probably wouldn't include it in a modern setting, where we're deeply indoctrinated with the idea that "words won't hurt me", but it might have a real effect in, say, an ancient Irish one where the effects of satire are understood to be real and powerful.
Single-Minded can represent certain types of trance states, but maybe it's better to leave this to the Autohypnosis Skill, which provides a similar effect. Speaking of that, I wonder if the two effects stack? Neither the Skill description nor the Advantage description rule the other out, so it would seem that they do. [EDIT: Looking into it, there is no official ruling yet, but the GURPS Line Editor has opined that the bonuses of the Autohypnosis Skill and Single-Minded Advantage should not stack, and that this should probably be incorporated into the rules. For all intents and purposes, the official stance is that they do not stack.]
Does Special Rapport exist in reality? There are certainly anecdotal reports of something like it. Sheldrake's experiments with dogs knowing about their owners returning home seem similar. It's questionable, but there is some reason to include it. Up to the Referee.
Visualization has a lot of experimental evidence in its favor. High-level athletes are not the only group of people who swear by the method. However, it seems to work best for people who are already at a high level of skill in the real world. Should it be a separate advantage, or just assumed as part of advanced levels of Skill? That's a difficult question. Should the Blessing and Cursing Enhancements from GURPS Powers be allowed? There is less evidence either way for this, but my experience would say that they should be. It's expensive enough that few characters in a realistic game are likely to have these abilities, but plausible enough to be worth allowing in theory. Cut the cost down by taking "Takes Extra Time" or the like.
I don't know about Wild Talent. Anecdotally, it does seem to exist to some degree, but I am not aware of any experimental evidence for something similar. I'd include it in a realistic game, but I can see arguments against it.
I'm not going to go through Perks in detail, though some of them clearly should exist in a realistic magic setting, like Autotrance.
Moving on to Disadvantages, let's just go through them in order, leaving out the ones we've already dealt with in the Luck cluster above.
Berserk matches up with some experiments that some groups have done with somafera "body-wild", which is specifically the idea that the human body can exceed its normal limits in particular circumstances. Advanced somafera practitioners relate that these abilities can be developed so that the loss of control is minimized. I refer the interested to Putting on the Wolf Skin and Scientific Magic, both by Wayland Skallagrimsson. Referees who accept these possibilities may make other Advantages available, using various Enhancements and Limitations. The prototypical ones would be Damage Resistance to Fire Only and Tough Skin, or Enhanced ST (or only Striking ST or Lifting ST) that costs FP, both connected via Accessibility to the Berserk Disadvantage, and then perhaps buying off the Berserk requirement (and Disadvantage) with further development.
Delusions are probably very common among real-world magicians. Certainly, in my experience, many magicians hold some very unorthodox beliefs to be true. Don't forget, though, that GURPS does specify that a Delusion might be factual in the end, it's just that most people don't believe it to be true and react to people who do accordingly.
Disciplines of Faith are a prerequisite to some of the "Technicians of the Sacred" practices described in GURPS Low Tech Companion 1: Philosophers and Kings.
Does Epilepsy allow a character to make use of the Dreaming Skill during waking hours? Up to the Referee. I'd say yes.
A lot of magicians develop something like Guilt Complex, since a lot of their worldview includes the notion that they have the ability to exert control over reality to some degree. In such cases, mishaps can be seen as personal mistakes and failings.
According to some researchers (here I am mainly thinking about T. M. Luhrmann, an anthropologist whose study, Persuasions of the Witch's Craft, is rather controversial in magical circles due to a justifiable perception that Luhrmann abused the trust of many of her informants, many magicians may suffer something very similar to Gullibility. I interpret Luhrmann's data in a very different way than she did, though. It doesn't help, when studying a social group, to apply one's own cultural assumptions to that group in interpreting the data from that group. Whatever, it's a book worth reading when trying to understand what exactly is going on in the real-world practices of magic. Like all of the ones I recommend, it needs to be understood in context, though, and not taken as Holy Writ.
Impulsiveness and Overconfidence might afflict some people who have too much reliance on their magical understanding and supposed powers. Similarly, people who rely excessively on divination might become Indecisive. Those who place too much significance on success and failure as magical outcomes might become Manic-Depressive, or even gain Low Self-Image. Obsessing over astronomical events might bring about Lunacy. Or maybe these things only reflect a deeper sensitivity to acausal connections.
Pacifism is far from universal among magicians in the real world, but some do make it a distinct part of their worldview. Note that, in reality, most human beings should have Pacifism: Reluctant Killer in any case. Not having that Disadvantage is usually described, informally at least, as "sociopathic" or "psychopathic", and it might be worthwhile to give such characters an automatic Reputation Disadvantage (perhaps -2 to all who have the Reluctant Killer Disadvantage and notice that the character is a "casual killer"). Maybe Post-Combat Shakes or even Combat Paralysis could be a reasonable alternative, too, if the Referee is willing. Certainly, other forms of Pacifism could replace these.
Strange events often do cluster around people who get into the practices of magic. Whether that is because the practitioners become adept at reframing otherwise innocuous events as unusual or they actually do happen more often is an open question. If an actual thing, then Weirdness Magnet might be appropriate, though for myself I dislike that Disadvantage immensely (isn't this already assumed to be the case in a roleplaying game?) and would not recommend it.
Finally, the following Skills are directly appropriate to any character in any setting who is said to practice "magic" in the sense of occult sciences. Magicians will also often develop other useful Skills, such as the Influence Skills, Artistic Skills, and the Prestidigitation-related Skills (and see that discussion below), as well as the Cinematic Martial Arts Skills and Musical Influence if they are available, but this list is for the more strictly magical ones.
Autohypnosis, Body Language, Breath Control, Cryptography (one magical text, Steganographia, is entirely a manual of cryptography presented in magical terms), Dreaming, Esoteric Medicine (if available, and perhaps it should be), Exorcism (see the discussion on spirits below), Fortune-Telling, Hypnotism, Meditation, Meteorology/TL4 (aka Weather Sense), Occultism (of course), Philosophy, Propaganda, Psychology, Religious Ritual, and Theology.
It's important to consider the "Technicians of the Sacred" section of GURPS Low Tech Companion 1: Philosophers and Kings. There are a number of skills described there which can be used to induce Autotrance and Dreaming effects in those lacking those Skills, including Religious Ritual, Dancing, Singing, Musical Instrument, and so on, even Erotic Art (which corresponds to a fairly large section of magical practice known as "sex magic").
Now, keep in mind that it was the magician John Dee who founded the world's first espionage agency (and his code name in it was 007!), and that previously espionage was treated as a largely esoteric and magical practice. Specifically, the deliberate obscurantism of the magical arts in history have been useful to those trying to control the flow of information. Even today, espionage agencies are frequently found skulking around esoteric sciences and philosophies.
Prestidigitation. In the past, magicians had to provide a show for their clients, going all the way back to the shamans of Siberia and other such "primitive" magicians. Magic is largely a practice that affects the mind, and so its effects are not immediately visible. This can reduce the effectiveness as the client stops trusting in the process and begins to fall back into previous patterns. To dramatize these rituals, some magicians have made use of "special effects" involving sleight of hand and misdirection, such as seeming to withdraw "disease" matter from the body of the client. Modern "skeptics" have pointed to these practices as indicating that the underlying techniques were therefore ineffective, with many modern stage magicians getting in on the scam of "debunking" anything that isn't described in purely materialist terms. They're mistaken, of course, but then as someone who doesn't accept the automatic supremacy of the materialist hypothesis, I would say that. Anyway, a magician, especially before the early 20th century, is very likely to have learned various prestidigitation skills like Sleight of Hand, Filch, Holdout, and the like, as well as Performance and such.
Spirits. Here we come to the biggest break between the materialist and the animist worldviews. The fact is that we can't measure spirits. On the other hand, we have fairly reliable methods of communicating with them which anyone who wants to take the effort can learn. The more sensitive among us can naturally feel their presence even without communicating directly. Materialists have attempted to explain these feelings with talk of standing sound waves, low-frequency sounds, and the like, but these seem to ignore the intermittent nature of some of these perceptions.
So do spirits exist? I would argue that they clearly do, as many people report being able to sense them, and that is repeatable enough, if not 100% reliable. Religions exist because many people are able to sense the presence of spiritual forces or entities (and then those perceptions are organized into social forces; the existence of the latter does not disprove the former). If some can't sense them, that doesn't disprove them any more than the fact that some people are blind or deaf disprove the existence of light and sound. They do seem to be very subtle forces, of course. I wouldn't go so far as to simulate them even with, say, ST 1 Telekinesis. Or maybe they are capable of such limited effects, as some hauntings correspond with physical stigmata such as scratches appearing on the afflicted, and of course some people have documented, inconclusively, some sort of "poltergeist" phenomena. Don't even get me started on the weirder end of what has been called the "Daimonic", though the interested can begin with books like Passport to Magonia by Jacques Vallée, Daimonic Reality by Patrick Harpur, or just about anything by John Keel (but especially Operation Trojan Horse, aka UFOs: Operation Trojan Horse).
How, then, to represent them in GURPS terms? The simplest method would be to start with the Astral Entity Meta-Trait, and to assume that Medium or Channeling are the only sure ways of communicating directly with them. Alternately, some of the Spirit Meta-Traits described in the Basic Set, GURPS Fantasy, or GURPS Horror provide a great place to start, and then the Probability Alteration Power from GURPS Psionic Powers can represent the more direct effects that some Spirits can use to affect the world. In any case, the ability to affect the material world should definitely be kept to a minimum, but that doesn't mean that it should be ruled out entirely. Even beyond that, though, these entities have the ability to communicate with those who are sensitive to them, as seen through Medium and Channeling, but perhaps also through Dreaming Skill. Dreaming communications may require interpretation via Fortune-Telling (Dream Interpretation), and are likely to be presented in symbolic terms, such that a Spirit wouldn't just say, "Hello, there are people preparing to raid your headquarters," but would instead be understood through a communication of dogs bursting through the windows of a dream house. How this is differentiated from the same dream being a sexual metaphor and wishful thinking is via the use of Fortune-Telling (Dream Interpretation).
Some such Spirits should be tied to locations, representing the Spirits of those locations, while others will be tied to objects, groups, or concepts. A great many Spirits are those of living or formerly-living persons, with Spirits of dead people commonly being called ghosts while those of living people have various names such as doppelganger, the Double, and the like. GURPS Spirits has some discussion of different sorts of Spirits. The most complex and powerful Spirits are frequently called Gods, Lwa, O-Kami, and similar terms that usually indicate deep respect and worshipfulness. In my own experience, apparently contacts with these great beings are usually with entities that are representative of them, rather than the beings Themselves. GURPS Voodoo: The Shadow War was particularly accurate, in my opinion, in framing all such contacts as being with Spirits of varying power level and able to appear in more than one example, such that one person could call for a Major Manifestation of a particular Lwa, while their friend could also call one at the same time, and the person they were opposing potentially could as well! These could be said to correspond to angels "messengers", while the Arch-Angels of monotheistic traditions would be in roughly the same position as the Gods of polytheistic ones. (I will refrain here from arguing how polytheistic assumptions seem to match observed reality better than monotheistic or atheistic ones. See A World Full of Gods by John Michael Greer—note, absolutely not the one of that title by Keith Hopkins, which is on an entirely different topic relating to the rise of Christianity in the ancient world—if you are interested in that topic.)
I'm not going to enter into a long discussion of herbalism (GURPS: Pharmacy (Herbal), at least in part, plus Meditation and other Skills described in "Technicians of the Sacred" in GURPS Low Tech Companion 1), entheogenic substances (again, see that "Technicians of the Sacred" section in GURPS Low Tech Companion 1), alchemy (GURPS: Chemistry/TL0-4 plus Meditation, as well as the occasional prestidigitation Skills), and the like.
Finally, it is useful to any real-world magician to have detailed and specific knowledge Skills. Everything from Intelligence Analysis to Naturalist to Market Analysis to Strategy and Tactics Skills, and many more besides, are going to be useful parts of the knowledge base of a real-world magician. Gambling is useful in calculating the odds and so advising a King or other decision-maker, which is a common enough place for magicians in history (again, see Dr. John Dee and his relationship with Queen Elizabeth I's court). Even outside of that, magicians were sought out by other people looking for advice on topics ranging from love to healing and beyond.
Because I think that it can be useful to end a long article with a bibliography of works that are useful but weren't referenced in the body, here are some works that are going to be particularly useful in understanding real-world magic that weren't referenced above:
Betz, Hans Dieter - The Greek Magical Papyri in Translation
Brennan, J. H. - Astral Doorways (also a game designer, known mainly for Timeship, which was notorious for blurring the lines between magical practices and roleplaying, and that during the Satanic Panic years)
Child, Alice B. and Irvin L. Child - Religion and Magic in the Life of Traditional Peoples
Couliano, Ioan - Eros and Magic in the Renaissance (if you read nothing else I've referenced, read this one)
Dukes, Ramsey - How to See Fairies
Dunn, Patrick - Magic, Power, Language, Symbol
Dunn, Patrick - Postmodern Magic
Greer, John Michael - Green Wizardry (attempts to be for the modern world what Picatrix and similar texts were, in part, to the early modern one, a collection of useful science and art that can help a community or individual in daily life; more importantly for these purposes, discusses in some detail what such texts were trying to transmit, and so what magicians were often expected to know)
Greer, John Michael - Mystery Teachings from the Living Earth
Keith, William H. Jr. - The Science of the Craft (a somewhat credulous, but definitely worthy, attempt at understanding magic through modern, cutting-edge science; written by a noted gamer and fiction author)
Lecouteux, Claude - Witches, Werewolves, and Fairies (really, anything by Lecouteux is worth looking into; pick a title that interests you and go from there, and if you can read the French originals, more power to you)
Marsh, Clint - The Mentalist's Handbook
Paper, Jordan - The Deities Are Many (along with Greer's A World Full of Gods, an excellent primer of polytheistic theology; where Greer's work is an apologetic and even a polemic, Paper's is a confessional; I should probably point out here that "polytheistic" and "animistic" are pretty well synonymous in the world outside of academia and the people who build their worldviews on the frequently artificial and theoretical distinctions of the academy—which is to say that I can't think of a real-world instance of a polytheistic religion which isn't also animistic, nor any pre-modern example of animism which is not also conceptually polytheistic)
Paxson, Diana - Trance-Portation
Pócs, Éva - Between the Living and the Dead
Skelton, Robin - Spellcraft (especially in conjunction with Thompson, below)
Skinner, Stephen - Techniques of Graeco-Egyptian Magic
Skinner, Stephen - Techniques of Solomonic Magic
Smith, Dave - Quantum Sorcery (another credulous approach to mixing science and magic)
Smith, Morton - Jesus the Magician
Stone, Kelly L. - Thinking Write
Thompson, Christopher Scott - A God Who Makes Fire (especially in conjunction with Skelton, above)
Whitcomb, Bill - The Magician's Reflection
Wier, Dennis R. - Trance: From Magic to Technology
Wilby, Emma - Cunning Folk and Familiar Spirits
Anyway, I hope this is helpful to you in some way.